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

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

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);
}

The find_subsequence function can also be made generic:

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

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