Fix #74264: grapheme_sttrpos() broken for negative offsets

[cmb69]

We must not assume that usearch_last() gives the proper result for
negative offsets. Instead we'd need to continue to search backwards
(usearch_previous) until we find a proper match. However, apparently
searching backwards is broken, so we work around by searching forward
from the start of the string until we pass the offset_pos, and then
use the previous result.

[nikic]

However, apparently searching backwards is broken

Broken how?

[cmb69]

For the test string "déjàààà", usearch_last() for "à" reports the last character (6) which is correct. A following usearch_previous() reports the third but last character (4), though. The same happens with usearch_preceding() passing 6 as position. I tested with recent ICU 66.1 and 68.2 on Windows.

[nikic]

@cmb69 Looking through the docs, I believe this should be a matter of setting the USEARCH_OVERLAP attribute.

[cmb69]

Thanks, that improves the backward search, but there is still an issue at the end of the string; e.g. grapheme_strrpos('déjàààà', 'à', -2) returns 6, but that should be 5.

[nikic]

I've tried a lot of different variants, but in the end I can only agree with your conclusion that usearch_previous/preceding are buggy. It looks like generally the starting offset of the match must be strictly lower than the starting position, but if the match is at the very end of the string, then the match can be at the starting position. I think we'll have to go back to your first implementation :(