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* Add doctests for the boyer_moore_search algorithm. * Update boyer_moore_search.py * [pre-commit.ci] auto fixes from pre-commit.com hooks for more information, see https://pre-commit.ci * Update boyer_moore_search.py * Update boyer_moore_search.py --------- Co-authored-by: Maxim Smolskiy <mithridatus@mail.ru> Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
105 lines
3.1 KiB
Python
105 lines
3.1 KiB
Python
"""
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The algorithm finds the pattern in given text using following rule.
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The bad-character rule considers the mismatched character in Text.
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The next occurrence of that character to the left in Pattern is found,
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If the mismatched character occurs to the left in Pattern,
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a shift is proposed that aligns text block and pattern.
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If the mismatched character does not occur to the left in Pattern,
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a shift is proposed that moves the entirety of Pattern past
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the point of mismatch in the text.
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If there is no mismatch then the pattern matches with text block.
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Time Complexity : O(n/m)
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n=length of main string
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m=length of pattern string
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"""
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class BoyerMooreSearch:
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"""
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Example usage:
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bms = BoyerMooreSearch(text="ABAABA", pattern="AB")
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positions = bms.bad_character_heuristic()
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where 'positions' contain the locations where the pattern was matched.
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"""
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def __init__(self, text: str, pattern: str):
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self.text, self.pattern = text, pattern
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self.textLen, self.patLen = len(text), len(pattern)
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def match_in_pattern(self, char: str) -> int:
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"""
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Finds the index of char in pattern in reverse order.
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Parameters :
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char (chr): character to be searched
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Returns :
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i (int): index of char from last in pattern
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-1 (int): if char is not found in pattern
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>>> bms = BoyerMooreSearch(text="ABAABA", pattern="AB")
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>>> bms.match_in_pattern("B")
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1
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"""
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for i in range(self.patLen - 1, -1, -1):
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if char == self.pattern[i]:
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return i
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return -1
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def mismatch_in_text(self, current_pos: int) -> int:
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"""
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Find the index of mis-matched character in text when compared with pattern
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from last.
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Parameters :
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current_pos (int): current index position of text
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Returns :
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i (int): index of mismatched char from last in text
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-1 (int): if there is no mismatch between pattern and text block
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>>> bms = BoyerMooreSearch(text="ABAABA", pattern="AB")
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>>> bms.mismatch_in_text(2)
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3
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"""
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for i in range(self.patLen - 1, -1, -1):
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if self.pattern[i] != self.text[current_pos + i]:
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return current_pos + i
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return -1
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def bad_character_heuristic(self) -> list[int]:
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"""
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Finds the positions of the pattern location.
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>>> bms = BoyerMooreSearch(text="ABAABA", pattern="AB")
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>>> bms.bad_character_heuristic()
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[0, 3]
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"""
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positions = []
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for i in range(self.textLen - self.patLen + 1):
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mismatch_index = self.mismatch_in_text(i)
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if mismatch_index == -1:
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positions.append(i)
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else:
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match_index = self.match_in_pattern(self.text[mismatch_index])
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i = (
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mismatch_index - match_index
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) # shifting index lgtm [py/multiple-definition]
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return positions
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if __name__ == "__main__":
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import doctest
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doctest.testmod()
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