bm in python

Author: KPatr1ck

python/33_bm/bm_.py

@@ -0,0 +1,120 @@
+#!/usr/bin/python
+# -*- coding: UTF-8 -*-
+
+SIZE = 256
+
+
+def bm(main, pattern):
+    """
+    BM算法
+    匹配规则：
+    1. 坏字符规则
+    2. 好字符规则
+    :param main:
+    :param pattern:
+    :return:
+    """
+    assert type(main) is str and type(pattern) is str
+    n, m = len(main), len(pattern)
+
+    if n <= m:
+        return 0 if main == pattern else -1
+
+    # bc
+    bc = [-1] * SIZE
+    generate_bc(pattern, m, bc)
+
+    # gs
+    suffix = [-1] * m
+    prefix = [False] * m
+    generate_gs(pattern, m, suffix, prefix)
+
+    i = 0
+    while i < n-m+1:
+        j = m - 1
+        while j >= 0:
+            if main[i+j] != pattern[j]:
+                break
+            else:
+                j -= 1
+
+        # pattern整个已被匹配，返回
+        if j == -1:
+            return i
+
+        # 1. bc规则计算后移位数
+        x = j - bc[ord(main[i+j])]
+
+        # 2. gs规则计算后移位数
+        y = 0
+        if j != m - 1:    # 存在gs
+            y = move_by_gs(j, m, suffix, prefix)
+
+        i += max(x, y)
+
+    return -1
+
+
+def generate_bc(pattern, m, bc):
+    """
+    生成坏字符哈希表
+    :param pattern:
+    :param m:
+    :param bc:
+    :return:
+    """
+    for i in range(m):
+        bc[ord(pattern[i])] = i
+
+
+def generate_gs(pattern, m, suffix, prefix):
+    """
+    好后缀预处理
+    :param pattern:
+    :param m:
+    :param suffix:
+    :param prefix:
+    :return:
+    """
+    for i in range(m-1):
+        k = 0   # pattern[:i+1]和pattern的公共后缀长度
+        for j in range(i, -1, -1):
+            if pattern[j] == pattern[m-1-k]:
+                k += 1
+                suffix[k] = j
+                if j == 0:
+                    prefix[k] = True
+            else:
+                break
+
+
+def move_by_gs(j, m, suffix, prefix):
+    """
+    通过好后缀计算移动值
+    需要处理三种情况：
+    1. 整个好后缀能和pattern的剩余字符匹配
+    2. 好后缀里存在 *后缀子串* 能和pattern的 *前缀* 匹配
+    3. 其他
+    :param j:
+    :param m:
+    :param suffix:
+    :param prefix:
+    :return:
+    """
+    k = m - 1 - j           # j指向从后往前的第一个坏字符，k是此次匹配的好后缀的长度
+
+    if suffix[k] != -1:             # 1. 整个好后缀在pattern剩余字符中仍有出现
+        return j - suffix[k] + 1
+    else:
+        for r in range(j+2, m):     # 2. 后缀子串从长到短搜索
+            if prefix[m-r]:
+                return r
+        return m                    # 3. 其他情况
+
+
+if __name__ == '__main__':
+    print('--- search ---')
+    m_str = 'dfasdeeeetewtweyyyhtruuueyytewtweyyhtrhrth'
+    p_str = 'eyytewtweyy'
+    print('[Built-in Functions] result:', m_str.find(p_str))
+    print('[bm] result:', bm(m_str, p_str))