Time-stamp: <1999-12-30 22:58:42 blp>

TODO
----

The way that data-in.c and data-out.c deal with strings is wrong.  Instead of
the way it's done now, we should make it dynamically allocate a buffer and
return a pointer to it.  This is a much safer interface.

Add libplot output driver.  Suggested by Robert S. Maier
<rsm@math.arizona.edu>: "it produces output in idraw-editable PS format, PCL5
format, xfig-editable format, Illustrator format,..., and can draw vector
graphics on X11 displays also".

Storage of value labels on disk is inefficient.  Invent new data structure.

Add an output flag which would cause a page break if a table segment could fit
vertically on a page but it just happens to be positioned such that it won't.

Fix spanned joint cells, i.e., EDLEVEL on crosstabs.stat.

Cell footnotes.

PostScript driver should emit thin lines, then thick lines, to optimize time
and space.

New functions?  var_name_or_label(), tab_value_or_label()

Should be able to bottom-justify cells.  It'll be expensive, though, by
requiring an extra metrics call.

Perhaps instead of the current lines we should define the following line types:
null, thin, thick, double.  It might look pretty classy.

Perhaps thick table borders that are cut off by a page break should decay to
thin borders.  (i.e., on a thick bordered table that's longer than one page,
but narrow, the bottom border would be thin on the first page, and the top and
bottom borders on middle pages.)

Support multi-line titles on tables. (For the first page only, presumably.)

Rewrite the convert_F() function in data-out.c to be nicer code.

In addition to searching the source directory, we should search the current
directory (for data files).  (Yuck!)

Fix line-too-long problems in PostScript code, instead of covering them up.
setlinecap is *not* a proper solution.

Need a better way than MAX_WORKSPACE to detect low-memory conditions.

When malloc() returns 0, page to disk and free() unnecessary data.

Remove ccase * argument from procfunc argument to procedure().

See if process_active_file() has wider applicability.

Looks like there's a potential problem with value labels--we use free_val_lab
from avl_destroy(), but free_val_lab doesn't decrement the reference count, it
just frees the label.  Check into this sometime soon.

Eliminate private data in struct variable through use of pointers.

Fix som_columns().

There needs to be another layer onto the lexer, which should probably be
entirely rewritten anyway.  The lexer needs to read entire *commands* at a
time, not just a *line* at a time.  This would vastly simplify the
(yet-to-be-implemented) logging mechanism and other stuff as well.
	   
Has glob.c been pared down enough?

Improve interactivity of output by allowing a `commit' function for a page.
This will also allow for infinite-length pages.

All the tests need to be looked over.  Some of the SET calls don't make sense
any more.

Implement thin single lines, should be pretty easy now.

SELECT IF should be moved before other transformations whenever possible.  It
should only be impossible when one of the variables referred to in SELECT IF is
created or modified by a previous transformation.

The manual: add text, add index entries, add examples.

The inline file should be improved: There should be *real* detection of whether
it is used (in dfm.c:cmd_begin_data), not after-the-fact detection.

Figure out a stylesheet for messages displayed by PSPP: i.e., what quotation
marks around filenames, etc.

Data input and data output are currently arranged in reciprocal pairs: input is
done directly, with write_record() or whatever; output is done on a callback
event-driven basis.  It would definitely be easier if both could be done on a
direct basis, with read_record() and write_record() routines, with a coroutine
implementation (see Knuth).  But I'm not sure that coroutines can be
implemented in ANSI C.  This will require some thought.  Perhaps 0.4.0 can do
this.

New SET subcommand: OUTPUT.  i.e., SET OUTPUT="filename" to send output to that
file; SET OUTPUT="filename"(APPEND) to append to that file; SET OUTPUT=DEFAULT
to reset everything.  There might be a better approach, though--think about it.

HDF export capabilities (http://hdf.ncsa.uiuc.edu).  Suggested by Marcus
G. Daniels <mgd@santafe.edu>.

From Zvi Grauer <z.grauer@csuohio.edu> and <zvi@mail.ohio.net>:

   1. design of experiments software, specifically Factorial, response surface
   methodology and mixrture design.  

   These would be EXTREMELY USEFUL for chemists, engineeris, and anyone
   involved in the production of chemicals or formulations.

   2. Multidimensional Scaling analysis (for market analysis) - 

   3. Preference mapping software for market analysis

   4. Hierarchical clustering (as well as partition clustering)

   5. Conjoint analysis

   6. Categorical data analsys ?

IDEAS
-----

In addition to an "infinite journal", we should keep a number of
individual-session journals, pspp.jnl-1 through pspp.jnl-X, renaming and
deleting as needed.  All of the journals should have date/time comments.

