Commits on May 1, 2021

1. compiler: fix lazy DFA false quits on ASCII text …

   One of the things the lazy DFA can't handle is Unicode word boundaries,
   since it requires multi-byte look-around. However, it turns out that on
   pure ASCII text, Unicode word boundaries are equivalent to ASCII word
   boundaries. So the DFA has a heuristic: it treats Unicode word
   boundaries as ASCII boundaries until it sees a non-ASCII byte. When it
   does, it quits, and some other (slower) regex engine needs to take over.
   
   In a bug report against ripgrep[1], it was discovered that the lazy DFA
   was quitting and falling back to a slower engine even though the
   haystack was pure ASCII.
   
   It turned out that our equivalence byte class optimization was at fault.
   Namely, a '{' (which appears very frequently in the input) was being
   grouped in with other non-ASCII bytes. So whenever the DFA saw it, it
   treated it as a non-ASCII byte and thus stopped.
   
   The fix for this is simple: when we see a Unicode word boundary in the
   compiler, we set a boundary on our byte classes such that ASCII bytes
   are guaranteed to be in a different class from non-ASCII bytes. And
   indeed, this fixes the performance problem reported in [1].
   
   [1] - BurntSushi/ripgrep#1860

   

   BurntSushi committed May 1, 2021
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