Re: [PATCH] improve string find algorithm

[Jonathan Wakely <jwakely at redhat dot com>]

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.