  netrik hacker's manual
>========================<

[This file contains a description of the layouting module. See hacking.txt
or hacking.html for an overview of the manual.]

_0. Overview_

The whole layouting is split up into several, fairly simple passes, which are
executed one after the other. See the notes in hacking.* for a discussion of
this approach.

The first pass is the _parse_syntax()_ function, which creates a _Syntax
Tree_ of the document. This tree contains all HTML elements and their content,
but the elements have no special meaning yet.

_dump_tree()_ can be used to output the syntax tree.

In the next pass ( _parse_elements()_ ), all element and attribute names
are looked up in tables and stored as enums to facialiate further processing.

If not compiled with -DXHTML_ONLY, an additional pass is inserted after
element parsing: In _5. sgml_rework()_ , the syntax tree is modified
to fix the wrong element nesting caused by missing end tags in SGML documents.

dump_tree() can be used again to dump all element and attribute types as
found in the lookup, and the possibly modified tree structure.

The third pass is the central processing step.  _parse_struct()_ interprets
the elements and their attributes, and creates a _Structure Tree_ , which
contains all the items that will be visible on the output page.

The fourth pass prepares the page for rendering. In _pre_render()_, all
items created in parse_struct() are assigned actual sizes and positions in
the output page. Also, a structure "page_map[]" is created, needed for fast
lookup what items are present in any given line.

All of the passes mentioned above are necessary to prepare the rendering,
and are executed from _layout()_ .

The actual rendering is done in render.c. However, this isn't done for the
whole page like the other layouting passes. Instead, every time some region
of the output page needs to be displayed, _render()_ is called to render
exactly that region.

Alternatively, The whole page can be dumped to the terminal line by line,
using _dump()_ .

The third function in render.c is _dump_items()_ .  This is not really a
rendering function; it only dumps the item tree, including the (coloured) text.

_1. layout.c_

This file forms the framework for the layouting process. It contains functions
to load a file and prepare it for rendering, but also to free the memory used
by a document when it is no longer needed.

_layout()_

layout() is given a URL of a file or web resource to load, and does all
actions necessary to be able to render the corresponding page.

Before starting any of the loading or layouting operations, a descriptor is
allocated where all the data structures created inside layout() will be stored.

The descriptor is a "struct Layout" pointer. It contains the following data:

- A pointer to the input ressource descriptor ("input")
- An additional pointer "url" to the effective page URL, necessary to hold the
  URL after the input resource descriptor is freed
- Pointers to all data structures necessary for the layouting ("syntax_tree",
  "item_tree", "page_map[]")
- Pointers to the "link_list" and "anchor_list" data structures

After allocating the descriptor, layout() first opens the resource with
init_load(). (Described in hacking-load.*)

Afterwards, _parse_syntax()_ , _parse_elements()_ , _5. sgml_rework()_
, _parse_struct()_ , and _pre_render()_ are called in sequence.
These functions are responsible for preparing the page for rendering.

The file loading itself is done inside parse_syntax(), which uses the load()
function from load.c (see hacking-load.*) to read a data block every time
the input buffer is empty. It processes the data in the buffer character by
character (keeping track of the current read position by "input->buf_ptr"),
and when it reaches the end it calls load() again to get the next data block.

After parse_struct(), the syntax tree is no longer needed. It is freed by
_free_syntax()_ .

At this point also the "link_list" and "anchor_list" data structures are
created using make_link_list() and make_anchor_list(). (See hacking-links.* )

_free_layout()_

When a page is unloaded (usally before loading a new page), this function
is called to free all the data structures created by the layouting process
to allow rendering (Item tree, page map, link list, anchor list), i.e. all
data stored in the "Layout" descriptor, except for "input" and "syntax_tree",
which are already freed during the layouting process. (s.a.) The descriptor
itself is also freed.

_2. parse-syntax.c_

The first thing to be done when layouting is parsing the syntax of the
input file. This is done by parse_syntax(). This function creates a syntax
tree. The pointer to the head of this tree is returned to layout() and stored
as "layout->syntax_tree" there.

_Syntax Tree_

Every node of "syntax_tree" is a structure of the type "Element" (defined
in "syntax.h") and corresponds to one HTML element. (An element in an HTML
document is represented by an HTML-tag, and the corresponding end tag, if any.)
End tags do not create tree nodes, as they only close elements already stored.

For the supplied "test/0.html":

   header text
 
   <html><head> </head>
   <body>
      <h1> heading </h1>
      <p>
         first paragraph of text; 
         includes  multiple spaces and newlines,
         <em> emphasized text </em>and 
         <strong> strong text </strong>
      </p>
      <p>
         <center>starting with an evil center tag,</center>
         this very long second paragraph contains some special characters (including a simple space...):
         &amp;; &lt;&gt;&quot;=/ plus a big gap&nbsp;&nbsp;&nbsp;and two unicode escapes
         (decimal: &#161; and hexal: &#xbf;)
         but also an anchor em<a name="anchor" href="">bed</a>ded inside a word
         (this anchor also is the only tag with parameters);
         and finally a blank row <br /> (a single tag)
      </p>
   </body>
   </html>

the syntax tree looks like this:

  ++>NULL
  +
+---+
| ! |-. <++
+---+ |   +
      v   +
 ,-----------.
("header text")
 `-+------+--'
   | html |-. <++++++++
   +------+ | <+      +
            v  +      +
        +------+  +------+   <+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
        | head |->| body |-. <++++++++++++++                                              +
        +------+  +------+ | <+            +                                              +
                           v  +            +                                              +     <+++++++++++++++++++++++++++++++++++++++++++++++++
                        +----++          +---+     <++++++++++++++++++++                +---+   <++++++++++++++++++++++++++++++++                +
                        | h1 |-. <+   ,->| p |---. <++                 +             ,->| p |-. <++++++++++++++++++             +                +    (back
                        +----+ |  +   |  +---+   |   +                 +             |  +---+ | <+                +             +                +    to top)
                               v  +   |          v   +                 +             |        |  +                +             +                +          ^
                          ,--------.  |  ,------------------.       ,----.           |        |  +          ,----------.   ,---------.   ,---------------.  |
                         (" heading") | (" first...newlines,")     (" and")          |        v  +         (" this...em") ("ded...row") (" (a single tag)") |
                          `--+---+-'  |  `-----+----+-------'     +-+----+-+         |   +--------+         `---+---+--'   `--+----+-'   `-----+---+-----'  |
                             | ? |----'        | em |-. <++    ,->| strong |-. <++   |   | center |-. <++    ,->| a |-. <+ ,->| br |---------->| ? |--------'
                             +---+             +----+ |   +    |  +--------+ |   +   |   +--------+ |   +    |  +---+ |  + |  +----+           +---+
                                                      v   +    |             v   +   |              v   +    |        v  + |
                                           ,----------------.  |     ,------------.  |   ,----------------.  |      ,---.+ |
                                          (" emphasized text") |    (" strong text") |  (" starting...tag,") |     ("bed") |
                                           `--------+---+---'  |     `-----+---+--'  |   `--------+---+---'  |      +---+  |
+++> "parent"                                       | ? |------'           | ? |-----'            | ? |------'      | ? |--'
---> "list_next"                                    +---+                  +---+                  +---+             +---+

(I'm really curious if anyone can read this ;-) )

The "Element" structure includes:

- The pointers "list_next" and "parent" describe the tree structure.
  "parent" points to the element (node) which contains this one in its content
  (text) area. "list_next" points to the next element as it appears in the
  input stream.
- The "closed" flag is a helper flag for _5. sgml_rework()_ and has
  no meaning outside of it.
- The union "name" describes what kind of element this node represents. ("html",
  "head" etc.) It can store the element name either as a pointer to a string
  (as appears in the input stream), or as an enum number.
- "attr_count" stores the number of attributes of this element. (Attributes
  are the parameters of an element, which appear inside the start tag, like:
  href="foo" etc.)
- "attr" points to an array of "Attr" structures. Each of these structures
  contains the data for one attribute; it consists of a union of type
  "Attr_name", which, like "Element_name", stores the attribute name either as a
  string or as an enum; and a union of type "Attr_value", which stores the
  value of the attribute. (String or number.)
- The "content" string stores the content. (The text between the tags.) Every
  element stores the content between the previous tag and the start tag of this
  element. Thus it does not store the content of the element itself, but part
  of the content of the *parent* element. This simplifies processing a bit,
  because this way no facility for storing content blocks divided by
  sub-elements is needed -- the sub-elements store the content themselves. The
  caveat is that a lot of dummy elements are needed to store the content if no
  further sub-element follows them. This is quite a big inefficiency, as nearly
  every real element also needs a dummy element to store its content. This
  should change in the future -- if we won't drop the syntax-tree in its
  present form at all... Which we will :-)

_Initialization_

Before starting parsing, we have to create the tree top. (We call it the global
element.) This is done by setting "cur_el" to NULL and calling add_element().

_add_element()_

This function creates a new node and inserts it into the syntax tree; thus it
has to set the "parent" and "list_next" pointers too, and adjust some pointes
of other nodes to point to this one.

"parent" is set to "cur_el", as any new tag is created while parsing the
content area of its parent. "list_next" is set to NULL, as the new node is
always the last one in the list. "list_next" of "last_el" (the last node
in the list up to now) is set to point to the new node; this is omitted if
"cur_el" is NULL, indicating that there are no other nodes yet.

_Parsing_

The parser itself works in a very simple way. It is some kind of state machine.
For every input character, one action is taken, selected by a dispatcher
depending on the current state (stored in "parse_mode") and the input char
itself. Several combinations (e.g. tag start) change the current state, thus
the following character(s) are parsed in a different mode. (Other actions
are taken.)

Sometimes a character that causes a mode switch has to be parsed in the new
mode itself. In this case the flag "recycle" is set after the mode change,
causing the dispatch to be repeated for the same char, but in the new mode.

Again, the parsing is not very efficient in the present implementation. (In
fact, it is by far the most time consuming part of the whole layouting.)
Especially the huge switch is quite slow. (Good compilers have a fairly
efficient implementation of the switch itself; however, it still causes many
unpredictable branches.) There are some possibilities to optimize this. The
bigger problem is that the inner loop is quite big, and may not fit into the
processor's instruction cache, thus making it terribly slow. Maybe splitting
the parsing into several simpler passes would help. However, we are planning
to switch to a completely different, (hopefully) much more efficient parser
system in the next major release...

The default parsing mode is "PM_CONTENT", which is the mode for parsing
element content. Any normal character encountered in this mode is simply
added to "text_buf" by "buf_add_char()". A ' ', '\t', '\n', '\r' or '\f'
aren't stored; we switch to "PM_BLANK" instead. Any following blank space
is ignored. As soon as a normal character occurs again, we store a single '
' and swich back to "PM_CONTENT".

input:
         first paragraph of text; 
         includes  multiple spaces and newlines,
                      ^
                      file position
 
text_buf: " first paragraph of text; includes mul"

_<pre> Blocks_

After a <pre> tag, the mode isn't switched back to "PM_CONTENT", but to
"PM_PRE". In this mode all blank space characters are stored to "text_buf"
as non-breakable spaces, except newlines which are stored directly.

The mode is ended and switched to "PM_CONTENT" again when a closing "</pre>"
tag is encountered.

_References_

An '&' indicates a character reference (unicode escape) or entity reference
(named escape), and starts the reference parsing mode "PM_AMP". On entering
this mode, the current write position in "text_buf" is saved to "amp_pos".

input: [...] &lt; [...]
             ^
text_buf: "[...] &"
                 ^^
                  text_buf_len
                 amp_pos

In this mode characters are added to "text_buf", too. The mode is switched
back to the previous mode (saved in "prev_mode_amp") when a ';' occurs.

input: [...] &lt; [...]
                ^
text_buf: "[...] &lt;"
                 ^   ^
                     text_buf_len
                 amp_pos

The text between the saved start position of the escape sequence and the current
positon is converted then. First the string is looked up in "ref_table[]",
which is a table of named characters, defined in facilities.c . If it isn't
there, but it seems to be a numerical character reference, it is converted
to an integer (ASCII value). If a replacement char was found either in the
table or by the number conversion, the reference is removed from "text_buf"
and the replacement char is inserted instead.

text_buf: "[...] <"
                  ^
                  text_buf_len

If no replacement was found, the string is left unchanged. (We consider this
behaviour better then of some other browsers, which simply leave unknown
characters out, and the user only wonders that something is missing...)
Probably we will mark unknown escapes by some attribute in the future.

_Tags_

A '<' starts tag parsing. There is a a whole bunch of tag parsing modes. The
one entered after the '<' is "PM_TAG_START", which indicates that the tag
name should follow next.

input: [...] but also an anchor em<a name="anchor" href="">bed</a>ded inside a word
                                  ^
text_buf: "[...] but also an anchor em"
tree:
     +
     +     
   +---+   
-->| p |-.     <== cur_el
   +---+ | <+         
         |  +         
         |  +         
         v  +         
    +--------+        
    | center |-. <++  
    +--------+ |   +  
               v   +  
    ,----------------.
   (" starting...tag,")
    `--------+---+---'   <-- last_el
             | ? |->NULL
             +---+

_Start Tags_

If the following character is an normal char, it's a start tag. (Or a single
tag, which is treated the same way for now.) "PM_TAG_NAME" is entered. A new
element node is created by _add_element()_ . Any content in front of this
new element, which was stored in "text_buf" up to now, is stored to the new
node's "text" field by "insert_buf".

input: [...] but also an anchor em<a name="anchor" href="">bed</a>ded inside a word
                                   ^
text_buf: ""
tree:
    +    
  +---+
->| p |-. <++++++++++++++++++
  +---+ | <+                +
        |  +  <==           +
        |  +          ,----------. 
        v  +         (" this...em")
   +--------+         `---+---+--'    <--
   | center |-. <++    ,->|   |->NULL
   +--------+ |   +    |  +---+
              v   +    |
   ,----------------.  |
  (" starting...tag,") |
   `--------+---+---'  |
            | ? |------'
            +---+       

Normal characters encounterd in "PM_TAG_NAME" mode (including the one that
started the mode) are stored to "text_buf".

input: [...] but also an anchor em<a name="anchor" href="">bed</a>ded inside a word
                                   ^
text_buf: "a"

A blank space character ends "PM_TAG_NAME" and switches to "PM_TAG", which
indicates that attributes may follow. "text_buf" is stored as the element name.

input: [...] but also an anchor em<a name="anchor" href="">bed</a>ded inside a word
                                    ^
text_buf: ""
tree:
    +    
  +---+
->| p |-. <++++++++++++++++++
  +---+ | <+                +
        |  +  <==           +
        |  +          ,----------. 
        v  +         (" this...em")
   +--------+         `---+---+--'    <--
   | center |-. <++    ,->| a |->NULL
   +--------+ |   +    |  +---+
              v   +    |
   ,----------------.  |
  (" starting...tag,") |
   `--------+---+---'  |
            | ? |------'
            +---+       

A following normal char is the beginning of an attribute name, and switches to
"PM_ATTR_NAME". Characters encounterd in "PM_ATTR_NAME" mode are also stored to
"text_buf".

input: [...] but also an anchor em<a name="anchor" href="">bed</a>ded inside a word
                                        ^
text_buf: "name"

The attribute name ends with an '=' or a blank char. A new entry is created
in the "attr[]" array, and "text_buf" is stored as the attribute name. Mode
is switched to "PM_ATTR_NAME_END" first, which indicates that the attribue
value should follow.

input: [...] but also an anchor em<a name="anchor" href="">bed</a>ded inside a word
                                         ^
text_buf: ""
attr:   name:   data:
        "name"  ""

If the attribute name was ended by an '=', mode is switched immediately to
"PM_ATTR_VALUE", otherwise as soon as an '=' is encountered. (After any amount
of whitespace.)

White space in "PM_ATTR_VALUE" mode (after the '=') is ignored too.

