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How can I find a subsequence in a &[u8] slice?

Submitted by: @import:stackoverflow-api·Mar 10, 2026·

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howslicesubsequencefindcan

Problem

I have a &[u8] slice over a binary buffer. I need to parse it, but a lot of the methods that I would like to use (such as str::find) don't seem to be available on slices.

I've seen that I can covert both by buffer slice and my pattern to str by using from_utf8_unchecked() but that seems a little dangerous (and also really hacky).

How can I find a subsequence in this slice? I actually need the index of the pattern, not just a slice view of the parts, so I don't think split will work.

Solution

Here's a simple implementation based on the windows iterator.

fn find_subsequence(haystack: &[u8], needle: &[u8]) -> Option {
    haystack.windows(needle.len()).position(|window| window == needle)
}

fn main() {
    assert_eq!(find_subsequence(b"qwertyuiop", b"tyu"), Some(4));
    assert_eq!(find_subsequence(b"qwertyuiop", b"asd"), None);
}

The find_subsequence function can also be made generic:

fn find_subsequence(haystack: &[T], needle: &[T]) -> Option
    where for &'a [T]: PartialEq
{
    haystack.windows(needle.len()).position(|window| window == needle)
}

Code Snippets

fn find_subsequence(haystack: &[u8], needle: &[u8]) -> Option<usize> {
    haystack.windows(needle.len()).position(|window| window == needle)
}

fn main() {
    assert_eq!(find_subsequence(b"qwertyuiop", b"tyu"), Some(4));
    assert_eq!(find_subsequence(b"qwertyuiop", b"asd"), None);
}

fn find_subsequence<T>(haystack: &[T], needle: &[T]) -> Option<usize>
    where for<'a> &'a [T]: PartialEq
{
    haystack.windows(needle.len()).position(|window| window == needle)
}

Context

Stack Overflow Q#35901547, score: 55

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