bug#32750: [PATCH 2/2] dfa: optmization of alternation in NFA

[Norihiro Tanaka]

Hi,

Even when similer states exists in alternation, DFA treats them as
separate items.  It may complicate the transition in NFA and cause
slowdown.  This change assembles the states into one.

For example, ab|ac is changed into a(b|c).

This change speeds-up matching for many branched pattern.  For
example, grep speeds-up more than 30x in following case.

  seq 10000 | sed 's/$/ abcdefghijklmnopqrstuvwxyz/; s/$/./' >in
  time -p env LC_ALL=C grep -vf in in

  (before) real 63.43 user 61.67 system 1.65
  (after)  real  1.64 user  1.61 system 0.03

  # If we do not add '.' at last in pattern, not dfa but kwset is used.

grep also speeds-up about 3x in following case.

  time -p env LC_ALL=C grep -vf /usr/share/dict/linux.words 
/usr/share/dict/linux.words

  (before) real  2.48 user  2.09 system 0.38
  (after)  real  7.69 user  6.32 system 1.29

Thanks,
Norihiro

0001-dfa-simplify-initial-state.patch
Description: Text document

0002-dfa-optmization-of-alternation-in-NFA.patch
Description: Text document