Next char must be either a '"' or a '\'', and switches to "PM_ATTR_DATA_QUOT"
or "PM_ATTR_DATA_APOS", respectively. In this modes characters are stored to
"text_buf" again.

input: [...] but also an anchor em<a name="anchor" href="">bed</a>ded inside a word
                                                ^
text_buf: "anchor"
attr:   name:   data:
        "name"  ""

A second '"' (or '\'', respectively) ends this mode. "text_buf" is stored as
the attribute value for the new "attr" entry.

input: [...] but also an anchor em<a name="anchor" href="">bed</a>ded inside a word
                                                 ^
text_buf: ""
attr:   name:   data:
        "name"  "anchor"

Mode is swiched back to "PM_TAG". Now blank space may follow (which is ignored),
followed by another attribute.

input: [...] but also an anchor em<a name="anchor" href="">bed</a>ded inside a word
                                                   ^

In "PM_TAG" mode also a '>' may occur, ending tag parsing and switching back
to the mode before tag parsing had begun. (PM_CONTENT or PM_BLANK.) In this
case, "cur_el" is set to "last_el"; this means descending in the syntax tree
to the newly created node.

input: [...] but also an anchor em<a name="anchor" href="">bed</a>ded inside a word
                                                          ^
text_buf: ""
attr:   name:   data:
        "name"  "anchor"
        "href"  ""
tree:
    +    
  +---+
->| p |-. <++++++++++++++++++
  +---+ | <+                +
        |  +                +
        |  +          ,----------. 
        v  +         (" this...em")
   +--------+         `---+---+--'     <==
   | center |-. <++    ,->| a |->NULL  <--
   +--------+ |   +    |  +---+
              v   +    |
   ,----------------.  |
  (" starting...tag,") |
   `--------+---+---'  |
            | ? |------'
            +---+       

A '>' may also occur in "PM_TAG_NAME" mode, meaning the element has no
attributes.

input: <html> <head> [...]
            ^

In this case creating the new node and storing the name, and descending into
the element are done in one step. (By "recycle".)

_End Tags_

If the first character after the '<' is a '/', the tag is an end tag, and we
switch to "PM_END_TAG_NAME". If any text is pending in "text_buf", we have to
store it somewhere. As an end tag normally does not create a new element node,
we have to create a dummy node for this. (Very inefficient, s.a.)

input: [...] but also an anchor em<a name="anchor" href="">bed</a>ded inside a word
                                                               ^
tree:
    +
  +---+
->| p |-. <++++++++++++++++++
  +---+ | <+                +
        |  +                +
        |  +          ,----------.
        v  +         (" this...em")
   +--------+         `---+---+--'    <==
   | center |-. <++    ,->| a |-. <+
   +--------+ |   +    |  +---+ |  +
              v   +    |        v  +
   ,----------------.  |      ,---.+
  (" starting...tag,") |     ("bed")
   `--------+---+---'  |      +---+        <--
            | ? |------'      | ? |->NULL
            +---+             +---+

Normal chars in "PM_END_TAG_NAME" mode are stored to "text_buf", too.
"PM_END_TAG_NAME" can be ended immediately by a '>', or by blank space
(switching to "PM_END_TAG_SPACE") followed by '>'.

The tag name extracted to "text_buf" is compared against the element name of
the current element, to see if the end tag matches, and then abdannoned. The
element is closed by ascending to the parent.

input: [...] but also an anchor em<a name="anchor" href="">bed</a>ded inside a word
                                                                 ^
tree:
    +
  +---+
->| p |-. <++++++++++++++++++
  +---+ | <+                +
        |  +    <==         +
        |  +          ,----------.
        v  +         (" this...em")
   +--------+         `---+---+--'
   | center |-. <++    ,->| a |-. <+
   +--------+ |   +    |  +---+ |  +
              v   +    |        v  +
   ,----------------.  |      ,---.+
  (" starting...tag,") |     ("bed")
   `--------+---+---'  |      +---+        <--
            | ? |------'      | ? |->NULL
            +---+             +---+

_Single Tags_

If a '/' appears instead of an attribute name in "PM_TAG" mode, "parse_mode"
is set to "PM_SINGLE_TAG", indicating an (XHTML) single tag.

input: [...] and finally a blank row <br /> (a single tag)
                                         ^

The '/' can also immediately follow the element name. (In "PM_TAG_NAME" mode.)

input: <hr/>
          ^

In this case, creating the node and switching to a single tag are done in
one step by "recycle".

In any case, a '>' has to follow, and switches back to normal mode just like
in a start tag, only it does not descend (set the new node as "cur_el") --
a single tag has no content area; the content following a single tag still
belongs to the parent.

input: [...] and finally a blank row <br /> (a single tag)
                                          ^
tree:
    +
  +---+   <++++++++++++++++++++++++++++++++
->| p |-. <++++++++++++++++++             +
  +---+ | <+                +             +
        |  +   <==          +             +
        |  +          ,----------.   ,---------. 
        v  +         (" this...em") ("ded...row")
   +--------+         `---+---+--'   `--+----+-'      <--
   | center |-. <++    ,->| a |-. <+ ,->| br |->NULL
   +--------+ |   +    |  +---+ |  + |  +----+   
              v   +    |        v  + |
   ,----------------.  |      ,---.+ |
  (" starting...tag,") |     ("bed") |
   `--------+---+---'  |      +---+  |
            | ? |------'      | ? |--'
            +---+             +---+

_Comments_

In "PM_TAG_START" mode (after the '<'), also an '!' can follow, indicating
that we have not any tag at all, but either a comment, a DOCTYPE declaration,
or a CDATA section. "parse_mode" is set to "PM_EXCLAM" in this case.

input: some text <!--a test-comment--> and more text
                  ^

If a '-' follows, it's a comment. Mode is switched to "PM_COMMENT_START".

input: some text <!--a test-comment--> and more text
                   ^

Now a second '-' has to follow, switching to "PM_COMMENT". In this mode any
characters but a '-' are simply ignored.

input: some text <!--a test-comment--> and more text
                          ^
text_buf: "some text"

A '-' switches to "PM_COMMENT_END1", which means that it *may* be the
comment end.

input: some text <!--a test-comment--> and more text
                           ^

However, if it is followed by any other char than a second '-', mode is
switched back to "PM_COMMENT".

input: some text <!--a test-comment--> and more text
                            ^

A second '-' in "PM_COMMENT_END1" switches to "PM_COMMENT_END2", which means
that now the comment really ends.

input: some text <!--a test-comment--> and more text
                                    ^

Now the '>' has to follow, and switches back to parsing mode before the
beginning of the comment ("prev_mode_tag").

input: some text <!--a test-comment--> and more text
                                     ^

_DOCTYPE Declarations_

If a normal char occurs in "PM_EXCLAM", we assume it is the "D" in "<!DOCTYPE".

input: garbage <!DOCTYPE somedoc> more garbage
                 ^

We treat DOCTYPE declarations as comments. Any characters but '>' are ignored.

input: garbage <!DOCTYPE somedoc> more garbage
                               ^
text_buf: "garbage"

A '>' returns to normal mode.

input: garbage <!DOCTYPE somedoc> more garbage
                                ^

This isn't a very reliable detection, as according to the grammer, an unescaped
'>' may appear in some system literal inside the declaration. However, we
assume that this won't happen... (We would have to parse the whole declaration
otherwise.)

_CDATA Sections_

A '[' in "PM_EXCLAM" mode starts a CDATA section, indicated by
"PM_CDATA_START". If there is a pending blank ("prev_mode_tag" is "PM_BLANK"),
it has to be stored *before* the CDATA.

input: some text <![CDATA[a tricky ]> CDATA section]]> and more text
                   ^
text_buf: "some text "

Following normal chars (should) belong to the "CDATA" string, and are ignored.

input: some text <![CDATA[a tricky ]> CDATA section]]> and more text
                        ^
text_buf: "some text "

A second '[' in "PM_CDATA_START" mode switches to "PM_CDATA", indicating that
the actual data will follow.

input: some text <![CDATA[a tricky ]> CDATA section]]> and more text
                         ^

Any characters in "PM_CDATA" mode but '>' are stored directly to "text_buf".

input: some text <![CDATA[a tricky ]> CDATA section]]> and more text
                                   ^
text_buf: "some text a tricky ]"

When a '>' occurs, the previous two chars (in "text_buf") are tested against
"]]". If they do not match, the '>' is simply stored just as any other
character.

input: some text <![CDATA[a tricky ]> CDATA section]]> and more text
                                    ^
text_buf: "some text a tricky ]>"

If they match, the last two characters are removed from "text_buf" (they
belong to the CDATA terminator), and mode is switched back to "PM_CONTENT". (It
doesn't need to be switched back to the mode before the CDATA section, as any
pending blanks already have been stored, and a CDATA section can't start in
other modes than "PM_TEXT" or "PM_BLANK".)

input: some text <![CDATA[a tricky ]> CDATA section]]> and more text
                                                     ^
text_buf: "some text a tricky ]> CDATA section]]"

input: some text <![CDATA[a tricky ]> CDATA section]]> and more text
                                                      ^
text_buf: "some text a tricky ]> CDATA section"

_Processing Instructions_

The '<' may also be followed by a '?', indicating a processing instruction.
Mode is switched from "PM_TAG_START" to "PM_INSTR".

input: some text <?a fake? processing instruction??> more text
                  ^

Processing instructions are also treated as comments. Any chars but '?' are
ignored in "PM_INSTR".

input: some text <?a fake? processing instruction??> more text
                        ^
text_buf: "some text"

A '?' switches to "PM_INSTR_END", indicating this *may* be the end of the
processing instruction.

input: some text <?a fake? processing instruction??> more text
                         ^

If a normal char follows the '?', mode is switched back to "PM_INSTR".

input: some text <?a fake? processing instruction??> more text
                          ^

If a second '?' follows, "PM_INSTR_END" is kept, as the first one isn't the
end of the processing instruction, but the new one could be.

input: some text <?a fake? processing instruction??> more text
                                                  ^

A '>' in PM_INSTR_END really ends the processing instruction, and switches to
"prev_mode_tag".

input: some text <?a fake? processing instruction??> more text
                                                   ^

_SGML Mode_

When compiled without the "-DXHTML_ONLY" option, a few cases more are possible.

_Unclosed Tags_

In SGML, not every element has to have an end tag.

When an end tag is encountered, we ascend in the syntax tree not only once,
but until an element is found that matches the end tag. Thus, all elements
in between are automatically closed.

input:
<body>
   <p>
      some text
      <hr>
   </p>
   ^
 
+------+
| body |-.<+
+------+ | +
         v +
       +---+
       | p |-.<+
       +---+ | +
             v +
        ,-------.
       (some text)
        `-+----+'
      --> | hr |->NULL
      ==> +----+

input:
<body>
   <p>
      some text
      <hr>
   </p>
      ^
 
+------+
| body |-.<+   <==
+------+ | +
         v +
       +---+
       | p |-.<+
       +---+ | +
             v +
        ,-------.
       (some text)
        `-+----+'
      --> | hr |->NULL
          +----+

_Unquoted Attribute Values_

When a normal char occurs in "PM_ATTR_VALUE" mode, "PM_ATTR_DATA_NOQUOTE"
is entered.

input: <sometag someattribute=somevalue minimized third="nothing">
                              ^

This mode is just like  "PM_ATTR_DATA_QUOT" or "PM_ATTR_DATA_APOS", only it
is ended by a blank or the tag end.

input: <sometag someattribute=somevalue minimized third="nothing">
                                       ^

_Mimimized Attributes_

In SGML, attributes without a value are possible. This is recognized when a
normal char or the tag end occurs in "PM_ATTR_NAME_END" mode instead of the '='.

input: <sometag someattribute=somevalue minimized third="nothing">
                                                  ^

The attribute is ended immediately. "text_buf[]" (which is empty in this case)
is stored just like at the end of an unquoted attribute value. Mode is set to
"PM_TAG", and the current character (the tag end or beginning of next attribute)
is processed in this mode.

_SGML Comments_

Comments also allow more complicated syntax. For one, blank space is
possible between the "--" ending the comment string and the '>' ending the
declaration. Thus, blank space in PM_COMMENT_END2 is ignored.

Moreover, another comment string may follow the end. Thus, a '-' in
PM_COMMENT_END2 switches back to PM_COMMENT_START, similary to the '-' after
the "<!".

input: <!--comment start--  --second comment string in same declaration--  >
                            ^

Finally, SGML also allows empty declarations ("<!>"), which are also a kind
of comment. Thus a '>' in PM_EXCLAM switches immediately to PM_COMMENT_END2
and recycles.

input: <!>
         ^

_Finishing_

Parsing is ended by EOF. This should only appear in "PM_CONTENT" or "PM_BLANK"
mode (not inside some tag, comment, CDATA section or chracter/entity reference),
and only if the current element is the global one (not while parsing some
element's content).

The "list_next" pointer of the last node is set to point back to the tree top.
This faciliates easier processing in the following steps.

_Error Handling_

When using -DXHTML_ONLY, every syntax error encountered causes netrik to
print an error message and immediately quit. (The XML standard requires this.)

Without -DXHTML_ONLY, netrik is more tolerant.

_Workarounds_

Netrik uses simple workarounds for some of the most common cases of broken HTML:

If some illegal char occurs in a entity/character reference, it's probably
not really a reference, but an unescaped '&'. We keep the whole sequence
literally and switch back to "prev_mode_amp".

input: x = a & b
              ^
text_buf: "x = a & "

Similar for illegal characters in "PM_TAG_START", which indicate an unescaped
'<'. We store a '<' and switch back to "prev_mode_tag".

input: if(a < b)
             ^
text_buf: "< "

As SGML comments have a quite complicated syntax, reasonable error handling
is also a bit more complicated.

If someting else then '>' (end of comment declaration), '-' (beginning of
second comment string), or blank space follows in PM_COMMENT_END2 (after a
"--"), then the "--" was probably not intended to have any special meaning,
but simply to be part of the comment. Thus, mode is switched back to PM_COMMENT.

input: <!-- some broken -- comment -->
                           ^

The same is done for unexpected characters in PM_COMMENT_START mode, which
is most common for "---" inside a comment.

input: <!-- some broken --- comment -->
                            ^

There is one exception to this, however: If a '>' follows in PM_COMMENT_START
mode, and it was preceeded not only by one '-' (the one which started
PM_COMMENT_START) but two or more, then the the '>' together with the last two
'-' was probably intended as as an XML-like "-->" comment end.

input: <!--- anything --->
                        ^
parse_mode: PM_COMMENT_START
dash_count: 3

The "dash_count" variable keeps track of how many dashes have been encountered
in a row; it is incremented every time a '-' apprears in some of the comment
parsing modes, and is reset to 0 every time some other character is encountered.

"dash_count" is also used in another situation: If a '>' follows in PM_COMMENT
or PM_COMMENT_END1, normally it is part of the comment. The '>' is ignored
and mode stays PM_COMMENT. (Or is switched back from PM_COMMENT_END1.)

input: <!-- comment with > and -> in it -->
                                ^

However, if there were two or more dashes in front of the '>', this "-->"
combination was probably also intended as a comment end. A comment consisting
of a series of dashes is a typical example:

input: <!------>
              ^
parse_mode=PM_COMMENT
dash_count=6

However, only a little warning can be printed in this case -- this is valid
SGML, and *has* to be treated as part of the comment, even if it's probably
not what the page author intended! Printing an error and using a workaround
would mean presumably to violate the standard in favour of broken pages,
which is probably not a very good idea...

All other syntax errors are simply ignored, hoping for the best.

_html_error()_

Whenever some syntax error is detected (no matter whether workarounds are
available), html_error() is called, with several parameters describing the
error. This function is responsible for everything that needs to be done when
an error occurs.

First, it prints an error message. The message text is passed from
parse_syntax(), and used as the format string for printf(). If the error
message requires additional arguments, they are passed at the end of the
parameter list when calling html_error().

If the parsing mode requires that, it quits immediately afterwards. The
mode is determined by the config variable "cfg.parser", which is an
"enum Parser_mode", with the possible values CORRECT_HTML, NORMAL_HTML,
BROKEN_HTML and IGNORE_BROKEN. The parser quits only in CORRECT_HTML mode,
or when -DXHTML_ONLY is enabled. If the input resource from which the page is
loaded is a pipe from wget (see hacking-load.* ), the pipe is closed before
quitting to assure a cleaner exit.

In all other modes, an additonal message passed from parse_syntax() is printed
afterwards, informing in which way netrik will handle the error. (workaround,
ignore etc.)

Finally the error level passed from parse_syntax() is compared against the
highest error level up to know, and the new higest level is returned.

_Warning messages_</h4>

parse_syntax keeps track of most severe syntax error that was found while
parsing the page in "err_level", which is of type "enum Syntax_error" and
can have the following values:

- SE_NO: no errors were found
- SE_BREAK: the user issued an interrupt (SIGINT) while loading the document.
  This isn't really an error, but can be handled very convenient this way...
- SE_WORKAROUND: only errors were found which seem uncritical and workarounds
  were applied
- SE_HEAVY: there were heavy syntax errors, for which no workaround is known,
  and quite likely may cause the page to be layouted incorrectly (some parts of
  text may be missing etc.)
- SE_CRITICAL: the parser was not in normal text mode at the page end; this
  means that some important char was missing (e.g the '"' at the end of
  attribute values or the '>' closing a tag), and the following text was
  misinterpreted in will be rendered incorrectly.
- SE_FAIL: This isn't really a syntax error. It is not used inside
  parse_syntax() itself; it's only set before returning when a file loading
  error was detected, for the sake of the calling function.
- SE_NODATA: Similar to SE_FAIL. This is set if EOF is returned by load()
  before *any* data has been read.

