Revised gperf 2.7.2 patch

[Bruce Lilly]

In February 2002, I posted some patches to gperf 2.7.2 to address
a number of issues:

1. efficient case-independent matching of keywords
2. aoutomatic determination of optimal key positions

A revised patch is available at
http://users.erols.com/blilly/mailparse/gperf.patch

Item #1 above has not changed.  Item #2 has been
substantially improved; the method now used is much
faster and handles some pathological cases that were
not handled in the earlier patch.

Additional issues are also handled:

3. more efficient code when all keywords have the same length
4. more efficient code when the -l option is used
5. correct handling of automatic key position determination
   when -n option is used

Details:

Vanilla gperf 2.7.2 generates a test like:

 if (len <= MAX_WORD_LENGTH && len >= MIN_WORD_LENGTH)

Clearly that is inefficient when all keywords have the
same length (MAX_WORD_LENGTH == MIN_WORD_LENGTH). The
patch generates the following in such cases:

  if (len == MIN_WORD_LENGTH)

which requires a single comparison rather than two.

When the -l option is used, vanilla gperf checks the
length via lengthtable and a string comparison, then
makes an additional check for a terminating '\0';
that additional check is unnecessary (because of the
lengthtable check).

The revised automatic optimal key position determining
code ensures that at least one key position is used if
the -n option is specified or if all keywords have the
same length.