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Re: [PATCH] improve string find algorithm


On 06/01/17 16:26 +0100, Jakub Jelinek wrote:
On Fri, Jan 06, 2017 at 03:16:12PM +0000, Jonathan Wakely wrote:
On 06/01/17 16:09 +0100, Jakub Jelinek wrote:
> On Fri, Jan 06, 2017 at 08:42:15AM -0600, Aditya Kumar wrote:
> > Yes, we do.
> > Sorry for the mistake, it happened because I first wrote this for
> > libcxx (https://reviews.llvm.org/D27068) and while porting that line
> > got missed.
>
> Shouldn't find at least in the case where it is narrow char string
> just use C library memmem?  That implements a Two-Way searching algorithm
> with some improvements from Boyer-Moore.
> Otherwise, consider what even your modified version will do for
>
> #include <string>
> int main() {
> (std::string(10000000, 'a')+"b").find(std::string(1000000, 'a')+"b");
> }

Yes, I have an incomplete local patch that checks for memmem and uses
it where possible. This change would still be valuable for non-GNU
targets though.

The description of Two-Way algorithm is in
http://www-igm.univ-mlv.fr/~lecroq/string/node26.html#SECTION00260
Boyer-Moore in
http://en.wikipedia.org/wiki/Boyer-Moore_string_search_algorithm

And in std::boyer_moore_searcher in <functional> :-)
We also have std::boyer_moore_horspool_searcher in <functional>.

We could try allocating memory in std::string::find() and fall back to
the dumb algorithm if it fails, but we don't want to throw bad_alloc.



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