After the whole page is parsed, a warning message is printed if some error
was found. The message text depends on the error level. The error level is
also passed back to main(), which then waits for a keypress before starting
the pager, so the message will be seen.

In IGNORE_BROKEN mode the warning is suppressed, and "err_level" is reset. In
BROKEN_HTML mode only SE_WORKAROUND errors are ignored, while more critical
errors still cause a warning.

SE_BREAK is set if EOF is returned, but at the same time "input->user_break"
has been set, indicating that it's not really EOF, but transfer was interrupted
by the user. Other errors are supressed in this mode, as a user break during
loading might cause several syntax errors (unclosed elements etc.) with the
page itself being not to blame for.

SE_NODATA is set if EOF is returned by load() before any data has been read.
(This can be caused by failure to open the resource, but also by an empty
file/http response.) It's handled like a normal syntax error; the only
difference is that it can't be masked even by IGNORE_BROKEN. The syntax tree
consists only of the global element; it will be correctly rendered to an
empty page.

SE_FAIL isn't set during parsing. Before returning, parse_syntax() checks
whether "input->type" is RES_FAIL; if it is, an error message is printed,
and SE_FAIL is set (so main() knows an error occured). However, this test is
only necessary if SE_NODATA isn't set; otherwise, an error message has already
been printed and an error code would be returned anyways. As EOF is returned
by load() also when an error occured, SE_NODATA is already set for most errors;
SE_FAIL is only used if the error occurs after some data could be read.

_free_syntax()_

This function is responsible for freeing the memory used by the syntax tree
when it is no longer needed.

The whole tree is traversed by "list_next", and the element nodes are freed
one by one.

As the "list_next" pointer is necesary to find the next node, but not longer
available after freeing the current node, it is saved in "next_el" before
freeing. At the the beginning of the next iteration this is copied to "cur_el".

Before freeing the element node itself, all dynamic data belonging to the
node has to be freed.

_3. dump-tree.c_

dump_tree() is primarily used for dumping the syntax tree generated by
parse_sytax() for debugging purposes. The reason it resides in an own file is
that it could be easily modified to be a really useful function for dumping a
HTML document's structure. This may be implemented in the future, if someone
shows interest...

The implementation of dump_tree() is quite straightforward, as the function
only needs to print every node in the order it occured in the HTML file.

For every node, first the text is printed. (If "dump_content" is given.) The
reason it is printed in front of the node itself is because it's also the
content in front of the element in the original HTML file.

Next, the current tree depth is shown by a number of '|'. The current depth
is always stored in "depth".

Afterwards the element name is printed, and all attributes with their values.
Depending on "elements_parsed", either the raw values extracted from the
document are printed, or the transformed values generated by parse_elements().

The next node is reached by the "list_next" pointer of the current node. Before
going to the next node the tree depth of the new node needs to be calculated.
This is done by assuming that the next node is below the current one, and than
going up, until we find the parent of the new node. This idea is explained
in more detail in _6. parse-struct.c_ .

_4. parse-elements.c_

parse_elements() is responsible for making out the elements and attributes
from the syntax tree. (Extraced by parse_syntax().)

All elements but the first one (which is always ELEMENT_GLOBAL) are processed
one by one; the tree is traversed via "list_next". For every element, the
name is looked up in "element_table[]" by comparing to all entries, in a loop.

"element_table[]" contains all names of ordinary elements, then the "?"
representing ELEMENT_NO, and finally "!" representing ELEMENT_GLOBAL. (It
also contains other properties of the elements; more on this in _Processing_
in _6. parse-struct.c_ .)

The last two aren't checked against the element name.

As soon as a match is found, the loop is left and the entry number is
stored to "syntax_tree" in place of the string. The entry number is an "enum
Element_type", defined in syntax.h; it tells the element type in the following
processing passes.

If none of the ordinary entries matched, the entry nuber, which now is
ELEMENT_NO (as this one follows after the ordinary entries), is stored
anyhow, indicating that the element is unknown. If no element name was stored
("cur_el->name.str" is NULL), indicating a dummy tag, ELEMENT_NO is set also.

After the element name, all attribute names are processed in a loop. They
are looked up in "attr_table[]" the same way the element name is.

The attribute value isn't processed at all yet.

_5. sgml_rework()_

Before the syntax tree is further processed, sgml_rework() (from sgml.c)
is applied. (Unless compiled with -DXHTML_ONLY.)

This function is responsible for fixing the problems arising from the fact
that SGML allows certain end tags to be left out; thus the syntax parser
doesn't recognize the elements' ends, and stores all following elements as
children, even if they should actually be at the same level. (e.g. the list
items in a list.) sgml_rework() goes over the complete (broken) tree, finds
such situations, and unnests the elements, thus creating a correct syntax tree.

It won't be covered in too much detail here, as this is only a temporary
solution; it will become obsolete with the planned new parser(s).

The recognition of the missing element ends is done by the "element_group"
enum in "element_table[]". This Enum has the values GROUP_SINGLE for single
tag elements (elements which mustn't have any content), GROUP_OBLIGATE for
all elements where the end tag can't be left out, and several others for
various kinds of elements with optional end tag.

The whole tree is scanned element by element (using "list_next"), and each one
is tested to fullfill one of the offending conditions. Two things have to be
handled: Unclosed single tag elements, and unclosed optional end tag elements.

The second situation is more complicated. If the element is of some type with
optional end tag, it could terminate a previous (unclosed) element from the
same group; e.g. a <li> will terminate the previous <li>. It doesn't terminate
elements from other groups, though; a <td> inside a <tr> doesn't terminate the
<tr>, for example. Thus, the group of the current element needs to be tested
against the group of the parent (all elements following an unclosed one will
be stored as its children!); if they are the same, the parent is actually
an element that should be terminated at the position where the child starts,
and the child should follow it, at the same depth. This means that the child
has to be "lifted" out of the parent. However, we don't do that immediately;
we only set the "closed" flag of the parent element for now, and the lifting
will be done later.

However, it's not enough to test only the immediate parent: The element may
follow some other unclosed element, and thus be a child of it, e.g. a <tr>
following a <td>, which is inside the previous <tr>. This also needs to be
recognized, and *both* the previous <tr> and the <td> have to be closed. Thus,
not only the immediate parent's group is compared, but all ancestors are
scanned. The scanning only stops on an element with obligate end tag --
as the element's end is always known for these, nothing will be ever stored
inside that element that doesn't belong there, and nothing should be lifted
out. (In nested tables for example, the <tr>s and <td>s of the inner table
shouldn't mess with the ones of the outer table -- this is ensured by the
scanning of the inner table's rows and columns stopping at the inner <table>
element.) The "closed" flag is set for all the closed ancestors.

Some more handling is necessary due to the fact that an element node always
stores the content which appears *before* the element. When an element is
lifted, the content musn't be lifted also -- it appeared *before* the element,
and thus also before the previous element's end, so it has to stay where it is.
We have to create a new dummy tag inside the closed element therefore, taking
the place of the lifted element and storing its content.

However, this isn't done when the parent was already closed. This happens if
the parent is a single tag element. These elements end right where they start,
not at the beginning of the next element; the content also has to be lifted
out. (Nothing is allowed to stay inside a single tag element!)

The actual lifting is done after processing the element: If the parent is
closed, we have to "leave" it. (This is done by setting the "parent" to the
previous grandparent -- this way, the element is no longer a child of the old
parent, but a sibling.) Thus the element that closed it's parent is lifted right
afterwards; all following elements of the parent will be lifted also, after
being processed. Any preceeding elements (as well as the possibly created new
dummy) won't be lifted on the other hand, as they won't be processed anymore.

Single tag elements are handled more or less the other way round: They are
not closed by some child (which turns out to actually be a sibling), but
close *themselfs*, as soon as they are encountered. This way all children
will be lifted out, no matter what.

No other processing is necessary for single tag elements, as they won't ever
terminate some other element.

_6. parse-struct.c_

After the syntax tree was generated by parse_syntax(), we have to "understand"
it. This is done by parse_struct(), which is the central pass of the layouting
process. In this function the syntax tree, which contains a nearly 1:1
reproduction of the HTML file, is converted to an item tree, which contains a
representation of what will be actually shown as the output of the browser --
text blocks, blank rows, boxes grouping severel other items.

_Structure Tree_

For 0.html, we have to convert the syntax tree:

  ++>NULL
  +
+---+
| ! |-. <++
+---+ |   +
      v   +
 ,-----------.
("header text")
 `-+------+--'
   | html |-. <++++++++
   +------+ | <+      +
            v  +      +
        +------+  +------+   <+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
        | head |->| body |-. <++++++++++++++                                              +
        +------+  +------+ | <+            +                                              +
                           v  +            +                                              +     <+++++++++++++++++++++++++++++++++++++++++++++++++
                        +----++          +---+     <++++++++++++++++++++                +---+   <++++++++++++++++++++++++++++++++                +
                        | h1 |-. <+   ,->| p |---. <++                 +             ,->| p |-. <++++++++++++++++++             +                +    (back
                        +----+ |  +   |  +---+   |   +                 +             |  +---+ | <+                +             +                +    to top)
                               v  +   |          v   +                 +             |        |  +                +             +                +          ^
                          ,--------.  |  ,------------------.       ,----.           |        |  +          ,----------.   ,---------.   ,---------------.  |
                         (" heading") | (" first...newlines,")     (" and")          |        v  +         (" this...em") ("ded...row") (" (a single tag)") |
                          `--+---+-'  |  `-----+----+-------'     +-+----+-+         |   +--------+         `---+---+--'   `--+----+-'   `-----+---+-----'  |
                             | ? |----'        | em |-. <++    ,->| strong |-. <++   |   | center |-. <++    ,->| a |-. <+ ,->| br |---------->| ? |--------'
                             +---+             +----+ |   +    |  +--------+ |   +   |   +--------+ |   +    |  +---+ |  + |  +----+           +---+
                                                      v   +    |             v   +   |              v   +    |        v  + |
                                           ,----------------.  |     ,------------.  |   ,----------------.  |      ,---.+ |
                                          (" emphasized text") |    (" strong text") |  (" starting...tag,") |     ("bed") |
                                           `--------+---+---'  |     `-----+---+--'  |   `--------+---+---'  |      +---+  |
                                                    | ? |------'           | ? |-----'            | ? |------'      | ? |--'
                                                    +---+                  +---+                  +---+             +---+

to this structure tree:

***> "string"                                                                             (back  <+++++++++
xxx> "first_child"                                                                      to first          +
+++> "parent"                                                                             item)        +-----+
===> "next"                                                                                         ,->| box |-->NULL
---> "list_next"                                                                                    |  +-----+<==#
                                                                                                    |  x  ^  #===#
  xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx  +
  x                                                                                                 |     +
  x  ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
  x  +         +           +          +           +          +           +          +          +    |
  v  +         +           +          +           +          +           +          +          +    |
+------+   +-------+   +------+   +-------+   +------+   +-------+   +------+   +------+   +------+ |
| text |-->| blank |-->| text |-->| blank |-->| text |-->| blank |-->| text |-->| text |-->| text |-'
+----*-+==>+-------+==>+----*-+==>+-------+==>+----*-+==>+-------+==>+----*-+==>+----*-+==>+----*-+==>NULL
  x  *         x         x  *         x         x  *         x         x  *       x  *       x  * 
  v  *         v         v  *         v         v  *         v         v  *       v  *       v  *   ,--------------.
NULL *       NULL      NULL *       NULL      NULL *       NULL      NULL *     NULL *     NULL **>("(a single tag)")
     v                      v                      v                      v          *              `--------------'
 ,-----------.          ,-------.           ,------------.       ,---------------.   *   ,----------------.
("header text")        ("heading")         ("first...text")     ("starting...tag,")  **>("this...blank row")
 `-----------'          `-------'           `------------'       `---------------'       `----------------'

which, in turn, is a representation of this output page:

+-------------------------------------------------------------------------------------------------+
|+-----------+                                                                                    |
||header text|                                                                                    |
|+-----------+                                                                                    |
|                                                                                                 |
|+-------+                                                                                        |
||heading|                                                                                        |
|+-------+                                                                                        |
|                                                                                                 |
|+-----------------------------------------------------------------------------------------------+|
||first paragraph of text; includes multiple spaces and newlines, emphasized text and strong text||
|+-----------------------------------------------------------------------------------------------+|
|                                                                                                 |
|+---------------------------------+                                                              |
||starting with an evil center tag,|                                                              |
|+---------------------------------+                                                              |
|+-----------------------------------------------------------------------------------------------+|
||this very long second paragraph contains some special characters (including a simple space...):||
|| &; <>"=/ plus a big gap###and two unicode escapes (decimal: ¡ and hexal: ¿) but also an anchor||
|| embedded inside a word (this anchor also is the only tag with parameters); and finally a blank||
|| row                                                                                           ||
|+-----------------------------------------------------------------------------------------------+|
|+--------------+                                                                                 |
||(a single tag)|                                                                                 |
|+--------------+                                                                                 |
+-------------------------------------------------------------------------------------------------+

Note that there are no actual sizes or postions for the items, and no line
breaks inside the text items; this is all done in a later processing step
(pre-render.c). The item tree at this point only represents the structure of
the output page. (The line breaks in the fifth text block aren't really there;
we have inserted them in the figure because the text block is a bit too long
to put in a single line...)

The item tree looks complicated at first, but it's a quite trivial example
when taking a closer look. (This is becuase at the time of creating the 0.html
file used here, netrik wasn't able to do anything more complicated...) However,
it should be sufficient to get the idea...

Every node of the item tree consists of an "Item" structure. This structure
is declared in "items.h". It contains:

- The pointers "list_next", "next", "parent", and "first_child" connect the
  nodes inside the tree. "list_next" points to the next node in the order they
  are generated. Note that in contrast to the element tree, in the item tree
  any children are generated *before* the parent. "next" points to the next
  item at the same tree depth and in the same branch, i.e. the next sibling.
  "parent" points to the parent item, and "first_child" to the first sub-item.
  More pointers are necessary than for the element tree, because the item tree
  is traversed in several differnt ways while processing.
- The "center" flag indicates whether the item is centered. As not all items
  use this, and the exact meaning varies between different item types, it may
  be reasonable to move this to the item specific data. We'll decide on this as
  soon as enought HTML facilities are implemented.
- "x_start", "x_end", "y_start" and "y_end" define a square area inside the
  layouted page, in which the item is displayed. In some processing steps
  "x_end" end "y_end" are also "abused" to store the minimal size of the item.
- "type" is an enum storing what kind of item this node represents. (Currently
  ITEM_TEXT, ITEM_BOX, ITEM_FORM, ITEM_BLANK, ITEM_BLOCK_ANCHOR or
  ITEM_INLINE_ANCHOR.)
- "data" is an union storing all data specific to different item types.
  Currently this can be the pointer to a text string for text items, to an
  (block or inline) anchor struct, or to a form paramters struct.

_add_item()_

New items are created by add_item(). This function allocates a new "Item"
structure, and sets some pointers. The item isn't inserted into the tree
directly; it's only inserted into a list of all items at the current tree
depth. This is a single linked list maintained by the "first_item" and
"last_item" pointers of "state" (there is one such list for each tree depth),
and the "next" pointers of the item structures. The only other pointers set
are "list_next". The "parent" and "first_child" pointers aren't set; this is
done later when the items are actually inserted into the tree, while ascending
from the current depth. (For the first item in every tree depth the "parent"
is explicitly set to NULL, indicating that there is no parent yet.)

This function is called directly to create box items, and from add_string() to
create text items. Under certain conditions it also creates a blank item before
the actual text item or box item; more on this later, under _Blank Lines_ .

When called with the "virtual" flag, this function behaves slightly different:
No line break/blank line handling is done; the status remains unchanged. This
is for creating the _Virtual Boxes_ used for anchors.

_String_

The actual text data of text items is stored in a different place. Every
text item points to a "String" structure (also declared in "items.h"). This
structure consists of a normal C-string containing the text itself, and an
array of "Div" structures, containing all attribute information. (Color etc.) A
"String" can consist of several divisions with different attributes. Every
"Div" structure stores the attributes for one such division, and the ending
position of the division inside the string. (More exactly: the position *after*
the end of the div -- which is the starting position of the next div.) The
end of the last division is also used to find out the string length. This is
quite inefficient...

