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671e570c35
* Implement prefix function, knuth-morris-pratt another usage
* fixup! Format Python code with psf/black push
* Fix style
* updating DIRECTORY.md
* Update prefix_function.py
Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: Christian Clauss <cclauss@me.com>
66 lines
1.6 KiB
Python
66 lines
1.6 KiB
Python
"""
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https://cp-algorithms.com/string/prefix-function.html
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Prefix function Knuth–Morris–Pratt algorithm
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Different algorithm than Knuth-Morris-Pratt pattern finding
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E.x. Finding longest prefix which is also suffix
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Time Complexity: O(n) - where n is the length of the string
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"""
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def prefix_function(input_string: str) -> list:
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"""
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For the given string this function computes value for each index(i),
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which represents the longest coincidence of prefix and sufix
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for given substring (input_str[0...i])
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For the value of the first element the algorithm always returns 0
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>>> prefix_function("aabcdaabc")
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[0, 1, 0, 0, 0, 1, 2, 3, 4]
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>>> prefix_function("asdasdad")
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[0, 0, 0, 1, 2, 3, 4, 0]
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"""
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# list for the result values
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prefix_result = [0] * len(input_string)
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for i in range(1, len(input_string)):
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# use last results for better performance - dynamic programming
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j = prefix_result[i - 1]
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while j > 0 and input_string[i] != input_string[j]:
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j = prefix_result[j - 1]
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if input_string[i] == input_string[j]:
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j += 1
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prefix_result[i] = j
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return prefix_result
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def longest_prefix(input_str: str) -> int:
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"""
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Prefix-function use case
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Finding longest prefix which is sufix as well
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>>> longest_prefix("aabcdaabc")
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4
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>>> longest_prefix("asdasdad")
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4
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>>> longest_prefix("abcab")
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2
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"""
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# just returning maximum value of the array gives us answer
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return max(prefix_function(input_str))
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if __name__ == "__main__":
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import doctest
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doctest.testmod()
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