Qualifiers for variables giving type--categorical, ordinal, ...

Analysis Wizard

Consider consequences of xmalloc(), fail(), hcf() in interactive
use:
a. Can we safely just use setjmp()/longjmp()?
b. Will that leak memory?
i. I don't think so: all procedure-created memory is either
garbage-collected or globally-accessible.
ii. But you never know... esp. w/o Checker.
c. Is this too early to worry? too late?

Need to implement a shared buffer for funny functions that require relatively
large permanent transient buffers (1024 bytes or so), that is, buffers that are
permanent in the sense that they probably shouldn't be deallocated but are only
used from time to time, buffers that can't be allocated on the stack because
they are of variable and unpredictable but usually relatively small (usually
line buffers).  There are too many of these lurking around; can save a sizeable
amount of space at very little overhead and with very little effort by merging
them.

Clever multiplatform GUI idea (due partly to John Williams): write a GUI in
Java where each statistical procedure dialog box could be downloaded from the
server independently.  The statistical procedures would run on (the/a) server
and results would be reported through HTML tables viewed with the user's choice
of web browsers.  Help could be implemented through the browser as well.

Design a plotting API, with scatterplots, line plots, pie charts, barcharts,
Pareto plots, etc., as subclasses of the plot superclass.

HOWTOs
------

1. How to add an operator for use in PSPP expressions:

a. Add the operator to the enumerated type at the top of expr.h.  If the
operator has arguments (i.e., it's not a terminal) then add it *before*
OP_TERMINAL; otherwise, add it *after* OP_TERMINAL.  All these begin with OP_.

b. If the operator's a terminal then you'll want to design a structure to hold
its content.  Add the structure to the union any_node.  (You can also reuse one
of the prefab structures, of course.)

c. Now switch to expr-prs.c--the module for expression parsing.  Insert the
operator somewhere in the precedence hierarchy.

(1) If you're adding a operator that is a function (like ACOS, ABS, etc.) then
add the function to functab in `void init_functab(void)'.  Order is not
important here.  The first element is the function name, like "ACOS".  The
second is the operator enumerator you added in expr.h, like OP_ARCOS.  The
third element is the C function to parse the PSPP function.  The predefined
functions will probably suit your needs, but if not, you can write your own.
The fourth element is an argument to the parsing function; it's only used
currently by generic_str_func(), which handles a rather general syntax for
functions that return strings; see the comment at the beginning of its code for
details.

(2) If you're adding an actual operator you'll have to put a function in
between two of the operators there already in functions `exprtype
parse_*(any_node **n)'.  Each of these stores the tree for its result into *n,
and returns the result type, or EX_ERROR on error.  Be sure to delete all the
allocated memory on error before returning.

d. Add the operator to the table `op_desc ops[OP_SENTINEL+1]' in expr-prs.c,
which has an entry for every operator.  These entries *must* be in the same
order as they are in expr.h.  The entries have the form `op(A,B,C,D)'.  A is
the name of the operator as it should be printed in a postfix output format.
For example, the addition operator is printed as `plus'.  B is a bitmapped set
of flags:

* Set the 001 bit (OP_VAR_ARGS) if the operator takes a variable number of
arguments.  If a function can take, say, two args or three args, but no other
numbers of args, this is a poor way to do it--instead implement the operator as
two separate operators, one with two args, the other with three.  (The main
effect of this bit is to cause the number of arguments to be output to the
postfix form so that the expression evaluator can know how many args the
operator takes.  It also causes the expression optimizer to calculate the
needed stack height differently, without referencing C.)

* Set the 002 bit (OP_MIN_ARGS) if the operator can take an optional `dotted
argument' that specified the minimum number of non-SYSMIS arguments in order to
have a non-SYSMIS result.  For instance, MIN.3(e1,e2,e3,e4,e5) returns a
non-SYSMIS result only if at least 3 out of 5 of the expressions e1 to e5 are
not missing.

Minargs are passed in the nonterm_node structure in `arg[]''s elements past
`n'--search expr-prs.c for the words `terrible crock' for an example of this.

Minargs are output to the postfix form.  A default value is output if none was
specified by the user.

You can use minargs for anything you want--they're not limited to actually
describing a minimum number of valid arguments; that's just what they're most
*commonly* used for.