The "String" structure also contains "line_table[]", which holds the
positions of all line breaks inside the string; more on this in <a
href="#preRender">_7. pre-render.c_ .

Finally, it contains an array of "Link" structures, which describe all the
links (and form elements) inside this text block. See hacking-links.* for this.

_Processing_

Many properties of the various element types are data-driven. This presently
includes the line break/blank line handling, and elements creating a box
around all children. This properties are stored to the same "element_table[]"
as the name strings used by parse_elements(). In future probably more properties
will be data driven, making the code simpler, and also necessary for handling
style sheets, which allow changing of almost all formatting properties.

The item tree is generated while traversing the element tree. Processing of
each element is done in two steps: One step is done before entering an element
(descending in the element tree), and the second step is done after leaving
the element (ascending). Between those two steps, the same is done for all
sub-elements. You guess it: This is a recursive algorithm. Only we haven't
implemented it recursively, as mentioned in chapter 0. In this function
the pseudo-recursive implementation is most evident; we even have to use
a pseudo-stack.

The processing could also be split into a couple of much simpler passes using
some temporary data structures, e.g. one generating the "normal" items, one
generating blank items, one generating the strings, one "optimizing" the tree
(lifting items where possible). This would be much easier to understand,
and probably it would have been a good idea for the beginning. However,
it would be much less efficient; that's why we won't step back to such an
implementation after already having the present one.

_Pre-processing_

The first action in every iteration of the outer loop is to do the first
processing step (s.a.) for the current element ("cur_el") -- in every iteration
exactly one element is pre-processed.

  ++>NULL
  +
+---+
| ! |-. <++
+---+ |   +
      v   +
 ,-----------.
("header text")
 `-+------+--'  <-- cur_el   <== depth
   | html |-. <++++++++
   +------+ | <+      +
            v  +      +
        +------+  +------+
        | head |->| body |-.
        +------+  +------+ |

First, we store any text from the current element node to the current open
text item by add_string().

state[0]->first_item -->NULL   (depth 0)
state[0]->last_item **>NULL
---
+------+         <-- first_item
| text |-->NULL  <** last_item    <== depth=1
+----*-+==>NULL  
  x  *     
  v  *     
NULL *     
     v     
 ,-----------. 
("header text")
 `-----------' 

The next thing is processing of line breaks and paragraph breaks, depending what
kind of element we have. More on this later, in <a href="#textBlocks">_Text
Blocks_ and <a href="#blankLines">_Blank Lines_ .

_Recursing_

Then we recurse into the element (descend in the element tree).

  ++>NULL
  +
+---+
| ! |-. <++
+---+ |   +
      v   +
 ,-----------.
("header text")
 `-+------+--'  <-- cur_el
   | html |-. <++++++++
   +------+ | <+      +
            v  +      +
        +------+  +------+
        | head |->| body |-.     <== depth
        +------+  +------+ |

state[0]->first_item -->NULL   (depth 0)
state[0]->last_item **>NULL
---
+------+         <-- state[1].first_item   (depth 1)
| text |-->NULL  <** state[1].last_item
+----*-+==>NULL  
  x  *     
  v  *     
NULL *     
     v     
 ,-----------. 
("header text")
 `-----------' 
---
first_item -->NULL    <== depth=2
last_item **>NULL

This is done by push_state(). This function doubles the top of stack, and
returns a pointer to the newly created entry, which is used as the current
state. ("first_item" and "last_item" aren't copied, but set to NULL; "id_attr"
and "link_type" are set to -1.)

The stack stores all variables which are specific to every tree depth. Note
that the stack uses the depths from the element tree, not from the item tree.
The depths in the item tree are completely different, and change while
processing -- which is one of the most tricky parts about parse_struct().

Currently the data stored is:

- Text attributes ("text_mode" and "high").
- The nesting depth "list_depth" of item lists. (Determines the indent of list
  items.)
- The two pointers "first_item" and "last_item", necessary to maintain the list
  of all items at a given depth.
- The type of the link or form control created by the element in the
  "link_type" enum (a value of -1 indicates there is no link at all)
- The URL of links or value of form elements in the "link_value" string
- The "form_enabled" flag indicates whether a form element is to be submitted to the
  server
- The name of a possible <select> element, bequeathed to its <option> elements
- The kind of the <select> element, also bequeathed
- "link_start" stores the position where a link or inline anchor beginns inside
  the current string
- "link_item" stores the text item in which a link/anchor beginns
- For elements with an anchor, "id_attr" stores which attribute contains the
  anchor id (or name)

After descending, first "link_start" is set to the current string end, so any
text generated inside this element will become part of the link or anchor,
if the element creates one. If there is no string item open, 0 is stored,
so the link will begin at the start of the string if a new string beginns
inside the element.

Aferwards, some element type specific handling is done. Mostly this is
outputing of special element indicators. (This could be made data-driven,
and probably it will do so soon.)

For some element types, also values of the current state are modified; this
would be the argument passing in a real recursive implementation.

_Ascending_

The last step of every outer loop iteration is returning from recursion
(ascending in the element tree). But in contrast to descending, ascending
isn't done once per outer loop iteration. Instead, there is an inner loop,
that ascends as often as needed to reach the level of the next element --
this can be zero, once or several times.

We know how long we need to ascend by starting at the level of the current
element and looking for the parent of the new element; as soon as we find it,
we know we needn't ascend any more. "depth" is adjusted every time we ascend,
and all actions for leaving the element (unrecursing) are taken. ("depth"
is always one below the depth of "new_el".)

If the next element is a child of the current one, no ascending is necessary.
We have descended one step in the pre-processing step of the current iteration,
and this is ok; we keep it. ("cur_el" is alredy the parent of "list_next".)

 ,-----------.
("header text")
 `-+------+--'
   | html |-. <++++++++
   +------+ | <+      +
            v  +      +
        +------+  +------+
        | head |->| body |-. <++++++++++++++   
        +------+  +------+ | <+            +   
                           v  +            +   
                        +----++          +---+ 
            cur_el  --> | h1 |-. <+   ,->| p |->
            new_el  xx> +----+ |  +   |  +---+ 
                               v  +   |
                          ,--------.  |
                         (" heading") |
                          `--+---+-'  |      <== depth
               list_next **> | ? |----'
                             +---+     

If the next element is at the same level as the current one (single tags or
other elements with no sub-elements), we need to ascend exactly one time,
to get back to the level of the current element, after we have descended in
the pre-processing step.

 ,-----------.
("header text")
 `-+------+--'
   | html |-. <++++++++
   +------+ | <+      +
            v  +      +
        +------+  +------+ <**
    --> | head |->| body |-. <++++++++++++++
    xx> +------+  +------+ | <+            +   
                           v  +            +   
                        +----++          +---+ 
                        | h1 |-. <+   ,->| p |->      <==
                        +----+ |  +   |  +---+ 
                               v  +   |
                          ,--------.  |
                         (" heading") |
                          `--+---+-'  |
                             | ? |----'
                             +---+     

 ,-----------.
("header text") <xx
 `-+------+--'
   | html |-. <++++++++
   +------+ | <+      +
            v  +      +
        +------+  +------+ <**
    --> | head |->| body |-. <++++++++++++++       <==
        +------+  +------+ | <+            +   
                           v  +            +   
                        +----++          +---+ 
                        | h1 |-. <+   ,->| p |->
                        +----+ |  +   |  +---+ 
                               v  +   |
                          ,--------.  |
                         (" heading") |
                          `--+---+-'  |
                             | ? |----'
                             +---+     

Of course it's a bit of overhead first to descend into an element, just to
ascend from it right after. But it saves a lot of code for special handling
of such childless elements. Many of the actions of both the first step and the
second step have to be done for them also -- putting them together and leaving
out the descending and ascending wouldn't save that much, while complicating
the code quite a lot. We may consider some way in the future if profiling
shows this would be rewarding.

If the next element is above the current one, we have to ascend more than once.
(Once to get back to the level of the current one, the others to ascend to
the new level.)

 ,-----------.
("header text")     
 `-+------+--'
   | html |-. <++++++++
   +------+ | <+      +
            v  +      +
        +------+  +------+    
        | head |->| body |-. <++++++++++++++          
        +------+  +------+ | <+            +   
                           v  +            +   
                        +----++          +---+ 
                        | h1 |-. <+   ,->| p |-> <**
                        +----+ |  +   |  +---+ 
                               v  +   |
                          ,--------.  |
                         (" heading") |
                          `--+---+-'  |
                         --> | ? |----'
                         xx> +---+     
 
                                            <==

 ,-----------.
("header text")     
 `-+------+--'
   | html |-. <++++++++
   +------+ | <+      +
            v  +      +
        +------+  +------+    
        | head |->| body |-. <++++++++++++++          
        +------+  +------+ | <+            +   
                           v  +            +   
                        +----++          +---+ 
                    xx> | h1 |-. <+   ,->| p |-> <**
                        +----+ |  +   |  +---+ 
                               v  +   |
                          ,--------.  |
                         (" heading") |
                          `--+---+-'  |        <==
                         --> | ? |----'
                             +---+     

 ,-----------.
("header text")     
 `-+------+--'
   | html |-. <++++++++
   +------+ | <+      +
            v  +      +
        +------+  +------+ <xx
        | head |->| body |-. <++++++++++++++          
        +------+  +------+ | <+            +   
                           v  +            +   
                        +----++          +---+ 
                        | h1 |-. <+   ,->| p |-> <**     <==
                        +----+ |  +   |  +---+ 
                               v  +   |
                          ,--------.  |
                         (" heading") |
                          `--+---+-'  |
                         --> | ? |----'
                             +---+     

In every ascending iteration, first we do some element type specific handling
again, mostly outputing element end indicators. Then we pop the previous
state from the stack.

 ++>NULL
 +
 + .--------------------------------.
 + v                                |
+---+        <xx                    |
| ! |-. <++  <**                    |
+---+ |   +                         |
      v   +                         |
 ,-----------.                      |
("header text")                     |          <==
 `-+------+--'                      |
   | html |-. <+++++++++++          |
   +------+ |            +          |
            |          [...]      [...]
          [...]          +          |
            |    ,---------------.  |
            |   (" (a single tag)") |
            |    `-----+---+-----'  |  <--
            `--------->| ? |--------'
                       +---+

state[0]->first_item -->NULL   (depth 0)
state[0]->last_item **>NULL
---         ,-- first_item
+------+ <--'     +------+   +------+ <** last_item
| text |--[...]-->| text |-->| text |-->NULL              <== depth=1
+----*-+==[...]==>+----*-+==>+----*-+==>NULL
  x  *              x  *       x  * 
  v  *              v  *       v  *   ,--------------.
NULL *            NULL *     NULL **>("(a single tag)")
     v                 *              `--------------'
 ,-----------.         *   ,----------------.
("header text")        **>("this...blank row")
 `-----------'             `----------------'

 ++>NULL <xx
 +
 + .--------------------------------.
 + v                                |
+---+                               |
| ! |-. <++  <**                    |          <==
+---+ |   +                         |
      v   +                         |
 ,-----------.                      |
("header text")                     |
 `-+------+--'                      |
   | html |-. <+++++++++++          |
   +------+ |            +          |
            |          [...]      [...]
          [...]          +          |
            |    ,---------------.  |
            |   (" (a single tag)") |
            |    `-----+---+-----'  |  <--
            `--------->| ? |--------'
                       +---+

first_item -->NULL        <== depht=0
last_item **>NULL
---         ,-- state[1].first_item
+------+ <--'     +------+   +------+ <** state[1].last_item     (depth 1)
| text |--[...]-->| text |-->| text |-->NULL
+----*-+==[...]==>+----*-+==>+----*-+==>NULL
  x  *              x  *       x  * 
  v  *              v  *       v  *   ,--------------.
NULL *            NULL *     NULL **>("(a single tag)")
     v                 *              `--------------'
 ,-----------.         *   ,----------------.
("header text")        **>("this...blank row")
 `-----------'             `----------------'

_Inserting into Item Tree_

Afterwards, the probably most interesting part follows: The sub-items
created inside the element we are just leaving, are inserted into the item
tree properly.

If the element we are leaving enforces a box (looked up in "element_table[]"),
a new box item is created. (Box items are always created when leaving the
element, and thus *after* all items inside the box.)

                                  -->    +-----+
                                  **> ,->| box |-->NULL     <== depht=0
                                      |  +-----+==>NULL
                                      |      
                                      |      
                                      |      
                                      |      
                                      |
                                      |
+------+ <--      +------+   +------+ | <**           (depth 1)
| text |--[...]-->| text |-->| text |-'
+----*-+==[...]==>+----*-+==>+----*-+==>NULL
  x  *              x  *       x  * 
  v  *              v  *       v  *   ,--------------.
NULL *            NULL *     NULL **>("(a single tag)")
     v                 *              `--------------'
 ,-----------.         *   ,----------------.
("header text")        **>("this...blank row")
 `-----------'             `----------------'

The "parent" pointers of all immediate children are set to the new box item,
and "first_child" of the new item is set to the first of them.

                                  -->    +-----+
                                  **> ,->| box |-->NULL     <== depht=0
                                      |  +-----+==>NULL
                                      |  x  ^
  xxxxxxxx[...]xxxxxxxxxxxxxxxxxxxxxxxxxxx  +
  x                                   |     +
  x  +++++[...]++++++++++++++++++++++++++++++
  x  +                +          +    |
  v  +                +          +    |
+------+ <--      +------+   +------+ | <**           (depth 1)
| text |--[...]-->| text |-->| text |-'
+----*-+==[...]==>+----*-+==>+----*-+==>NULL
  x  *              x  *       x  * 
  v  *              v  *       v  *   ,--------------.
NULL *            NULL *     NULL **>("(a single tag)")
     v                 *              `--------------'
 ,-----------.         *   ,----------------.
("header text")        **>("this...blank row")
 `-----------'             `----------------'

If the element does not create a box, things are more tricky: We have to
"lift" all sub-elements to the new level. This is done by concatenating the
list of elements of the depth we are leaving to the list of elements of the
depth we are entering.

state[0]->first_item -->NULL   (depth 0)
state[0]->last_item **>NULL
---
+------+         <-- state[1].first_item   (depth 1)
| text |-->NULL  <** state[1].last_item
+----*-+==>NULL  
  x  *     
  v  *     
NULL *     
     v     
 ,-----------. 
("header text")
 `-----------' 
---
state[2]->first_item -->NULL   (depth 2)
state[2]->last_item **>NULL
---       ,-- state[3].first_item
+------+<-'+-------+ <** state[3].last_item    (depth 3)
| text |-->| blank |-->NULL           
+----*-+==>+-------+==>NULL  
  x  *         x      
  v  *         v      
NULL *       NULL     
     v     
 ,-------. 
("heading")
 `-------' 
---
+------+        <-- first_item
| text |-->NULL <** last_item      <== depth=4
+----*-+==>NULL
  x  *       
  v  *       
NULL *       
     v       
 ,------------. 
("first...text")
 `------------' 

  +------+   <+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
->| body |-. <++++++++++++++                                              +
  +------+ | <+            +                                              +
 xx^       v  +            +                                              +   
        +----++          +---+     <++++++++++++++++++++                +---+ 
==>     | h1 |-. <+   ,->| p |---. <++                 +         **> ,->| p |->
depth=3 +----+ |  +   |  +---+   |   +                 +             |  +---+ 
               v  +   |          v   +                 +             |
          ,--------.  |  ,------------------.       ,----.           |
         (" heading") | (" first...newlines,")     (" and")          |
          `--+---+-'  |  `-----+----+-------'     +-+----+-+         |
             | ? |----'        | em |-. <++    ,->| strong |-. <++   |
             +---+             +----+ |   +    |  +--------+ |   +   |
                                      v   +    |             v   +   |
                           ,----------------.  |     ,------------.  |
                          (" emphasized text") |    (" strong text") |
                           `--------+---+---'  |     `-----+---+--'  |
                                    | ? |------'       --> | ? |-----'
                                    +---+                  +---+      

state[0]->first_item -->NULL   (depth 0)
state[0]->last_item **>NULL
---
+------+         <-- state[1].first_item   (depth 1)
| text |-->NULL  <** state[1].last_item
+----*-+==>NULL  
  x  *     
  v  *     
NULL *     
     v     
 ,-----------. 
("header text")
 `-----------' 
---
state[2]->first_item -->NULL   (depth 2)
state[2]->last_item **>NULL
---       ,-- first_item
+------+<-'+-------+   +------+ <** last_item
| text |-->| blank |-->| text |-->NULL      <== depth=3
+----*-+==>+-------+==>+----*-+==>NULL
  x  *         x         x  *       
  v  *         v         v  *       
NULL *       NULL      NULL *       
     v                      v       
 ,-------.          ,------------. 
("heading")        ("first...text")
 `-------'          `------------' 

In the second case, we need an additional step if the element we are leaving
was a <center> element (or some element with an align="center" attribute):
In this case all sub-elements have to be centered one by one. (Elements
creating a box are centered as a whole.)

