kmp in python

Author: jerryderry

python/34_kmp/kmp.py

@@ -0,0 +1,45 @@
+"""
+ KMP algorithm
+
+ Author: Wenru Dong
+"""
+
+from typing import List
+
+def kmp(s: int, pattern: int) -> int:
+ m = len(pattern)
+ partial_match_table = _get_partial_match_table(pattern)
+ j = 0
+ for i in range(len(s)):
+ while j and s[i] != pattern[j]:
+ j = partial_match_table[j - 1] + 1
+ j += 1
+ if j == m:
+ return i - m + 1
+ return -1
+
+
+def _get_partial_match_table(pattern: int) -> List[int]:
+ # Denote pi^k(i) as pi applied to i for k times,
+ # i.e., pi^2(i) = pi(pi(i)).
+ # Then we have the result:
+ # pi(i) = pi^k(i-1) + 1,
+ # where k is the smallest integer such that
+ # pattern[pi^k(i-1)+1] == pattern[i].
+ 
+ m = len(pattern)
+ pi = [0] * m
+ pi[0] = k = -1 # We use k here to represent pi^k(i)
+ for i in range(1, m):
+ while k >= 0 and pattern[k + 1] != pattern[i]:
+ k = pi[k]
+ k += 1
+ pi[i] = k
+ return pi
+
+
+if __name__ == "__main__":
+
+ s = "abc abcdab abcdabcdabde"
+ pattern = "bcdabd"
+ print(kmp(s, pattern), s.find(pattern))