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Re: Improving regexp-opt

From: Miguel V. S. Frasson
Subject: Re: Improving regexp-opt
Date: Wed, 17 Apr 2019 21:59:38 -0300

> Furthermore, even when the result is noticeably shorter, have you
> compared the performance of the regexp-matcher?  I expect that you won't
> be able to see a measurable difference there.

For the very particular regexp for cXr, cXXr and cXXXr, the shorter
"\\(?:c[ad]\\(?:[ad][ad]?\\)?r\\)" takes 40% of the time compared to the longer

But most of the time the match time is equivalent, and although I made an
improvement in the computation of regexp-opt with FA, it still has complexity
of greater order.

For the example for
'("cond" "if" "when" "unless" "while"
  "let" "let*" "progn" "prog1" "prog2"
  "save-restriction" "save-excursion" "save-window-excursion"
  "save-current-buffer" "save-match-data"
  "catch" "throw" "unwind-protect" "condition-case")
now it takes "just" 50 times of the computation time of current regexp-opt
(instead of 70 times that it took before).

So I admit that it has little benefit.

> IOW it's just not a good deal.

I agree.

> As I said, if you really want to improve on regexp-opt, you have to go
> through a *real* DFA and that means not returning a regexp but a DFA, so
> it's a completely different beast from `regexp-opt`.

I don't understand what you mean by real DFA in emacs and how to
implement that.

best regards.


Miguel Vinicius Santini Frasson

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