Finally, we have to do some more line break/blank line handling.

_Text Blocks_

One single text block can be created by several elements. Every series of text
parts not interrupted by elements requirering line breaks (or blank lines)
around them is stored to a single text item; it can even contain newline
characters created by <br> elements. This is done by calling add_string()
every time text data is encountered.

The color for the text added by some element is determined from the current
"state->text_mode" and "state->high". The normal color for the text mode
is looked up in "color_map[]", and than its bit 3 is negated if the text is
highlighted. This is more or less a hack; we'll have to replace this by some
serious attribute handling at some point...

_add_string()_

This function concatenates the new text to the "string" of "string_item",
which is a global variable pointing to the current open text item. If the
new text has other attributes than the last division of the "string" so far,
a new division is created for the new text; otherwise, the text is simply
added to the last division.

text_item->string:
            div[0].end      div[1].end   div[2].end
                     v               v   v
text: "[...]newlines, emphasized text and"
       `~~~~~v~~~~~~'`~~~~~~~v~~~~~~'`~v'
                 div[1].color=MAGENTA  div[2].color=WHITE
    div[0].color=WHITE

+------------------------------------------------+
|+-----------+                                   |
||header text|                                   |
|+-----------+                                   |
|                                                |
|+-------+                                       |
||heading|                                       |
|+-------+                                       |
|                                                |
|+----------------------------------+            |
||[...]newlines, emphasized text and|            |

 ,------------------.       ,----.          
(" first...newlines,")     (" and")         
 `-----+----+-------'     +-+----+-+        
       | em |-. <++    ,->| strong |-. <++  
       +----+ |   +    |  +--------+ |   +  
              v   +    |             v   +  
   ,----------------.  |     ,------------. 
  (" emphasized text") |    (" strong text")
   `--------+---+---'  |     `-----+---+--' 
            | ? |------'       --> | ? |-->
            +---+                  +---+      

text_item->string:
            div[0].end      div[1].end   div[2].end  div[3].end
                     v               v   v           v
text: "[...]newlines, emphasized text and strong text"
       `~~~~~v~~~~~~'`~~~~~~~v~~~~~~'`~v'`~~~~~v~~~~'
                 div[1].color=MAGENTA          div[3].color=STRONG WHITE
      div[0].color=WHITE               div[2].color=WHITE

+------------------------------------------------+
|+-----------+                                   |
||header text|                                   |
|+-----------+                                   |
|                                                |
|+-------+                                       |
||heading|                                       |
|+-------+                                       |
|                                                |
|+----------------------------------------------+|
||[...]newlines, emphasized text and strong text||

A new division can also be enforced, by an additional call of add_text()
with NULL as text before adding the next text part. (This is necessary to
prevent multiple consecutive links from being merged into one div, as link
highlighting is done div-wise.)

Appending to an existing text item is only possible if the last created item
was a text item, and there was no breaking element. (Neither the last one,
nor the new one, nor anyone in between.) Otherwise, a new text item has to
be created, and a new "String" structure for it. This is the only place where
new text items are created.

|                                   |
|+---------------------------------+|
||starting with an evil center tag,||
|+---------------------------------+|

  +---+
->| p |-. <++++++++++++++++++      
  +---+ | <+                +      
        |  +                +      
        |  +          ,----------. 
        v  +         (" this...em")
   +--------+         `---+---+--'  <--
   | center |-. <++    ,->| a |->
   +--------+ |   +    |  +---+
              v   +    |
   ,----------------.  |
  (" starting...tag,") |
   `--------+---+---'  |
            | ? |------'
            +---+       

|                                   |
|+---------------------------------+|
||starting with an evil center tag,||
|+---------------------------------+|
|+---------------------+            |
||this very...anchor em|            |

If there is a space at the beginning of a string, it is discarded while
creating the string, for a text block always starts with a word.

_Line Breaks_

As soon as an element forcing a line break is either entered or left (there
are no elements creating a break only before or only behind it), "string_item"
is set to NULL, indicating that no more text can be added to the last text
item, and new text has to create a new one.

  +---+
->| p |-. <++++++++++++++++++      
  +---+ | <+                +      
        |  +                +      
        |  +          ,----------. 
        v  +         (" this...em")
   +--------+ <xx     `---+---+--'  <**
   | center |-. <++    ,->| a |->
   +--------+ |   +    |  +---+
              v   +    |
   ,----------------.  |
  (" starting...tag,") |
   `--------+---+---'  | <--            <==
            | ? |------'
            +---+       

|                                   |
|+---------------------------------+|
||starting with an evil center tag,||

  +---+ <xx
->| p |-. <++++++++++++++++++      
  +---+ | <+                +      
        |  +                +      
        |  +          ,----------. 
        v  +         (" this...em")
   +--------+         `---+---+--'  <**      <==
   | center |-. <++    ,->| a |->
   +--------+ |   +    |  +---+
              v   +    |
   ,----------------.  |
  (" starting...tag,") |
   `--------+---+---'  | <--
            | ? |------'
            +---+       

|                                   |
|+---------------------------------+|
||starting with an evil center tag,||
|+---------------------------------+|

_Blank Lines_

Probably the most tricky part is handling of blank lines. Similar to line
breaks, items needing blank lines have them before *and* after them. However,
when two such items meet, they have only *one* blank line between them.
Furthermore, a blank line is *never* inserted before the first or after the
last item inside a box. That's why blank lines cannot be simply inserted when
entering an element causing blank lines, or when leaving it. Instead, only
"requests" for blank lines are stored in "para_blank", and if certain conditions
are met, a blank line is inserted before the next item (inside add_item()).

Actually, the blank line is inserted not in front of the new item, but after
the last item, at the same tree depth as that one. This is important, to ensure
that a blank line generated in front of a box is actually inserted *outside*
of the box, not inside it, as would be the case if it was inserted at the
current depth. The global "blank_depth" variable is responsible for this, and
is set to the current depth every time a blank line request is generated. The
blank item is then inserted directly to the "state" structure at "blank_depth"
in add_item(). This is surely very bad programming style ;-)

The blank line requests are managed by the global "para_blank" variable. A value
of 1 indicates that a blank line is needed, and will be generated by the next
add_item(). A value of 0 indicates that no blank line is needed in any case;
this situation can only occur when there are no items inside the current box
yet. A value of -1 indicates that a blank line *may* be necessary. That is
the case when we already have some items inside the current box, but the last
item does not need a blank line; a blank line needs to be inserted only when
the following item wants one.

When entering an element needing a blank line, a request is generated
("para_blank" set to 1 and "blank_depth" is stored) if "para_blank" was -1,
indicating that there are already items in the current box.

      +
  +------+
->| body |-. <++++++++++++++    
  +------+ | <+            +    
           v  +            +    
        +----++          +---+  
   -->  | h1 |-. <+   ,->| p |->
 cur_   +----+ |  +   |  +---+  
 tag           v  +   |
          ,--------.  |
         (" heading") |
          `--+---+-'  |
             | ? |----'
             +---+     

+------------------+
|+-----------+     |
||header text|     |
|+-----------+     |
??? (para_blank=-1)

      +
  +------+
->| body |-. <++++++++++++++    
  +------+ | <+            +    
           v  +            +    
        +----++          +---+  
   -->  | h1 |-. <+   ,->| p |->
   ==>  +----+ |  +   |  +---+  
 blank_        v  +   |
 depth    ,--------.  |
         (" heading") |
          `--+---+-'  |
       **>   | ? |----'
  list_next  +---+     
(new "cur_el")

+------------------+
|+-----------+     |
||header text|     |
|+-----------+     |
| (para_blank=1)   |
       ...
|+-------+         | (will be created later)
||heading|         |
|+-------+         |

If "para_blank" is already 1, the request is left unchanged; a blank line
will be inserted already.

|                  |
|+-------+         |
||heading|         |
|+-------+         |
| (para_blank=1)   |

   |
   v
+----+ <==       +---+ <--
| h1 |-. <+   ,->| p |---. <++
+----+ |  +   |  +---+   |   +
       v  +   |          v   +
  ,--------.  |  ,------------------. 
 (" heading") | (" first...newlines,")
  `--+---+-'  |  `-----+----+-------' 
     | ? |----'    **> | em |-.
     +---+             +----+ |

|                  |
|+-------+         |
||heading|         |
|+-------+         |
| (para_blank=1)   |
       ...
|+------------+    |
||first...text|    |
|+------------+    |

If "para_blank" is 0 nothing is done, even if the element would normally need
a blank line: No blank line is ever inserted in front of the first item inside
a box.

+-----------+
(para_blank=0)

+---+
| ! |-. <+
+---+ |  +
      v  +
    +---++
--> | p |-. <+
    +---+ |  +
          v  +
    ,---------.
   ("some text")
    `--+---+--'
   **> | ? |->
       +---+

+-----------+
     ...
|+---------+|
||some text||
|+---------+|

When some item (text or box) is inserted by add_item(), "para_blank" is always
set to -1, as now there is at least one item in the current box.

+------------------+
|+-----------+     |
||header text|     |
|+-----------+     |
| (para_blank=1)   |

      +
  +------+
->| body |-. <++++++++++++++    
  +------+ | <+            +    
           v  +            +    
        +----++          +---+  
        | h1 |-. <+   ,->| p |->
        +----+ |  +   |  +---+  
               v  +   |
          ,--------.  |
         (" heading") |
          `--+---+-'  |
         --> | ? |----'
             +---+     

+------------------+
|+-----------+     |
||header text|     |
|+-----------+     |
|                  |
|+-------+         |
||heading|         |
|+-------+         |
??? (para_blank=-1)

When leaving an element needing a blank line, a request is generated also, and
will be handled in the next add_item(). Otherwise, the current state is kept.

      +
  +------+
->| body |-. <++++++++++++++    
  +------+ | <+            +    
           v  +            +    
        +----++          +---+  
    xx> | h1 |-. <+   ,->| p |-> <**
new_el  +----+ |  +   |  +---+  
               v  +   |
          ,--------.  |
         (" heading") |
          `--+---+-'  |
         --> | ? |----'
             +---+     

+------------------+
|+-----------+     |
||header text|     |
|+-----------+     |
|                  |
|+-------+         |
||heading|         |
|+-------+         |
??? (para_blank=-1)

      +
  +------+ <xx
->| body |-. <++++++++++++++    
  +------+ | <+            +    
           v  +            +    
        +----++          +---+  
    ==> | h1 |-. <+   ,->| p |-> <**
        +----+ |  +   |  +---+  
               v  +   |
          ,--------.  |
         (" heading") |
          `--+---+-'  |
         --> | ? |----'
             +---+     

+------------------+
|+-----------+     |
||header text|     |
|+-----------+     |
|                  |
|+-------+         |
||heading|         |
|+-------+         |
| (para_blank=1)   |

When entering an element generating a box, "para_blank" is reset to 0.

|                |
|+---------+     |
||some text|     |
|+---------+     |
??? (para_blank=-1)

  +--------+            +------+
->| center |-.       ,->| form |-.  <--
  +--------+ |       |  +------+ |
             v       |           |
        ,---------.  |           |
       ("some text") |           v
        `--+---+--'  |         +---+
           | ? |-----'    **>  | p |->
           +---+               +---+

|                |
|+---------+     |
||some text|     |
|+---------+     |
|+--------------+|
(para_blank=0)

However, it's left unchanged if there is already a request, indicating that
a blank line will be inserted *in front* of the box by the next add_item().

|                |
|+---------+     |
||some text|     |
|+---------+     |
| (para_blank=1) |

  +---+            +------+
->| p |-. <==   ,->| form |-.  <--
  +---+ |       |  +------+ |
        v       |           |
   ,---------.  |           |
  ("some text") |           v
   `--+---+--'  |         +---+
      | ? |-----'    **>  | p |->
      +---+               +---+

|                |
|+---------+     |
||some text|     |
|+---------+     |
| (para_blank=1) |
|+--------------+|

When leaving a box creating element, any request generated inside the box is
discarded. We never insert a blank line after the last item of a box.

 ++>NULL
 + .--------------------------------.
 + v                                |
+---+        <xx                    |
| ! |-. <++  <**                    |
+---+ |   +                         |
      v   +                         |
 ,-----------.                      |
("header text")                     |
 `-+------+--'                      |
   | html |-. <+++++++++++          |
   +------+ |            +          |
            |          [...]      [...]
          [...]          +          |
            |    ,---------------.  |
            |   (" (a single tag)") |
            |    `-----+---+-----'  |  <--
            `--------->| ? |--------'
                       +---+

|+--------------+  |
||(a single tag)|  |
|+--------------+  |
| (para_blank=1)   |

 ++>NULL <xx
 + .--------------------------------.
 + v                                |
+---+                               |
| ! |-. <++  <**                    |
+---+ |   +                         |
      v   +                         |
 ,-----------.                      |
("header text")                     |
 `-+------+--'                      |
   | html |-. <+++++++++++          |
   +------+ |            +          |
            |          [...]      [...]
          [...]          +          |
            |    ,---------------.  |
            |   (" (a single tag)") |
            |    `-----+---+-----'  |  <--
            `--------->| ? |--------'
                       +---+

|+--------------+  |
||(a single tag)|  |
|+--------------+  |
+------------------+
??? (para_blank=-1)

However, if the last request was generated *before* descending into the
element creating the box (only possible if the box is empty), it is kept,
and the blank is inserted when the box is added.

|                |
|+---------+     |
||some text|     |
|+---------+     |
| (para_blank=1) |
|+--------------+|

  +---+            +------+ <xx      +---+   
->| p |-. <==   ,->| form |-.     ,->| p |->  <**
  +---+ |       |  +------+ |     |  +---+
        v       |           |     |
   ,---------.  |           |     |
  ("some text") |           v     |
   `--+---+--'  |        +------+ |
      | ? |-----'   -->  | span |-'
      +---+              +------+

|                |
|+---------+     |
||some text|     |
|+---------+     |
|                |
|+--------------+|
|+--------------+|

_<br> elements_

<br> elements used to break text blocks as described above. This however
turned out to be a bug: some elements (e.g. <a>) can span over a <br>, and
these were handled incorrectly with that approach.

Now simply a '\n' is added to the text block when <br> is encountered, and
then correctly handled while _Breaking String into Lines_ .

_Links_

When a link element is encountered (any <a> element having a "href"),
"cur_state->link_type" is set (to FORM_NO), indicatating that the
element creates a link. The URL (from the "href" attribute) is saved in
"cur_state->link_value".

The link is then stored when leaving the link element. (After processing all
sub-elements.) The link start is set to "cur_state->link_start", which was the
string end position while entering the link element; the link end is set to
the current string end position. This way the link spans all text generated
inside the element.

There is some additional handlig necessary to workaround broken links, however.
(This used to be important to make forms work with SGML at all; now that full
SGML support is implemented, it probably only helps a few broken pages...)

When a link doesn't end in the same string as it started (checked by
"cur_state->link_item", which was the current string item when entering the
link element), it is stored in the starting string, not the current one.
However, if there was no active text item when the link started (indicating
the link starts at the beginning of a text block), we can't determine the
starting string. In this case, we store it in the current string -- normally,
this is the right thing to do; if the link spans multiple strings, on the
other hand, at least the last part ist stored this way.

Some magic is necessary to handle nested links: The link which is stored later
is the outer one, i.e. it starts *before* the previously stored inner link... To
handle this in a useful fashion, we also have to *store* it before the inner
link. Thus, instead of simply appending it at the end of the string's link
list, we have to shift all inner links one position behind, and put the new
(outer) link at the free position created in front of them.

_Forms_

Form elements are handled very similar, and by the same code. The only
difference is that "link_type" is set to some form type instead of FORM_NO,
and the initial "value" of the element is stored in the structure tree so it
will be submitted to the server, and (for some form control types) displayed
on the page via _set_form()_ (see hacking-links.*).

Also, "form_enabled" is set for all elements that will be submitted to the
server. It is always set for elements that are submitted unconditionally
(text/password/hidden input fields), and set to the initial state for elements
that are submitted depending on their state (radio buttons, checkboxes,
<select> options). Submit buttons initially aren't enabled.

