pub fn knuth_morris_pratt(st: String, pat: String) -> Vec<usize> {
    if st.is_empty() || pat.is_empty() {
        return vec![];
    }

    let string = st.into_bytes();
    let pattern = pat.into_bytes();

    // build the partial match table
    let mut partial = vec![0];
    for i in 1..pattern.len() {
        let mut j = partial[i - 1];
        while j > 0 && pattern[j] != pattern[i] {
            j = partial[j - 1];
        }
        partial.push(if pattern[j] == pattern[i] { j + 1 } else { j });
    }

    // and read 'string' to find 'pattern'
    let mut ret = vec![];
    let mut j = 0;

    for (i, &c) in string.iter().enumerate() {
        while j > 0 && c != pattern[j] {
            j = partial[j - 1];
        }
        if c == pattern[j] {
            j += 1;
        }
        if j == pattern.len() {
            ret.push(i + 1 - j);
            j = partial[j - 1];
        }
    }

    ret
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn each_letter_matches() {
        let index = knuth_morris_pratt("aaa".to_string(), "a".to_string());
        assert_eq!(index, vec![0, 1, 2]);
    }

    #[test]
    fn a_few_separate_matches() {
        let index = knuth_morris_pratt("abababa".to_string(), "ab".to_string());
        assert_eq!(index, vec![0, 2, 4]);
    }

    #[test]
    fn one_match() {
        let index =
            knuth_morris_pratt("ABC ABCDAB ABCDABCDABDE".to_string(), "ABCDABD".to_string());
        assert_eq!(index, vec![15]);
    }

    #[test]
    fn lots_of_matches() {
        let index = knuth_morris_pratt("aaabaabaaaaa".to_string(), "aa".to_string());
        assert_eq!(index, vec![0, 1, 4, 7, 8, 9, 10]);
    }

    #[test]
    fn lots_of_intricate_matches() {
        let index = knuth_morris_pratt("ababababa".to_string(), "aba".to_string());
        assert_eq!(index, vec![0, 2, 4, 6]);
    }

    #[test]
    fn not_found0() {
        let index = knuth_morris_pratt("abcde".to_string(), "f".to_string());
        assert_eq!(index, vec![]);
    }

    #[test]
    fn not_found1() {
        let index = knuth_morris_pratt("abcde".to_string(), "ac".to_string());
        assert_eq!(index, vec![]);
    }

    #[test]
    fn not_found2() {
        let index = knuth_morris_pratt("ababab".to_string(), "bababa".to_string());
        assert_eq!(index, vec![]);
    }

    #[test]
    fn empty_string() {
        let index = knuth_morris_pratt("".to_string(), "abcdef".to_string());
        assert_eq!(index, vec![]);
    }
}