Splitter

Tests and benchmarks a handful of ways to split a string in Rust.

What we're trying to do

I wanted a Rust function that trims a string up to and including the first instance of a specified character.

So fn remove_through_first_char("word1 word2 word3", ' ') would return "word2 word3". If the specified character isn't in the given string, return the string as inputted.

An initial, naive attempt

My first working version of the function was this.

fn remove_through_first_char(l: &str, ch: char) -> String {
    if l.contains(ch) {
        let mut word_vec = l.split(ch).collect::<Vec<&str>>();
        word_vec.remove(0);
        return word_vec.join(&ch.to_string());
    } else {
        l.to_string()
    }
}

Basically we check if the character ch is present with a contains. If it is, we split on the given character ch into a vector (word_vec), remove the first element, then finally join the vector into a fresh new String.

While this version works in that it passes the tests in src/lib.rs, it is inefficient, mostly because it performs unnecessary allocations and copies strings. We can do better!

Other approaches

Sergey Bugaev contributed not 1 but ultimately 4 other implementations of the function that all perform this same task more efficiently. They're all located in src/lib.rs.

Measuring performance of various implementations of the function

Using the Rust benchmarking library Criterion, we benchmarked each of the functions with varying input data sizes. You can see this code in benches/splitters.rs.

You should be able to run the benchmarks yourself by running cargo bench, but spoilers below.

(After running cargo bench, graphs like this one are printed to /target/criterion/Splitters/report/index.html.)

A winner emerges

The clear winner, "V4", is this version, which uses the memchr library:

pub fn remove_through_first_char_variant_4(s: &str, ch: char) -> &str {
    match memchr(ch as u8, s.as_bytes()) {
        None => s, // not found => return the whole string
        Some(pos) => &s[pos + 1..],
    }
}

Using the function in a non-trivial project

I'm now using this V4 implementation in a tool called Tidy.

Contributions

Do you have another way to perform the task described above that you think might be more efficient? Improvements to the existing functions? Feel free to create a pull request!