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语言: Python
分类: 算法分析
最后更新于 2018-10-11 23:20
gistfile1.txt
原始数据 复制代码
#计算最长公共字串
PRIME = 72057594037927931
DOMAIN = 128
def roll_hash(old_val, out_digit, in_digit, last_pos):
val = (old_val - out_digit*last_pos+DOMAIN*PRIME)%PRIME
val = (val*DOMAIN) % PRIME
return (val + in_digit) %PRIME
def matches(s, t, i, j, k):
for d in range(k):
if s[i+d] != t[j+d]:
return False
return True
def rabin_karp_matching(s, t):
hash_s = 0
hash_t = 0
len_s = len(s)
len_t = len(t)
last_pos = pow(DOMAIN, len_t -1)%PRIME
if len_s < len_t:
return -1
for i in range(len_t):
hash_s = (DOMAIN*hash_s+ord(s[i]))%PRIME
hash_t = (DOMAIN*hash_t+ord(t[i]))%PRIME
for i in range(len_s - len_t+1):
if hash_s == hash_t:
if matches(s, t, i, 0, len_t):
return i
if i<len_s - len_t:
hash_s = roll_hash(hash_s, ord(s[i]),ord(s[i+len_t]), last_pos)
return -1
def rabin_karp_factor(s, t, k):
last_pos = pow(DOMAIN, k-1)%PRIME
pos = {}
assert k>0
if len(s) < k or len(t)<k:
return None
hash_t = 0
for j in range(k):
hash_t = (DOMAIN*hash_t + ord(t[j]))%PRIME
for j in range(len(t)-k+1):
if hash_t in pos:
pos[hash_t].append(j)
else:
pos[hash_t] = [j]
if j < len(t)-k:
hash_t = roll_hash(hash_t, ord(t[j]),ord(t[j+k]),last_pos)
hash_s = 0
for i in range(k):
hash_t = (DOMAIN*hash_t+ord(s[i]))%PRIME
for i in range(len(s)-k+1):
if hash_t in pos:
for j in pos[hash_s]:
if matches(s, t, i, j, k):
return (i, j)
if i<len(s)-k:
hash_s = roll_hash(hash_s, ord(s[i]),ord(s[j+k]),last_pos)
return None
if __name__ =='__main__':
src_string = 'Hello, Google, hello, Microsoft, hello, Python'
des_string = 'Google'
print(rabin_karp_matching(src_string, des_string))

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