Some special handling is necessary for <select> options: They do not have an
own "name" attribute to store in "Link->name" like other form elements; the
"name" for all options is given in the <select> element instead. Thus it needs
to be passed to all the options by the state stack, in "select_name". The same
for the link type (FORM_OPTION or FORM_MULTIOPTION) of the option links: It
depends on the presence of the "multiple" attribute in the <select> element,
and is passed in "select_type" for that reason.

_Virtual Items_

There are two kinds of "virtual" Items: ITEM_BLOCK_ANCHOR and
ITEM_INLINE_ANCHOR. As the names suggest, both types are presently used for
anchors. The difference is that block anchors are created by block elements
with an "id" attribute, and span one or more block elements, while inline
anchors are created inside text blocks by the classical "a" element or by
any inline element with an "id", and span only a text part.

"virtual" means that these items do not affect layouting like other items;
they only create some additional structure.

_Block Anchors_

As block anchors span multiple other elements, they have to act as boxes,
containing a series of virtual children. In contrast to real box items,
they do not get their size assigned by the parent, and do not assign size
to their virtual children. The children get their size assigned by the real
parent directly, and the size of the virtual box is determined afterwards,
from the outer bounds of all virtual children.

In the present implementation, virtual box items aren't inserted normally
as parents of their virtual children into the item tree. They are simply
inserted as normal items after their virtual children, at the same tree
depth. The virtual box is created only by special pointers, handled only in
the necessary places; the normal tree traversal functions do not know about
them. This is a hack; the idea was to integrate them into the existing system
changing as little as possible. Most probably this will be replaced by a
clean implementation using real parent/children relations in the future.

virtual tree:
                                                                  +-----+
                                                               ,->| box |
                                                               |  +-----+
                                                               |    x ^
  xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx +
  x +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
  v +                                      +              +    |
+------+                           +--------------+   +------+ |
| text |-.                      ,->| block anchor |-->| text |-'
+------+=|======================|=>+--------------+==>+------+==>NULL
         |                      |    x     ^
         |    xxxxxxxxxxxxxxxxxxxxxxxx     +
         |    x  +++++++++++++++++++++++++++
         |    v  +         +    |
         |  +------+   +------+ |
         `->| text |-->| text |-'
            +------+   +------+==>NULL

real tree:
 
°°°> virtual child                                              +-----+
                                                             ,->| box |
                  virtual box                                |  +-----+
           ,----------^----------.                           |    x ^
  xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx +
  x +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
  v +          +          +              +              +    |
+------+   +------+   +------+   +--------------+   +------+ |
| text |-->| text |-->| text |-->| block anchor |-->| text |-'
+------+==>+------+==>+------+==>+--------------+==>+------+==>NULL
             ^                          °
             °                          °
             °°°°°°°°°°°°°°°°°°°°°°°°°°°°

_Inline Anchors_

As inline anchors are contained within text elements, they have to act as
children of text items. Text item however can't have children in the present
implementation; thus, inline anchors presently are also only virtual children.

Similar to the virtual boxes created by block anchors, these virtual children
are stored after their virtual parent, at the same tree depth. This is a hack
just as the virtual boxes, and will be replaced also.

Additionally to the virtual parent (and the name of the anchor), inline anchors
need to store the position of their start and end inside the parent string.

virtual tree:
 
%%%> anchor_start,anchor_end                                                +-----+
                                                                         .->| box |
                                                                         |  +-----+
                                                                         |    x ^
  xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx +
  x +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
  v +                                                    +          +    |
+------+                                             +------+   +------+ |
| text |-.                                        .->| text |-->| text |-'
+------+=|========================================|=>+------+==>+------+==>NULL
         |                                        |    x ^ *
         |                                        |    x + *   ,--------------------.                           
         |    xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx + **>("anchor1 and anchor2.")                          
         |    x     ++++++++++++++++++++++++++++++++++++++     `--------------------'                           
         |    x     +                   +         |             ^      ^    ^      ^
         |    x     +                   +         |             %      %    %      %
         |    x     +    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%    %      %
         |    x     +    %              +    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
         |    v     +    %              +    %    |
         |  +---------------+   +---------------+ |
         `->| inline anchor |-->| inline anchor |-'
            +---------------+==>+---------------+==>NULL

real tree:
 
°°°> virtual parent                                                       +-----+
                                                                       .->| box |
                                                                       |  +-----+
                                                                       |    x ^
  xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx +
  x +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
  v +          +              +                   +               +    |
+------+   +------+   +---------------+   +---------------+   +------+ |
| text |-->| text |-->| inline anchor |-->| inline anchor |-->| text |-'
+------+==>+------+==>+---------------+==>+---------------+==>+------+==>NULL
             *  ^             °%                  °%
%%%%%%%%%%%%%*%%°%%%%%%%%%%%%%°%                  °%
%            *  °             °                   °%
%            *  °°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°%
%            v                                     %
%  ,--------------------.                          %
% ("anchor1 and anchor2.")                         %
%  `--------------------'                          %
%   ^      ^    ^      ^                           %
%   %      %    %      %                           %
%%%%%%%%%%%%    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

Links are very similar to inline anchors, and probably will be stored the same
way as anchors in the future, once ITEM_INLINE is cleanly implemented. The
present different handling of links and anchors is by purpose -- to see which
method turns out to be better. (Presently, it looks much like the anchor-method
is better.)

_Finishing_

After all elements of the syntax tree are traversed, the structure tree is
finalized by setting "next" of the tree top (the global item) to point back
to itself, and "parent" point to the first item.

parse_struct() returns a "struct Item" pointer to the tree top. This is passed
as argument to the following passes by main().

Is is a bit strange that "parent" of the tree top points to the first item.
However, it is important to have some simple way of accessing the first item,
as some of the following passes start with the fist item; but only the pointer
to the tree top is passed on the following passes. Maybe we should use some
function that finds the first item by following "first_child" instead --
as this is done only once in every processing pass, it wouldn't be too big
an unefficiency.

_free_items()_

This function is responsible for freeing the memory used by the item tree.

Just as _free_syntax()_ , the tree is traversed by "list_next", and every
node is freed, including all data belonging to it.

For text items, the associated strings are freed also; for anchor items,
the anchor data is freed.

_7. pre-render.c_

So far, the item tree only represents the structure of the page, i.e. the
dependencies of the items. The Items have no actual sizes nor positions yet.

# ################################################################################################
 
# +-----------
# |header text
 
# ***
# *
 
# +-------
# |heading
 
# ***
# *
 
# +-----------------------------------------------------------------------------------------------
# |first paragraph of text; includes multiple spaces and newlines, emphasized text and strong text
 
# ***
# *
 
# +---------------------------------
# |starting with an evil center tag,
 
# +-----------------------------------------------------------------------------------------------
# |this very long second paragraph contains some special characters (including a simple space...):
# | &; <>"=/ plus a big gap###and two unicode escapes (decimal: ¡ and hexal: ¿) but also an anchor
# | embedded inside a word (this anchor also is the only tag with parameters); and finally a blank
# | row                                                                                           
 
# +--------------
# |(a single tag)

pre_render() assigns coordinates inside the layouted page to all items. It
also breaks the text blocks into lines.

This process is reversible (it doesn't alter any data stored before; it only
creates additional data), and thus resizing the output area of the viewer
will be possible without regenerating the item tree. (And without reloading
the file.)

The coordinates are stored to "x_start", "x_end", "y_start" and "y_end" of
the item structures. For every text item, the positions of all line breaks
are stored in "line_table[]". This is the only way line breaks are indicated;
the text string itself isn't altered. Thus the rendering function has to look
at this table while generating output. ( _8. render.c_ )

Besides of assigning the coordinates, pre_render() also generates the
"page_map[]". This is a structure telling which items occupy every line of
the output page. For every line, a table is generated, containing references
to all elements that show up in this line. It's described more thoroughly in
_create_map()_ .

The processing is split into five smaller passes, executed successively by
pre_render(). It also could be done in one single, pseudo-recursive pass. Most
probably this would be more efficient; it would be harder to understand, too.
Maybe we will change this at some point...

_calc_width()_

In the first pass, the minimal x-width of all items is calculated. This is done
by traversing the item tree by "list_next" (bottom to top). This ensures that
the sizes of all children are calculated before the parent. In every iteration
one item is processed. The actions taken depends on the type of the item.

For text items, the minimal width (stored in "x_end") is set to the width of
the longest word in the text block, as in other browsers. Note that we *can*
generate narrower text blocks; netrik can break words that do not fit on a
line, and probably also scrolling of items will be possible in the future.
(AFAIK no other browser does any of that, although it was recommended by the
W3C for years...)

# ###############[...]
 
# +------+
# |header|
   ^     ^x_end=6
   0
 
# ***
# *
 
# +-------
# |heading
[...]

For blank lines, the minimal width is set to 0 -- blank lines do not need
any width; anchor items as well.

# ###############[...]
 
# +------+
# |header|
 
# **
# **
   ^x_end=0
 
# +-------
# |heading
[...]

Finding the longest word is quite easy. The whole text block is processed char
by char in a loop. For each char, the current word length "len" is incremented.
When the word ends (space or string end encountered), "len" is reset to 0.
"longest" keeps the current maximum, and is stored to "x_end" after the whole
text block was processed.

For box items, the minimal width is the one of the widest sub-item.

# ##############
 
# +------+     #
# |header|     #
 
# **           #
# **           #
 
# +-------+    #
# |heading|    #
 
# **           #
# **           #
 
# +----------+ #
# |emphasized| #
 
# **           #
# **           #
 
# +--------+   #
# |starting|   #
 
# +------------#
# |parameters);#
 
# +------+     #
# |single|     #
   ^           ^x_end
   0

As all sub-items are processed before the parent, we already know the
widths of the sub-items when processing a box item. We simply go trough all
immediate children (start with "first_child" and go on by "next") and look
for the maximum.

_assign_width()_

The second pass assigns the x-coordinates (x_start and x_end) to all items.
(Presently, this is trivial: all items have the same coordinates as their
parent, which is the global item...) For text items, the positions of line
breaks are also calculated.

_Traversing Item Tree Top to Bottom_

In this pass, the tree is traversed top to bottom, as the coordinates of
the sub-items depend on the coordinates of the parent. This is a bit more
complicated than traversing bottom to top, as there is no equivalent of the
"list_next" pointer for this.

If the current item has children, we proceed with the first child. (Descend.)

                                                         <+++
                                                            +
                                                         +-----+
                                                      ,->| box |-->NULL
                                                      |  +-----+<==#
                                                      |    x ^ #===#
                         xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx +
                         x +++++++++++++++++++++++++++++++++++
                         v +                     +    |
       cur_item xx>    +-----+                +-----+ |
                    ,->| box |-.           ,->| box |-'
    new cur_item    |  +-----+=|===========|=>+-----+==>NULL
          |         |    x ^   |           |    x ^
  xxxxxxxx|xxxxxxxxxxxxxxx +   |    xxxxxxxxxxxxx +
  x  +++++|+++++++++++++++++   |    x  ++++++++++++
  v  +    |    +    |          |    v  +   |
+------+<-'+------+ |          |  +------+ |
| text |-->| text |-'          `->| text |-'
+------+==>+------+==>NULL        +------+==>NULL
  x          x                      x
  v          v                      v
  NULL       NULL                   NULL

If the current item has no children, we go to the "next" item.

                                                         <+++
                                                            +
                                                         +-----+
                                                      ,->| box |-->NULL
                                                      |  +-----+<==#
                                                      |    x ^ #===#
                         xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx +
                         x +++++++++++++++++++++++++++++++++++
                         v +                     +    |
                       +-----+                +-----+ |
                    ,->| box |-.           ,->| box |-'
                    |  +-----+=|===========|=>+-----+==>NULL
                    |    x ^   |           |    x ^
  xxxxxxxxxxxxxxxxxxxxxxxx +   |    xxxxxxxxxxxxx +
  x  +++++++++++++++++++++++   |    x  ++++++++++++
  v  +         +    |          |    v  +   |
+------+   +------+ | <--      |  +------+ |
| text |-->| text |-'          `->| text |-'
+------+==>+------+==>NULL        +------+==>NULL
  x  ^xx     x                      x
  v          v                      v
  NULL       NULL                   NULL

If there is no "next" item (we are already at the last item of this depth),
we have to ascend before we can go to the next item.

                                                         <+++
                                                            +
                                                         +-----+
                                                      ,->| box |-->NULL
                                                      |  +-----+<==#
                                                      |    x ^ #===#
                         xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx +
                         x +++++++++++++++++++++++++++++++++++
                         v +                     +    |
                       +-----+                +-----+ |
                --> ,->| box |-.           ,->| box |-'
                    |  +-----+=|===========|=>+-----+==>NULL
                    |    x ^   |           |    x ^
  xxxxxxxxxxxxxxxxxxxxxxxx +   |    xxxxxxxxxxxxx +
  x  +++++++++++++++++++++++   |    x  ++++++++++++
  v  +         +    |          |    v  +   |
+------+   +------+ | <xx      |  +------+ |
| text |-->| text |-'          `->| text |-'
+------+==>+------+==>NULL        +------+==>NULL
  x          x                      x
  v          v                      v
  NULL       NULL                   NULL

                                                         <+++
                                                            +
                                                         +-----+
                                                      ,->| box |-->NULL
                                                      |  +-----+<==#
                                                      |    x ^ #===#
                         xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx +
                         x +++++++++++++++++++++++++++++++++++
                         v +                     +    |
                       +-----+         -->    +-----+ |
                    ,->| box |-.           ,->| box |-'
                    |  +-----+=|===========|=>+-----+==>NULL
                    |    x ^   |           |    x ^
  xxxxxxxxxxxxxxxxxxxxxxxx +   |    xxxxxxxxxxxxx +
  x  +++++++++++++++++++++++   |    x  ++++++++++++
  v  +         +    |          |    v  +   |
+------+   +------+ | <xx      |  +------+ |
| text |-->| text |-'          `->| text |-'
+------+==>+------+==>NULL        +------+==>NULL
  x          x                      x
  v          v                      v
  NULL       NULL                   NULL

If we still do not have a "next" item after ascending, we ascend again.

                                                         <+++
                                                            +
                                                         +-----+
                                                      ,->| box |-->NULL
                                                      |  +-----+<==#
                                                      |    x ^ #===#
                         xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx +
                         x +++++++++++++++++++++++++++++++++++
                         v +                     +    |
                       +-----+         -->    +-----+ |
                    ,->| box |-.           ,->| box |-'
                    |  +-----+=|===========|=>+-----+==>NULL
                    |    x ^   |           |    x ^
  xxxxxxxxxxxxxxxxxxxxxxxx +   |    xxxxxxxxxxxxx +
  x  +++++++++++++++++++++++   |    x  ++++++++++++
  v  +         +    |          |    v  +   |
+------+   +------+ |          |  +------+ |  <xx
| text |-->| text |-'          `->| text |-'
+------+==>+------+==>NULL        +------+==>NULL
  x          x                      x
  v          v                      v
  NULL       NULL                   NULL

                                                         <+++
                                                            +
                                                         +-----+
                                                  --> ,->| box |-->NULL
                                                      |  +-----+<==#
                                                      |    x ^ #===#
                         xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx +
                         x +++++++++++++++++++++++++++++++++++
                         v +                     +    |
                       +-----+                +-----+ |
                    ,->| box |-.           ,->| box |-'
                    |  +-----+=|===========|=>+-----+==>NULL
                    |    x ^   |           |    x ^
  xxxxxxxxxxxxxxxxxxxxxxxx +   |    xxxxxxxxxxxxx +
  x  +++++++++++++++++++++++   |    x  ++++++++++++
  v  +         +    |          |    v  +   |
+------+   +------+ |          |  +------+ |  <xx
| text |-->| text |-'          `->| text |-'
+------+==>+------+==>NULL        +------+==>NULL
  x          x                      x
  v          v                      v
  NULL       NULL                   NULL

After all items have been processed, we'll ascend until we get back to the
top item, which's "next" pointer points back to itself; Thus we stay at the
tree top after following "next", and the main loop is terminated.

