Merge pull request wangzheng0822#185 from jerryderry/bm-python

Boyer-Moore string-search algorithm in python

Author: wangzheng0822

python/33_bm/bm.py

@@ -0,0 +1,77 @@
+"""
+ Boyer-Moore string-search algorithm.
+
+ Author: Wenru Dong
+"""
+
+from typing import List, Tuple
+
+SIZE = 256
+
+def _generate_bad_character_table(pattern: str) -> List[int]:
+ bc = [-1] * SIZE
+ for i, char in enumerate(pattern):
+ bc[ord(char)] = i
+ return bc
+
+
+def _generate_good_suffix_table(pattern: str) -> Tuple[List[bool], List[int]]:
+ m = len(pattern)
+ # prefix[k] records whether the last k-character suffix of pattern
+ # can match with the first k-character prefix of pattern.
+ # suffix[k] records the starting index of the last substring of
+ # pattern that can match with the last k-character suffix of pattern.
+ prefix, suffix = [False] * m, [-1] * m
+ # For each substring patter[:i+1], we find the common suffix with
+ # pattern, and the starting index of this common suffix.
+ # This way we can re-write previous suffix[k] to record the index
+ # as large as possible, hence the last substring.
+ for i in range(m - 1):
+ j = i # starting index of the common suffix
+ k = 0 # length of the common suffix
+ while j >= 0 and pattern[j] == pattern[~k]:
+ j -= 1
+ k += 1
+ suffix[k] = j + 1
+ if j == -1: prefix[k] = True
+ return (prefix, suffix)
+
+
+def _move_by_good_suffix(bad_character_index: int, suffix: List[int], prefix: List[bool]) -> int:
+ k = len(suffix) - 1 - bad_character_index
+ if suffix[k] != -1: return bad_character_index - suffix[k] + 1
+ # Test from k - 1
+ for r, can_match_prefix in enumerate(reversed(prefix[:k]), bad_character_index + 2): 
+ if can_match_prefix: return r
+ return len(suffix)
+
+
+def bm(s: str, pattern: str) -> int:
+ bc = _generate_bad_character_table(pattern)
+ prefix, suffix = _generate_good_suffix_table(pattern)
+ n, m = len(s), len(pattern)
+ i = 0
+ while i <= n - m:
+ j = m - 1 # bad character index in pattern
+ while j >= 0: 
+ if s[i + j] != pattern[j]: break
+ j -= 1
+ if j < 0: return i
+ 
+ x = j - bc[ord(s[i + j])]
+ y = 0
+ if j < m - 1:
+ y = _move_by_good_suffix(j, suffix, prefix)
+ i += max(x, y)
+ return -1
+
+
+if __name__ == "__main__":
+
+ s = "Here is a simple example"
+ pattern = "example"
+ print(bm(s, pattern))
+
+ s = "abcdcccdc"
+ pattern = "cccd"
+ print(s.find(pattern) == bm(s, pattern))