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51 lines
1.5 KiB
Python
51 lines
1.5 KiB
Python
def rabin_karp(pattern, text):
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"""
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The Rabin-Karp Algorithm for finding a pattern within a piece of text
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with complexity O(nm), most efficient when it is used with multiple patterns
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as it is able to check if any of a set of patterns match a section of text in o(1) given the precomputed hashes.
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This will be the simple version which only assumes one pattern is being searched for but it's not hard to modify
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1) Calculate pattern hash
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2) Step through the text one character at a time passing a window with the same length as the pattern
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calculating the hash of the text within the window compare it with the hash of the pattern. Only testing
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equality if the hashes match
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"""
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p_len = len(pattern)
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p_hash = hash(pattern)
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for i in range(0, len(text) - (p_len - 1)):
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# written like this t
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text_hash = hash(text[i:i + p_len])
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if text_hash == p_hash and \
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text[i:i + p_len] == pattern:
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return True
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return False
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if __name__ == '__main__':
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# Test 1)
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pattern = "abc1abc12"
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text1 = "alskfjaldsabc1abc1abc12k23adsfabcabc"
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text2 = "alskfjaldsk23adsfabcabc"
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assert rabin_karp(pattern, text1) and not rabin_karp(pattern, text2)
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# Test 2)
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pattern = "ABABX"
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text = "ABABZABABYABABX"
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assert rabin_karp(pattern, text)
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# Test 3)
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pattern = "AAAB"
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text = "ABAAAAAB"
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assert rabin_karp(pattern, text)
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# Test 4)
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pattern = "abcdabcy"
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text = "abcxabcdabxabcdabcdabcy"
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assert rabin_karp(pattern, text)
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