* Set the 004 bit (OP_FMT_SPEC) if the operator has an argument that is a
format specifier.  (This causes the format specifier to be output to the
postfix representation.)

Format specs are passed in the nonterm_node structure in the same way as
minargs, except that there are three args, in this order: type, width, # of
decimals--search expr-prs.c for the words `is a crock' for an example of this.

* Set the 010 bit (OP_ABSORB_MISS) if the operator can *ever* have a result of
other than SYSMIS when given one or more arguments of SYSMIS.  Operators
lacking this bit and known to have a SYSMIS argument are short-circuited to
SYSMIS by the expression optimizer.

* If your operator doesn't fit easily into the existing categories,
congratulations, you get to write lots of code to adjust everything to cope
with this new operator.  Are you really sure you want to do that?

C is the effect the operator has on stack height.  Set this to `varies' if the
operator has a variable number of arguments.  Otherwise this 1, minus the
number of arguments the operator has.  (Since terminals have no arguments, they
have a value of +1 for this; other operators have a value of 0 or less.)

D is the number of items output to the postfix form after the operator proper.
This is 0, plus 1 if the operator has varargs, plus 1 if the operator has
minargs, plus 3 if the operator has a format spec.  Note that minargs/varargs
can't coexist with a format spec on the same operator as currently coded.  Some
terminals also have a nonzero value for this but don't fit into the above
categories.

e. Switch to expr-opt.c.  Add code to evaluate_tree() to evaluate the
expression when all arguments are known to be constants.  Pseudo-random
functions can't be evaluated even if their arguments are constants.  If the
function can be optimized even if its arguments aren't all known constants, add
code to optimize_tree() to do it.

f. Switch to expr-evl.c.  Add code to evaluate_expression() to evaluate the
expression.  You must be absolutely certain that the code in evaluate_tree(),
optimize_tree(), and evaluate_expression() will always return the same results,
otherwise users will get inconsistent results, a Bad Thing.  You must be
certain that even on boundary conditions users will get identical results, for
instance for the values 0, 1, -1, SYSMIS, or, for string functions, the null
string, 1-char strings, and 255-char strings.

g. Test the code.  Write some test syntax files.  Examine the output carefully.

NOTES ON SEARCH ALGORITHMS
--------------------------

1. Trees are nicer when you want a sorted table.  However, you can always
sort a hash table after you're done adding values.

2. Brent's variation of Algorithm D is best when the table is fixed: it's
memory-efficient, having small, fixed overhead.  It's easier to use
when you know in advance how many entries the table will contain.

3. Algorithm L is rather slow for a hash algorithm, however it's easy.

4. Chaining is best in terms of speed; ordered/self-ordering is even
better.

5. Rehashing is slow.

6. Might want to decide on an algorithm empirically since there are no
clear mathematical winners in some cases.

7. gprof?  Hey, it works!

MORE NOTES/IDEAS/BUGS
---------------------

The behavior of converting a floating point to an integer when the value of the
float is out of range of the integer type is UNDEFINED!  See ANSI 6.2.1.3.

What should we do for *negative* times in expressions?

Sometimes very wide (or very tall) columns can occur in tables.  What is a good
way to truncate them?  It doesn't seem to cause problems for the ascii or
postscript drivers, but it's not good in the general case.  Should they be
split somehow?  (One way that wide columns can occur is through user request,
for instance through a wide PRINT request--try time-date.stat with a narrow
ascii page or with the postscript driver on letter size paper.)

NULs in input files break the products we're replacing: although it will input
them properly and display them properly as AHEX format, it truncates them in A
format.  Also, string-manipulation functions such as CONCAT truncate their
results after the first NUL.  This should simplify the result of PSPP design.
Perhaps those ugly a_string, b_string, ..., can all be eliminated.

From Moshe Braner <mbraner@nessie.vdh.state.vt.us>: An idea regarding MATCH
FILES, again getting BEYOND the state of SPSS: it always bothered me that if I
have a large data file and I want to match it to a small lookup table, via
MATCH FILES FILE= /TABLE= /BY key, I need to SORT the large file on key, do the
match, then (usually) re-sort back into the order I really want it.  There is
no reason to do this, when the lookup table is small.  Even a dumb sequential
search through the table, for every case in the big file, is better, in some
cases, than the sort.  So here's my idea: first look at the /TABLE file, if it
is "small enough", read it into memory, and create an index (or hash table,
whatever) for it.  Then read the /FILE and use the index to match to each case.
OTOH, if the /TABLE is too large, then do it the old way, complaining if either
file is not sorted on key.

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