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Fix Bug [#14844] : Boyer-Moore bad character shift has no effect (for-loop variable reassignment) #14848

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Fix Bug [#14844] : Boyer-Moore bad character shift has no effect (for-loop variable reassignment) #14848

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25 changes: 19 additions & 6 deletions strings/boyer_moore_search.py
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Original file line number Diff line number Diff line change
Expand Up @@ -13,9 +13,12 @@

If there is no mismatch then the pattern matches with text block.

Time Complexity : O(n/m)
Time Complexity : O(n/m) average case with bad character heuristic
n=length of main string
m=length of pattern string

Note: The bad character shift requires a while loop so positions are
actually skipped. A for loop ignores loop-variable reassignment.
"""


Expand Down Expand Up @@ -78,23 +81,33 @@ def mismatch_in_text(self, current_pos: int) -> int:

def bad_character_heuristic(self) -> list[int]:
"""
Finds the positions of the pattern location.
Finds the positions of the pattern in text using the bad character
heuristic. A while loop is used so the shift actually skips
positions, achieving O(n/m) average performance instead of the
O(nm) brute-force that a for loop would produce.

>>> bms = BoyerMooreSearch(text="ABAABA", pattern="AB")
>>> bms.bad_character_heuristic()
[0, 3]
>>> bms2 = BoyerMooreSearch(text="AAAAAA", pattern="AA")
>>> bms2.bad_character_heuristic()
[0, 1, 2, 3, 4]
>>> bms3 = BoyerMooreSearch(text="ABCDEF", pattern="XY")
>>> bms3.bad_character_heuristic()
[]
"""

positions = []
for i in range(self.textLen - self.patLen + 1):
i = 0
while i <= self.textLen - self.patLen:
mismatch_index = self.mismatch_in_text(i)
if mismatch_index == -1:
positions.append(i)
i += 1
else:
match_index = self.match_in_pattern(self.text[mismatch_index])
i = (
mismatch_index - match_index
) # shifting index lgtm [py/multiple-definition]
# Use max to prevent shifting backwards
i = max(i + 1, mismatch_index - match_index)
return positions


Expand Down