KMP string matching

Author: EbTech

src/string_proc.rs

@@ -0,0 +1,67 @@
+// Palindrome substrings in O(n), Manacher's algorithm
+// length of odd palin centred at s[i] is len[2*i]
+// even palin btwn s[i],s[i+1]: len[2*i+1]
+// TODO: check for underflows
+// Alternative version:
+// for c in (..)
+//   while bla
+//     len[c] += 2;
+//     if len[c]-r+i == len[c-r+i] { len.push(len[c]-r+i); r += 1; }
+fn find_pals(text: &[u8]) -> Vec<usize> {
+    let mut len = Vec::with_capacity(2*text.len() - 1); 
+    len.push(1); len.push(0);
+    let mut i = 1;
+    while i < 2*text.len() - 2 {
+        let max_len = ::std::cmp::min(i+1, 2*text.len()-1-i);
+        while len[i] < max_len && text[(i-len[i]-1)/2] == text[(i+len[i]+1)/2] {
+            len[i] += 2;
+        }
+        let mut d = 1;
+        while len[i-d] < len[i]-d { len[i+d] = len[i-d]; d += 1; }
+        len[i+d] = len[i]-d;
+        i += d;
+    }
+    len
+}
+
+// fail[i] = len of longest proper prefix-suffix of pat[0...i]
+fn kmp_init(pat: &[u8]) -> Vec<usize> {
+    let mut fail = Vec::with_capacity(pat.len());
+    fail.push(0);
+    let mut j = 0;
+    for ch in &pat[1..] {
+        while j > 0 && pat[j] != *ch { j = fail[j-1]; }
+        if pat[j] == *ch { j += 1; }
+        fail.push(j);
+    }
+    fail
+}
+
+// matches[i] = len of longest prefix of pat matching with suffix of text[0...i]
+fn kmp_match(text: &[u8], pat: &[u8]) -> Vec<usize> {
+    let fail = kmp_init(pat);
+    let mut matches = Vec::with_capacity(text.len());
+    let mut j = 0;
+    for ch in text {
+        if j == pat.len() { j = fail[j-1]; }
+        while j > 0 && pat[j] != *ch { j = fail[j-1]; }
+        if pat[j] == *ch { j += 1; }
+        matches.push(j);
+    }
+    matches
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    
+    #[test]
+    fn test_string() {
+        let text = "abcbc".as_bytes();
+        let pat = "bc".as_bytes();
+        let matches = kmp_match(text, pat);
+        let pal_len = find_pals(text);
+        assert_eq!(matches, vec![0, 1, 2, 1, 2]);
+        assert_eq!(pal_len, vec![1, 0, 1, 0, 3, 0, 3, 0, 1]);
+    }
+}