字符串朴素匹配算法:bugfix

Rabin-krap算法实现

Author: shengshijun

string/rabin-karp.py

@@ -0,0 +1,46 @@
+#!/usr/bin/env python
+# -*- coding:UTF-8
+__author__ = 'shenshijun'
+"""
+首先计算pattern字符串的hash值，然后在从目标字符串的开头，计算相同长度字符串的hash值。若hash值相同，则表示匹配，若不同，则向右移动一位，计算新的hash值。整个过程，与暴力的字符串匹配算法很相似，
+但由于计算hash值时，可以利用上一次的hash值，从而使新的hash值只需要加上新字母的计算，并减去上一次的第一个字母的计算，即可。
+Rabin-Karp算法的预处理时间为O(m)，最坏情况下该算法的匹配时间为O((n-m+1)m)，期望复杂度O(m+n)
+"""
+
+
+def match(origin, pattern):
+    pattern_len = len(pattern)
+
+    def _hash(string, start=0):
+        hash_code = 0
+        for x in xrange(pattern_len):
+            hash_code += ord(string[start + x]) * 2 ** (pattern_len - x - 1)
+        return hash_code
+
+    def _refresh(old_hash, old_char, new_char):
+        return (old_hash - ord(old_char) * 2 ** (pattern_len - 1)) * 2 + ord(new_char)
+
+    def test_equal(start_index):
+        for x in xrange(pattern_len):
+            if origin[x + start_index] != pattern[x]:
+                return False
+        return True
+
+    origin_index = 0
+    pattern_hash = _hash(pattern)
+    origin_hash = _hash(origin)
+    while origin_index < len(origin) - pattern_len - 1:
+        if pattern_hash == origin_hash and test_equal(origin_index):
+            return origin_index
+        else:
+            print "origin hash:%s,pattern hash:%s" % (origin_hash, pattern_hash)
+            origin_hash = _refresh(origin_hash, origin[origin_index], origin[origin_index + pattern_len])
+            origin_index += 1
+
+
+def main():
+    print match("sbsfsdgdgfgasbssssfsfdfeferf", 'sb')
+
+
+if __name__ == "__main__":
+    main()

string/simple.py

@@ -5,22 +5,25 @@
 
 def match(origin, pattern):
     origin_index, pattern_index = 0, 0
+    match_flag = True
     pattern_len = len(pattern)
     while origin_index < len(origin):
         for pattern_index in xrange(pattern_len):
             if pattern[pattern_index] != origin[origin_index]:
                 origin_index -= (pattern_index - 1)
+                match_flag = False
                 break
             else:
                 origin_index += 1
                 pattern_index += 1
+                match_flag = True
 
-        if origin[origin_index] == pattern[pattern_index]:
+        if match_flag:
             return origin_index - pattern_index
 
 
 def main():
-    print match("absbsbshdhhd", 'sb')
+    print match("absabsvbshsbdhhd", 'sb')
 
 
 if __name__ == "__main__":