                                                         <+++
                                                            +
                                                         +-----+
                                                  --> ,->| box |-->NULL
                                                      |  +-----+<==#
                                                      |    x ^ #===#
                         xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx +
                         x +++++++++++++++++++++++++++++++++++
                         v +                     +    |
                       +-----+                +-----+ |
                    ,->| box |-.           ,->| box |-'
                    |  +-----+=|===========|=>+-----+==>NULL
                    |    x ^   |           |    x ^
  xxxxxxxxxxxxxxxxxxxxxxxx +   |    xxxxxxxxxxxxx +
  x  +++++++++++++++++++++++   |    x  ++++++++++++
  v  +         +    |          |    v  +   |
+------+   +------+ |          |  +------+ |  <xx
| text |-->| text |-'          `->| text |-'
+------+==>+------+==>NULL        +------+==>NULL
  x          x                      x
  v          v                      v
  NULL       NULL                   NULL

_Assigning x-Coordinates_

When processing an item, not the coordinates of "cur_item" are assigned,
but the coordinates of its *sub-items*. "cur_item" has had its coodinates
assigned while its parent was processed. This is an exact reversal of <a
href="#calcWidth">_calc_width()_ : Instead of "cur_item" getting its
size from its sub-items, the sub-items get their size from "cur_item".

The coordinates for the global item are assigned before calling assign_width().
x-start is set to 0, x-end is set to the output width passed from main(). (The
screen width when cfg.term_width is set; otherwise, either a constant value (in
--dump mode) or the maximum of screen width and page width.) Play around with
them if you like. (If set to a narrow box, you can see the word breaking work.)

##################################################################################
 
#------+                                                                         #
#header|                                                                         #
 
#*                                                                               #
#*                                                                               #
[...]
#------------+                                                                   #
#parameters);|                                                                   #
 
#------+                                                                         #
#single|                                                                         #
 ^x_start=0                                                                      ^x_end=80

For box (and form) items, the coordinates of all immediate children are
simply set to the ones of the box. (This will have to change at some point,
but it's ok for now...) It's done in a simple loop processing the children by
"first_child" and "next", just as in _calc_width()_ .

##################################################################################
 
+--------------------------------------------------------------------------------+
|header text                                                                     |
 
**********************************************************************************
*                                                                                *
[...]
+--------------------------------------------------------------------------------+
|this very long second paragraph contains some special characters (including a   |
|simple space...): &; <>"=/ plus a big gap###and two unicode escapes (decimal: ¡ |
|and hexal: ¿) but also an anchor embedded inside a word (this anchor also is the|
|only tag with parameters); and finally a blank row                              |
 
+--------------------------------------------------------------------------------+
|(a single tag)                                                                  |

Blank items have no children. Nothing has to be done. The same for anchor items.

Text items have no children, too; but they need to be broken into lines. (This
could be done in a seperate pass...)

_Breaking String into Lines_

The breaking into lines is a bit complicated, because netrik has the ability
to break words that do not fit on a single line. And: It does this smartly,
avoiding line breaks whenever possible.

The line breaking does not alter the string structure itself. The only output is
"line_table[]" (containing the positions of all line breaks inside the string),
and its length (The number of line breaks), which is stored as the height of
the text item in "y_end".

Before starting, some constants are calculated: The width of the text block
is the difference of x_end and x_start. The pointer to the start of the text
string is taken from the string structure and stored in "string_start".

The text block is processed word-wise. In each iteration of the outer loop
one word is processed.

First, "word_end" is retrieved. It points to the space, newline or string
terminator that ends the word to be processed. "next_word_start" is set to
"word_end+1", assuming that we will proceed with the next word (which starts
after the space); however, this may by altered later.

                     line_table[0]
                     v
string: "I am a very simple and stupid example sentence."
         ^           ^      ^  ^^
         s           l      w  en
string_start  line_start        next_word_start
                               word_end
                            word_start
output:
 x_start     x_end
 |<- width ->|
 v    12     v
+------------+
|I am a very |
|simple and stupid example sentence.
 ^      ^  ^^
 l      w  en

Then we test if the whole word fits into the current open line (which's
starting position is stored in "line_start").

                     v
string: "I am a very simple and stupid example sentence."
         ^           ^      ^  ^^^ 
         s           l      w  enb 
                                 line_start+width (line end/Break position)
                     |<- width ->|
                         (12)
+------------+
|I am a very |
|simple and stupid example sentence.
 ^      ^  ^^^ 
 l      w  enb 
            e<b -> no break

If it does, we do nothing, and simply proceed with the next word. "word_start"
is set to "next_word_start" before the next iteration.

                     v
string: "I am a very simple and stupid example sentence."
         ^           ^          ^
         s           l          n
                                ^
                                w

Otherwise, we have to generate a line break, and put the word on a new line.

                     v
string: "I am a very simple and stupid example sentence."
         ^           ^          ^^    ^^
         s           l          wb    en
+------------+
|I am a very |
|simple and stupid example sentence.
 ^          ^^    ^^
 l          wb    en
              e>=b -> break

Normally, we simply break the line at the current word, by setting "line_start"
to "word_start", and adding a line break at this position to "line_table[]".

                                line_table[1]
                     v          v
string: "I am a very simple and stupid example sentence."
         ^                      ^     ^^
         s                      w     en
                                ^
                                l
+------------+
|I am a very |
|simple and  |
|stupid example sentence.
 ^     ^^
 w     en
 ^
 l

However, things are more complicated due to the ability of breaking too long
words. For this, we have to test whether the current word fits on one line. If
it does, we simlply insert a line break as described above.

                     v          v
string: "I am a very simple and stupid example sentence."
         ^           ^          ^^    ^^    ^
         s           l          wb    en    f
                                            word_start+width (Fit on line)
                                |<- width ->|
+------------+
|I am a very |
|simple and stupid example sentence.
 ^          ^^    ^^    ^
 l          wb    en    f
                    e<f -> simple break

If it does not (if it's longer than the width of the text block), we have to
break the word.

                v      v       v        v
string: "I am a second example sentence with aVeryLongWord in me."
                                        ^    ^      ^    ^^^
                                        l    w      b    fen
+------------+
|I am a      |
|second      |
|example     |
|sentence    |
|with aVeryLongWord in me.
 ^    ^      ^    ^^^
 l    w      b    fen
                 e>=f -> break word

In this case, there are two possibilities: Either, we put the beginning of the
word on the current line, and the remainder on the next line(s). Or, we put
the first part to the new line already. This sometimes saves an unnecessary
word break:

+------------+
|I am a third|
|example     |
|sentence,   |
|with        |
|anEvenABitL\|
|ongerWord in|
|me.         |
+------------+

instead of:

+------------+
|I am a third|
|example     |
|sentence,   |
|with anEven\|
|ABitLongerW\|
|ord in me.  |
+------------+

We decide which way to go, by truncating the word of as many whole line
lengths ("width-1", because of the break chars) as possible, and testing if
the remaining part fits on the current line.

                v      v       v        v
string: "I am a second example sentence with aVeryLongWord in me."
                                        ^    ^ ^    ^     ^^
                                        l    w t    b     en
                                               trunc_word_end=word_end-(width-1)
                                               |<-      ->|
                                               width-1 (11)
+------------+
|I am a      |
|second      |
|example     |
|sentence    |
|with a\     |
 ^    ^ ^    ^
 l    w t    b
         t<=b -> fill line
 
|VeryLongWord| (truncated part)
             ^
             e

Or:

         v            v       v         v    v
string: "I am a third example sentence, with anEvenABitLongerWord in me."
                                        ^    ^      ^ ^          ^^
                                        l    w      b t          en
+------------+
|I am a third|
|example     |
|sentence,   |
|with        |
|anEvenABitL\|
 ^    ^ ^    ^
 l    w t    b
         t<=b -> fill line
 
|itLongerWord|
             ^
             e

For a longer word, the truncation is done more than once:

                      v       v         v
string: "I am another example sentence, with aTerriblyLongWordThatDoesn'tSeemToEnd in me."
                                        ^    ^   ^  ^       ^          ^          ^^
                                        l    w   T  b       t          t          en
                                                 (final)
+------------+
|I am another|
|example     |
|sentence,   |
|with aTe\   |
 ^    ^   ^  ^
 l    w   T  b  t<=b -> fill line
 
|rriblyLongW\|
             ^
             t
|ordThatDoes\|
             ^
             t
|n'tSeemToEnd in me.
             ^^
             en

or:

                      v
string: "This example has again aTerriblyLongWordThatDoesn'tSeemToEnd in some other sentence."
                      ^         ^ ^ ^          ^          ^          ^^
                      l         w b T          t          t          en
+------------+
|This example|
|has again aTe\
 ^         ^ ^ ^
 l         w b T
              t>b -> don't fill (put first word part in new line)

This is done in a loop. We also could do it by a single expression, but
evaluating a quite complicated expression is probably less efficient than a
simple loop, which is almost always executed only once. (Very few words are
longer than two lines...) Also, it is easier to understand, isn't it?...

If we decide on the first way, we fill up the current line with the beginning of
the word. (The last char is kept empty for the word break indicator displayed
in the output.) The line break position and "line_start" are set accordingly,
so that the rest of the word will be put on the next line.

                v      v       v        v          v
string: "I am a second example sentence with aVeryLongWord in me."
                                             ^     ^°     ^^
                                             w     lb     en
                                                    (old value)
+------------+
|I am a      |
|second      |
|example     |
|sentence    |
|with aVeryL\|
|ongWord in me.
 ^      ^^
 l      en

We also need to test if the remainder fits on the next line; if it does not,
we have to adjust "next_word_start" so that processing in the next loop
iteration will not continue with the next word, but with the part of the
current word that does not fit on the next line.

                      v       v         v          v
string: "I am another example sentence, with aTerriblyLongWordThatDoesn'tSeemToEnd in me."
                                             ^     ^°          ^                  ^^
                                             w     lb          b                  en
+------------+
|I am another|
|example     |
|sentence,   |
|with aTerri\|
|blyLongWordThatDoesn'tSeemToEnd in me.
 ^           ^                  ^^
 l           b                  en
                  e>=b -> more breaks necessary

                      v       v         v          v
string: "I am another example sentence, with aTerriblyLongWordThatDoesn'tSeemToEnd in me."
                                             ^     ^          ^^                  ^ 
                                             w     l          nb                  e 
+------------+
|I am another|
|example     |
|sentence,   |
|with aTerri\|
|blyLongWordThatDoesn'tSeemToEnd in me.
 ^          ^^                  ^
 l          nb                  e

If the second way was chosen, things are quite easy: The line break is
inserted at the beginning of the word, and "next_word_start" is *always*
adjusted to the end of the new line.

         v            v       v         v    v
string: "I am a third example sentence, with anEvenABitLongerWord in me."
                                             ^          ^^       ^°
                                             w          nb       en
                                             ^
                                             l
+------------+
|I am a third|
|example     |
|sentence,   |
|with        |
|anEvenABitLongerWord in me."
 ^          ^^       ^°
 l          nb       en

In any case, if "next_line_start" was adjusted we will scan for the word
end again in the next iteration; but starting with the part of the word that
does not fit on the new line. (This is unefficient of course, but this case
is quite rare, and it's not worth adding special code for handling this.)

         v            v       v         v    v
string: "I am a third example sentence, with anEvenABitLongerWord in me."
                                             ^          ^        ^^
                                             l          w        en
+------------+
|I am a third|
|example     |
|sentence,   |
|with        |
|anEvenABitLongerWord in me."
 ^          ^^       ^^
 l          wb       en

As the new line is already filled up with the previous word part, another
line break will always be inserted just in front of the remainder, and the
remainder will be put into another line.

         v            v       v         v    v
string: "I am a third example sentence, with anEvenABitLongerWord in me."
                                             ^          ^^       ^^
                                             l          wb       en
+------------+
|I am a third|
|example     |
|sentence,   |
|with        |
|anEvenABitLongerWord in me."
 ^          ^^       ^^
 l          wb       en
                e>b -> break

         v            v       v         v    v
string: "I am a third example sentence, with anEvenABitLongerWord in me."
                                                        ^°       ^^
                                                        wb       en
                                                        ^
                                                        l
+------------+
|I am a third|
|example     |
|sentence,   |
|with        |
|anEvenABitL\|
|ongerWord in me."
 ^        ^^
 l        en

Of course it's also possible that the remainder still doesn't fit on a line,
and has to be broken again.

                      v       v         v          v
string: "I am another example sentence, with aTerriblyLongWordThatDoesn'tSeemToEnd in me."
                                                   ^          ^^                  ^^
                                                   l          wb                  en
+------------+
|I am another|
|example     |
|sentence,   |
|with aTerri\|
|blyLongWordThatDoesn'tSeemToEnd in me.
 ^          ^^                  ^^
 l          wb                  en
                   e>b -> break

                      v       v         v          v
string: "I am another example sentence, with aTerriblyLongWordThatDoesn'tSeemToEnd in me."
                                                   ^          ^           ^       ^^
                                                   l          w           f       en
+------------+
|I am another|
|example     |
|sentence,   |
|with aTerri\|
|blyLongWordThatDoesn'tSeemToEnd in me.
 ^          ^           ^       ^^
 l          w           f       en
                          e>f -> word break

                      v       v         v          v
string: "I am another example sentence, with aTerriblyLongWordThatDoesn'tSeemToEnd in me."
                                                   ^          ^^       ^          ^^
                                                   l          wb       t          en
+------------+
|I am another|
|example     |
|sentence,   |
|with aTerri\|
|blyLongWordThatDoes\|
 ^          ^^       ^
 l          wb       t
                 t>b -> don't fill

                      v       v         v          v          v
string: "I am another example sentence, with aTerriblyLongWordThatDoesn'tSeemToEnd in me."
                                                              ^          ^^       ^
                                                              w          nb       e
                                                              ^
                                                              l
+------------+
|I am another|
|example     |
|sentence,   |
|with aTerri\|
|blyLongWord\|
|ThatDoesn'tSeemToEnd in me.
 ^          ^^       ^
 l          nb       e

This will be repeated, until the whole word is stored.

                      v       v         v          v          v
string: "I am another example sentence, with aTerriblyLongWordThatDoesn'tSeemToEnd in me."
                                                              ^          ^^       ^^
                                                              l          wb       en
                                                                            e>b -> break

                      v       v         v          v          v
string: "I am another example sentence, with aTerriblyLongWordThatDoesn'tSeemToEnd in me."
                                                              ^          ^        ^^
                                                              l          w        en
                                                                                   ^
                                                                                   f
                                                                            e<=f -> no word break

                      v       v         v          v          v          v
string: "I am another example sentence, with aTerriblyLongWordThatDoesn'tSeemToEnd in me."
                                                                         ^        ^^
                                                                         w        en
                                                                         ^
                                                                         l
+------------+
|I am another|
|example     |
|sentence,   |
|with aTerri\|
|blyLongWord\|
|ThatDoesn't\|
|SeemToEnd in me.
 ^        ^^
 l        en

Of course, a line break is always inserted in front of the word no matter if
it would fit on the old line, if the word follows a newline character.

                v      v       v        v      v      
string: "I am a stupid example sentence with a newline/in me."
         ^                                     ^       ^ ^^^
         s                                     l       w enb
+------------+
|I am a      |
[...]
|newline/in me.
 ^       ^ ^^^
 l       w enb
          e<b, but w follows newline -> break

+------------+
|I am a      |
[...]
|newline/    |
|in me.
 ^ ^^
 w en
 ^
 l

_calc_ywidth()_

The third pass is simple again. calc_ywidth() calculates minimal heights
(y-widths) for all items.

Presently this is complete overkill; assign_ywidth() doesn't really need this.
However, it will be necessary as soon as interesting elements are implemented
(tables) -- which hopefully won't be too long now...

Like _calc_width()_ , it traverses the tree bottom to top, and for every
item it stores the minimal height to "y_end".

Blank lines always have the height of 1. Anchor items have no height. (They
are virtual...)

Text items have their height assigned already in _assign_width()_ , while
<a href="#lineBreaking">_Breaking String into Lines_ .

##################################################################################
 
+--------------------------------------------------------------------------------+___
|header text                                                                     |__0
+--------------------------------------------------------------------------------+  1<--y_end
 
**********************************************************************************___
*                                                                                *__0
**********************************************************************************  1<--
[...]
+--------------------------------------------------------------------------------+___
|this very long second paragraph contains some special characters (including a   |  0
|simple space...): &; <>"=/ plus a big gap###and two unicode escapes (decimal: ¡ |  1
|and hexal: ¿) but also an anchor embedded inside a word (this anchor also is the|  2
|only tag with parameters); and finally a blank row                              |__3
+--------------------------------------------------------------------------------+  4<--
 
+--------------------------------------------------------------------------------+___
|(a single tag)                                                                  |__0
+--------------------------------------------------------------------------------+  1<--

The height of a box item is calculated by summing up the height of all its
sub-items. This is done by "first_child" and "next", just as seeking the
widest sub-item in _calc_width()_ .

