Read the linked article.
I also have a favourite string matching algorithm, the “Generic SIMD” from this post [1] by Wojciech Muła (I haven’t really read the other two SIMD algorithms since I wasn’t planning on working with intrinsics).
There were some good comments that post’s thread [2], including from burntsushi of ripgrep.
Ah, last weekend I was just updating my own string search algorithm to use Bitap (Shift-OR variant) instead of KMP, when the pattern length is < 64. It is faster. It is a very lovely algorithm indeed.
My search algorithm, HashChain [1] is a very fast sublinear algorithm, but is uses KMP (and now Bitap) to verify matches so it has a linear worst case (instead of quadratic, like Boyer Moore Horspool).
It is a really nice derivation of Bitap from first principles. I had not seen that before. Good job!
One small nit: he says the naive algorithm is linear - maybe it is for the average case, but it has a quadratic worst case complexity. Bitap is linear even for worst case.
Always been interested in using SIMD to speed up searching. So far though i have not found a really nice one.
Need to figure out what area they excel in - there is no one search that is the best for all types of data and pattern/search length.
I like burntsushi’s work because it’s well documented, split modularly into libraries/applications, and covers the gamut from low- to high-level. I use these three tools which he maintains:
Low level: memchr [1]
Medium level: Regex (Rust crate) [2];
High level: ripgrep [3];
Other names to look out for are the previously aforementioned Wojciech Muła, as well as Daniel Lemire (of simdjson [4][5]). Not SIMD-specific, but Data-Oriented Design can be a big help in terms of thinking about SIMD and cache-friendly data layout (as well as trimming down the work that the computer needs to do, generally). I’ve talked that to death, so I’ll just link those comments here [6].
Interesting algorithm.
I can see how this could be advantageous for moderate length patterns up to 64 bytes.
But for short registered sized patterns (4 or 8 bytes), I am somewhat skeptical of any significant advantage over a very tight, brute force register based loop comparing multi-byte chunks.
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).
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.
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…