Code for {n} times faster than C, where n = 128

Actually, n = 290 🤯

Benchmark Setup

Rust version: rustc 1.70.0 (90c541806 2023-05-31)
Run test: cargo test
Run benchmark: cargo bench

Machine: Apple MacBook Pro 14-inch (2021)
Processor: Apple M1 Pro
Memory: 16 GB

Input size: 1,000,000 characters
Input generation: s and p are chosen with randomly 50% probability.

Benchmark Result

Function	Time	Throughput	Relative speed
baseline_unicode	3.7511 ms	254.24 MiB/s	0.88
baseline	3.3316 ms	286.25 MiB/s	1
opt1_idiomatic	227.33 µs	4.0968 GiB/s	14.7
opt2_count_s	152.44 µs	6.1096 GiB/s	21.9
opt3_count_s_branchless	72.902 µs	12.775 GiB/s	45.7
opt4_simd	43.131 µs	21.593 GiB/s	77.2
opt5_simd_unrolled_2x	32.810 µs	28.385 GiB/s	101.5
opt5_simd_unrolled_4x	28.524 µs	32.650 GiB/s	116.8
opt5_simd_unrolled_8x	26.518 µs	35.120 GiB/s	125.64
opt5_simd_unrolled_10x	26.070 µs	35.724 GiB/s	127.8 🎉
opt5_simd_unrolled_12x	27.833 µs	33.461 GiB/s	119.7
opt5_simd_unrolled_16x	27.157 µs	34.293 GiB/s	122.7
opt6_chunk_count1	14.597 µs	63.802 GiB/s	228.2
opt6_chunk_exact_count 2	11.489 µs	81.060 GiB/s	290.0 🚀

Credit

Thanks u/DavidM603 and u/Sharlinator for contributing even faster and cleaner solutions.

Thanks @PeterFaiman for catching and fixing an overflow problem in multiple optimizations (#1).

Footnotes

1. Credit to Reddit user u/DavidM603. ↩

2. Credit to Reddit user u/Sharlinator. ↩