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Comment by skrellm | Hacker News Reader

I've also implemented a SIMD accelerated, but case in-sensitive (*) search algorithm as an stb-style single header library, also based on Wojciech Muła's work.

https://gitlab.com/bztsrc/fast_memcasemem

See the performance comparison in the README, it's about 6 times faster than libc.

(*) - full UTF-8 support, but only works for UNICODE codepoints where the UTF-8 encoding lengths are the same for lowercase and uppercase. There are only 27 out of 40576 pairs which aren't handled (listed in the README).

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> UNICODE codepoints where the UTF-8 encoding lengths are the same for lowercase and uppercase.

This very same property got me to post this [1], which sent me down the rabbithole of learning about Unicode in earnest and building my Unicode tool. Which may have an initial release some time this millennia... maybe.

1. https://news.ycombinator.com/item?id=42014045

Yeah, UNICODE messed this up, really badly.

Sometimes there's an offset (like with Latin, +/- 32), sometimes lowercase and uppercase is interleaved (eg. Latin extended), and sometimes they are in totally different blocks simply because they forgot to add both letter cases at once... (the distance of the codepoints affects UTF-8 encoding difference the most).

I've also paid attention to optimize the most common case where both UTF-8 encodings' first bytes are the same (that's 40508 pairs out of 40549). But no escape, it must handle the remaining 41 pairs specially in a slower code path, which would not be needed at all should UNICODE guys did their homework better.

One of my favourite ones in the post I linked is the "ﬀ" ligature. It uppercases to "FF", meaning in UTF-8 it goes from one encoded character to two, and from 3 bytes total to 2 bytes total.

Lots to read about here for those interested (and that's without getting into `Casefold`, `NFKC_Casefold`, simple vs complex case mappings, the CLDR, etc.):

https://www.unicode.org/versions/Unicode17.0.0/core-spec/cha...

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