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* flake8 --max-line-length=88
* fixup! Format Python code with psf/black push
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91 lines
2.5 KiB
Python
91 lines
2.5 KiB
Python
# Numbers of alphabet which we call base
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alphabet_size = 256
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# Modulus to hash a string
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modulus = 1000003
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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)
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given the precomputed hashes.
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This will be the simple version which only assumes one pattern is being searched
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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
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length as the pattern
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calculating the hash of the text within the window compare it with the hash
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of the pattern. Only testing equality if the hashes match
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"""
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p_len = len(pattern)
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t_len = len(text)
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if p_len > t_len:
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return False
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p_hash = 0
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text_hash = 0
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modulus_power = 1
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# Calculating the hash of pattern and substring of text
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for i in range(p_len):
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p_hash = (ord(pattern[i]) + p_hash * alphabet_size) % modulus
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text_hash = (ord(text[i]) + text_hash * alphabet_size) % modulus
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if i == p_len - 1:
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continue
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modulus_power = (modulus_power * alphabet_size) % modulus
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for i in range(0, t_len - p_len + 1):
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if text_hash == p_hash and text[i : i + p_len] == pattern:
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return True
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if i == t_len - p_len:
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continue
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# Calculate the https://en.wikipedia.org/wiki/Rolling_hash
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text_hash = (
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(text_hash - ord(text[i]) * modulus_power) * alphabet_size
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+ ord(text[i + p_len])
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) % modulus
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return False
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def test_rabin_karp():
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"""
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>>> test_rabin_karp()
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Success.
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"""
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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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# Test 5)
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pattern = "Lü"
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text = "Lüsai"
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assert rabin_karp(pattern, text)
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pattern = "Lue"
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assert not rabin_karp(pattern, text)
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print("Success.")
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if __name__ == "__main__":
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test_rabin_karp()
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