Re: [PATCH v2 0/5] Speed up uNN_chr and uNN_strchr with Boyer-Moore algorithm

[Paolo Bonzini]

On 07/29/2010 11:05 AM, Pádraig Brady wrote:
> > I would better recommend to use
> > the u8_strstr function.
>
> I wonder could we speed that up for UTF-8
> by just deferring to strstr() ?
> I've not tested this so feel free to bin it.

An alternative idea: check if there is more than 1 character using 
u8_mbtouc.  If there is one, use the faster u8_strchr algorithm (you can 
just use u8_strchr, even though that does a useless conversion back to 
UTF-8).  If there is more than one, use strstr directly.

And this gives another opportunity: we can now define u8_strstr and 
u16_strstr to always return a pointer to a valid UTF-8 sequence:

/* Find the first occurrence of NEEDLE in HAYSTACK.  For UTF-8 and
   UTF-16, the returned value will never point in the middle of a
   multibyte sequence or surrogate pair.  If this cannot be satisfied,
   for example due to an invalid NEEDLE, NULL is returned.  */

Why?  Because the only cases when you can get a false positive are: 1) 
when you have illegal sequences at the beginning of the needle, which 
now we could easily check; 2) when both the needle and haystack have the 
same illegal UTF-8 in it, which I think could be ignored.

For u16_strstr you need to look for an invalid surrogate character at 
the beginning of the needle.

Thoughts?  Would anybody give a shot to the above?

Paolo