Created
January 16, 2014 21:51
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A simple implementation of the Rabin-Karp substring search algorithm with a fairly naive running hash function.
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| class RollingHash: | |
| s = None | |
| start = 0 | |
| end = 0 | |
| hash = 0 | |
| def __init__(self, s, length): | |
| self.s = s | |
| self.end = length - 1 | |
| for i in range(length): | |
| self.hash += ord(s[i]) | |
| def update(self): | |
| if self.s is None: | |
| return | |
| self.hash -= ord(self.s[self.start]) | |
| self.start += 1 | |
| self.end += 1 | |
| if self.end <= len(self.s) - 1: | |
| self.hash += ord(self.s[self.end]) | |
| def search(s, ss): | |
| if len(ss) > len(s): | |
| return False | |
| sshash = RollingHash(ss, len(ss)) | |
| shash = RollingHash(s, len(ss)) | |
| while shash.end <= len(s) - 1: | |
| if sshash.hash == shash.hash: | |
| if ss == s[shash.start:(shash.end + 1)]: | |
| return True | |
| shash.update() | |
| return False | |
| print search("Hello, world", "o, w") |
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