##################################################################################
 
+--------------------------------------------------------------------------------+
|header text                                                                     |
+--------------------------------------------------------------------------------+ <--  1
 
********************************************************************************** 
*                                                                                * 
********************************************************************************** <-- +1
[...]                                                                                 [...]
+--------------------------------------------------------------------------------+ 
|this very long second paragraph contains some special characters (including a   | 
|simple space...): &; <>"=/ plus a big gap###and two unicode escapes (decimal: ¡ | 
|and hexal: ¿) but also an anchor embedded inside a word (this anchor also is the| 
|only tag with parameters); and finally a blank row                              | 
+--------------------------------------------------------------------------------+ <-- +4
 
+--------------------------------------------------------------------------------+ 
|(a single tag)                                                                  | 
+--------------------------------------------------------------------------------+ <-- +1
 
################################################################################## <## 13
                                                                                   item_tree->y_end

_assign_ywidth()_

The fourth pass is also fairly simple. It assigns y-coordinates to all items.
There is no initialisation necessary before calling it, as the y_end of the
global item is already set by calc_ywidth(), and y_start is always 0.

Like in _assign_width()_ , the tree is traversed top to bottom, and the
sizes of all sub-items are assigned while processing the parent.

Thus, text and blank items do not need any processing -- they do not have
any children.

For box items, the coordinates of all sub-items are assigned in a loop, like
in _assign_width()_ . Every item is put immediately after the previous one,
i.e. it starts where the previous one ends.  We keep track of the current
position by "y_pos". At the beginning it is initialized to "y_start" of the box.

##################################################################################
                                                                                   0 <##           <== y_pos
                                                                                     item_tree->y_start

For every item, y_start is set to the current y_pos.

                                                                                     y_end  y_start
                                                                                        |     |
+--###############################################################################____  v     v
|header text                                                                     |___0       <--
*                                                                                *___1 <--   <**  
|heading                                                                         |___2 <**   <----
*                                                                                *___3 <---- <****
|first paragraph of text; includes multiple spaces and newlines, emphasized text |   4 <**** <--  
|and strong text                                                                 |___5       
*                                                                                *___6 <--   <**  
|starting with an evil center tag,                                               |___7 <**   <----
|this very long second paragraph contains some special characters (including a   |   8 <---- <--    <==
|simple space...): &; <>"=/ plus a big gap###and two unicode escapes (decimal: ¡ |           cur_item->y_start=y_pos=8
|and hexal: ¿) but also an anchor embedded inside a word (this anchor also is the|
|only tag with parameters); and finally a blank row                              |
+--------------------------------------------------------------------------------+ <-- cur_item->y_end=4
 
+--------------------------------------------------------------------------------+ 
|(a single tag)                                                                  | 
+--------------------------------------------------------------------------------+ <----  1
 
################################################################################## <## 13

"y_end" of an item is determined by adding the y-size (stored in "y_end"
up to now) to "y_start" of the item.

+--###############################################################################____
|header text                                                                     |___0       <--
*                                                                                *___1 <--   <**  
|heading                                                                         |___2 <**   <----
*                                                                                *___3 <---- <****
|first paragraph of text; includes multiple spaces and newlines, emphasized text |   4 <**** <--  
|and strong text                                                                 |___5       
*                                                                                *___6 <--   <**  
|starting with an evil center tag,                                               |___7 <**   <----
|this very long second paragraph contains some special characters (including a   |   8 <---- <--    <==
|simple space...): &; <>"=/ plus a big gap###and two unicode escapes (decimal: ¡ |   9
|and hexal: ¿) but also an anchor embedded inside a word (this anchor also is the|  10
|only tag with parameters); and finally a blank row                              |__11
+--------------------------------------------------------------------------------+  12 <-- cur_item->y_end=cur_item->ystart+cur_item->y_end=12
 
+--------------------------------------------------------------------------------+
|(a single tag)                                                                  |
+--------------------------------------------------------------------------------+ <----  1
 
################################################################################## <## 13

"y_pos" is adjusted to the end of the item.

+--###############################################################################____
|header text                                                                     |___0       <--
*                                                                                *___1 <--   <**  
|heading                                                                         |___2 <**   <----
*                                                                                *___3 <---- <****
|first paragraph of text; includes multiple spaces and newlines, emphasized text |   4 <**** <--  
|and strong text                                                                 |___5       
*                                                                                *___6 <--   <**  
|starting with an evil center tag,                                               |___7 <**   <----
|this very long second paragraph contains some special characters (including a   |   8 <---- <--
|simple space...): &; <>"=/ plus a big gap###and two unicode escapes (decimal: ¡ |   9
|and hexal: ¿) but also an anchor embedded inside a word (this anchor also is the|  10
|only tag with parameters); and finally a blank row                              |__11
+--------------------------------------------------------------------------------+  12 <--          <==
 
+--------------------------------------------------------------------------------+
|(a single tag)                                                                  |
+--------------------------------------------------------------------------------+<----  1
 
################################################################################## <## 13

In the next iteration, "y_pos" -- which now points to the end of the current
item -- is used as the beginning of the new item.

+--###############################################################################____
|header text                                                                     |___0       <--
*                                                                                *___1 <--   <**  
|heading                                                                         |___2 <**   <----
*                                                                                *___3 <---- <****
|first paragraph of text; includes multiple spaces and newlines, emphasized text |   4 <**** <--  
|and strong text                                                                 |___5       
*                                                                                *___6 <--   <**  
|starting with an evil center tag,                                               |___7 <**   <----
|this very long second paragraph contains some special characters (including a   |   8 <---- <--
|simple space...): &; <>"=/ plus a big gap###and two unicode escapes (decimal: ¡ |   9
|and hexal: ¿) but also an anchor embedded inside a word (this anchor also is the|  10
|only tag with parameters); and finally a blank row                              |__11
|(a single tag)                                                                  |  12 <--   <----   <==
+--------------------------------------------------------------------------------+<----  1
 
################################################################################## <## 13

_Link Coordinates_

Coordinates of links and anchors (both x and y!) are presently also assigned
here. Probably it would be a better idea to do that in extra pass... As soon as
the current link implementation is dropped and anchors are cleanly implemented,
the assignment can be cleanly and logically split between assign_xwidth()
and assign_ywidth(), just as for other item types.

Link coordinates are assigned by the parent text item. The text block is
scanned, one link after the other, for the lines containing the link start
and end. (The first line that ends after the link start, and the first line
that ends at/after the link end.) As an optimization, the search for the next
link is started in the line where the previous ended, not from beginning --
links can't be nested.

After having the line, (and thus also the y coordinate), the x coordinate
is calculated by adding the link's relative position inside the line
(link[].start-line_start) to the x coordinate of the line start. (Which is
equal to the item start for normal text, but has to be calculated seperately
for every line in centered text items.)

Inline anchors are very similar; however, they get their coordinates while
processing the anchor item, not the text item containing them. (The text item
doesn't know anything about the anchors.) Anchors can be nested if created
by <span> or so; thus the above optimization isn't possible. (It would be
harder anyhow, due to the anchors being processed every one on its own.)

Block anchors need another processing, of course: If they are empty, they keep
the coordinates assigned to them by the parent box; otherwise, all (immediate)
virtual children are scanned for the minimum/maximum for each of the four
coordinates; these are assigned as the coordinates of the anchor virtual box.

_create_map()_

The last sub-pass generates the "page_map[]". (This could be also done inside
assign_ywidth()...) This map is necessary to quickly determine which elements
show up in the visible area of the page, when it is displayed in the viewer.

The page usage map stores references to all items that show up in any given
line of the output page. This is a very simple approach, but it should be
perfectly sufficient as long as netrik has no full graphic mode -- and this
will be probably for quite a while...

"page_map[]" is an array containing an "Item_list" structure for every line
of the output page. This structure is declared in items.h . It contains a
count of items in this line, and an array of pointers to the items.

The whole line map is allocated at the beginning. Afterwards, all items are
processed in a loop. For every item that is visible on the screen (presently
this are only text items), a reference to this item is stored to every line of
"page_map[]" between "y_start" and "y_end" of the item.

                                                                                      line|page_map[line]
+--###############################################################################__  ----+--------------
|header text                                                                     |__    0 | t0
*                                                                                *__    1 | -
|heading                                                                         |__    2 | t1
*                                                                                *______3_|_-____
|first paragraph of text; includes multiple spaces and newlines, emphasized text |      4 | t2  <-- cur_item->y_start
|and strong text                                                                 |______5_|_t2___
*                                                                                *__    6 | -   <-- cur_item->y_end
|starting with an evil center tag,                                               |__    7 | -
|this very long second paragraph contains some special characters (including a   |      8 | -
|simple space...): &; <>"=/ plus a big gap###and two unicode escapes (decimal: ¡ |      9 | -
|and hexal: ¿) but also an anchor embedded inside a word (this anchor also is the|     10 | -
|only tag with parameters); and finally a blank row                              |__    1 | -
|(a single tag)                                                                  |__   12 | -
###############################################################################--+  

"page_map[]" is returned to pre_render(), and from there to main(), where it is
passed to _dump()_ or to _render()_ (via _display()_ , see hacking-pager.*)
along with "item_tree".

_free_map()_

This function frees the memory allocated for the page map.

First it goes through the table line by line, and frees the associated "list"
for each one. Afterwards, it frees the table itself.

_8. render.c_

With the item tree and the page usage map prepared in pre_render(), we can
now actually render the page. There are two different rendering functions:

dump() renders the whole page and dumps the output to the terminal. The output
is layouted correctly, using all the coordinates and text attributes.

render() renders only a specified area of the page, and outputs it to the
curses screen.

_dump()_

The page is rendered line by line, using "page_map[]" to determine which
items we need to render in every line. Of course this isn't really necessary,
as we do not ever have more than one item in a line presently. However, using
"page_map[]" here is *not* overkill, just for a change ;-) On the contrary,
this is a pragmatic approach. Dumping the page item by item without using
"page_map[]" would be more efficient; however, it would be also more complicated
than dumping line by line.

In every line, we process all items (from "page_map[line]") one after
the other. If there is actually more then one item (which is impossible
presently...), they are printed one after the other. As long as they do
not overlap (they will at some point, but not too soon...), they shouldn't
interfere with each other.

At the beginning of processing an item, some constants are set. "item" and
"string" point to the processed item and its text area.

"string_line" is the line number inside the text block of the line we are just
printing. That is to say, it's the offset to the first line of the text block.

2                           |[...]                                            |
3                           |+-----------------------------------------------+|
4 <-- item->y_start=4     0 ||first paragraph of text; includes multiple     ||
5 <-- line=5  string_ --> 1 ||spaces and newlines, emphasized text and strong||
6             line=1      2 ||text                                           ||
7                           |+-----------------------------------------------+|
8                           |[...]                                            |

"text_start" tells where this line beginns inside the string. Normally, it
is taken directly from "line_table[string_line-1]", which is the position of
the line break at which the line starts.

                                                  line_table[0]
                                                  v
text: "first paragraph of text; includes multiple spaces and newlines, [...]"
                                                  ^
                                          text_start=line_table[string_line-1]

As line 0 of a text block doesn't start with a line break, its start isn't
stored in "line_table[]"; we have to test for this case and set "text_start"
to 0.

                                                  v
text: "first paragraph of text; includes multiple spaces and newlines, [...]"
       ^
       text_start

"text_end" tells where this line ends; it is set to the beginning of the next
line ("line_table[string_line]").

                     line_table[0]                               line_table[1]
                     v                                               v
text: "[...]multiple spaces and newlines, emphasized text and strong text"
                     ^                                               ^
                     text_start               text_end=line_table[string_line]

As the last line does not end with a line break, we have to test for this
case end set "text_end" to the string end.

                     v                                               v
text: "[...]multiple spaces and newlines, emphasized text and strong text"
                                                                     ^   ^
                                                            text_start text_end

Maybe we should store the beginning of the first line and the end of the last
line to "line_table[]" also. This would simplify the code (we wouldn't need
the tests), and the efficiency loss should be quite irrelevant.

"text_end" isn't declared as a constant, because it may be altered: If the last
character in the line turns out to be a blank (or newline), it isn't printed.
We accomplish this by decrementing "text_end" by one.

                     v                                               v
text: "[...]multiple spaces and newlines, emphasized text and strong text"
                     ^                                              ^
                     text_start                                     text_end

After knowing start and end of the line, we calculate "x_start", which is
the screen column at which the line will start. Normally, this is equal to
"x_start" of the text item.

      ||spaces and newlines, emphasized text and strong||
        ^
        x_start=item->x_start

If the text item is centered, we have to add an individual offset to each
line. The offset is half of the unused space inside the line, i.e. the
difference between the width of the text block and the width of the text in
this line.

        text_start                       text_end
        v                                v
      ||starting with an evil center tag,              ||
        ^                                              ^
        item->x_start                                  item->x_end
        |<---- text_end-text_start ----->|
        |<--------------- x_end-x_start -------------->|
                                         |<----------->|
                                              unused
                                         |<---->|
                                         unused/2

               v                                v
      ||       starting with an evil center tag,       ||
        ^      ^                                       ^
item->x_start  x_start=item->xstart+unused/2
        |<---->|
        unused/2

Then we set the cursor to the calculated col. This we do by a carrige return
(not really needed for the first item in a line...) and than going forward
as much character positions as needed.

Now we can output the text itself. We print all attribute divisions in a loop.
But first we have to find the first one that shows up in the current line.

                                               div[0].end     ...    div[3].end
                                               v               v   v          v
text: "first[...]multiple spaces and newlines, emphasized text and strong text"
       |                  ^                    |                          ^
                          text_start

In every iteration we print all the text between "div_start" end "div_end",
which normally point to the start and end of the current division. For the
first iteration, "div_start" is set to "text_start" -- we only want the part
of the div that shows up in the current text line.

                                               v               v   v          v
text: "first[...]multiple spaces and newlines, emphasized text and strong text"
                          ^                    ^
                       div_start=text_start    div_end=div[0].end
 
output:
first paragraph of text; contains multiple
spaces and newlines,

The next div starts where the current one ends.

                                               v               v   v          v
text: "first[...]multiple spaces and newlines, emphasized text and strong text"
                          ^                    ^               ^          ^
                          text_start           div_start       div_end

The last division is truncated to "text_end" -- as with the first one, we
only want the part that shows up in the current line.

                                               v               v   v          v
text: "first[...]multiple spaces and newlines, emphasized text and strong text"
                          ^                                    ^          ^
                                                       div_start       div_end
                                                                      =text_end

Finally, we test if the line ends with a word break, and print the break
character if it does. We know it does when the character at the line end is
a word character (not a space, newline, or string end), because every word
end is followed by the space separating it from the next word; if there is
no space at the line end, we are inside a broken word.

                    v                     v
text "Some sentence containing aVeryLongAndThusBrokenWord."
                                          ^
output:
Some sentence
containing aVeryLongAn\
...
dThusBrokenWord.

_render()_

render() works similar to _dump()_ . The main difference is how the output
is printed. However, there are a couple of differences in screen position
handling and other calculations also.

render() takes the starting positon of the rendered area inside the page, the
starting position on the screen, and the size of the rendered area as arguments.

The area is processed line by line, and every line is processed item by item,
just like in dump().

"x_start" describes the starting column relative to the beginning of the
rendered area, not the screen. (dump() always dumps whole lines, and thus there
is no difference.) If it turns out that the line starts before the rendered
area, it has to be truncated. the ending position "x_end" is calculated in
a similar fashion.

Before the line is printed (div by div, as in dump()), the cursor is set to
the start position of the line. The column is the starting position relative
to the area ("x_start"), plus the starting position of the area on the screen;
likewise the row.

Before anything is printed, we test if some part of the line shows up inside
the rendered area at all. The word break indicator is also printed only if
the line ends inside the area.

If render() was called with the "overpaint" flag, the requested area is
cleaned before rendering anything, so any garbage will be removed. (Areas not
containing any text aren't affected otherwise.) This is done by overwriting
the desired part of each line with a string of spaces.

_dump_items()_

dump_items() dumps the item tree, including the text of text items. The text is
printed with correct attributes, but ignoring any coordinates and line breaks.
This function is for debugging purposes, and may be called anywhere inside
or after pre_render() (anywhere after parse_struct()).

The reason this function is in render.c is that it needs the same screen
handling functions as _dump()_ . Moreover, it works in a very similar
fashion.

The difference is that it does not dump line by line, but item by item
(traversing the tree top to bottom). After printing some information about the
item itself, it dumps the text division by division in the same way dump()
does, only it doesn't need to care about positions or line breaks; it dumps
the whole string at once.

