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89 lines
2.6 KiB
Python
89 lines
2.6 KiB
Python
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"""
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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 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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def __init__(self, text, pattern):
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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):
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""" finds the index of char in pattern in reverse order
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Paremeters :
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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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"""
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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, currentPos):
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""" finds the index of mis-matched character in text when compared with pattern from last
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Paremeters :
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currentPos (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 mis-match between pattern and text block
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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[currentPos + i]:
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return currentPos + i
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return -1
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def bad_character_heuristic(self):
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# searches pattern in text and returns index positions
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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 = mismatch_index - match_index #shifting index
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return positions
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text = "ABAABA"
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pattern = "AB"
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bms = BoyerMooreSearch(text, pattern)
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positions = bms.bad_character_heuristic()
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if len(positions) == 0:
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print("No match found")
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else:
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print("Pattern found in following positions: ")
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print(positions)
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