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