It is interesting to note how it compares to "choose" (also in Rust) in the benchmarks.

single character

    hck           1.494 ± 0.026s
    hck (no-mmap) 1.735 ± 0.004s
    choose        4.597 ± 0.016s

multi character

    hck           2.127 ± 0.004s
    hck (no-mmap) 2.467 ± 0.012s
    choose        3.266 ± 0.011s

The single pass optimization trick[1] seems to be helping a lot in single character case.

Of course, doing away with a pass is suppossed to give 2x, and I am wondering whether the regex constraint lead to this "side-effect".

[1] fast mode - https://github.com/sstadick/hck/blob/master/src/lib/core.rs#... https://github.com/sstadick/hck/blob/master/src/lib/core.rs#...