mirror of
https://github.com/TheAlgorithms/Python.git
synced 2024-11-30 16:31:08 +00:00
89 lines
2.6 KiB
Python
89 lines
2.6 KiB
Python
"""
|
|
The algorithm finds the pattern in given text using following rule.
|
|
|
|
The bad-character rule considers the mismatched character in Text.
|
|
The next occurrence of that character to the left in Pattern is found,
|
|
|
|
If the mismatched character occurs to the left in Pattern,
|
|
a shift is proposed that aligns text block and pattern.
|
|
|
|
If the mismatched character does not occur to the left in Pattern,
|
|
a shift is proposed that moves the entirety of Pattern past
|
|
the point of mismatch in the text.
|
|
|
|
If there no mismatch then the pattern matches with text block.
|
|
|
|
Time Complexity : O(n/m)
|
|
n=length of main string
|
|
m=length of pattern string
|
|
"""
|
|
|
|
|
|
class BoyerMooreSearch:
|
|
|
|
|
|
def __init__(self, text, pattern):
|
|
self.text, self.pattern = text, pattern
|
|
self.textLen, self.patLen = len(text), len(pattern)
|
|
|
|
|
|
def match_in_pattern(self, char):
|
|
""" finds the index of char in pattern in reverse order
|
|
|
|
Paremeters :
|
|
char (chr): character to be searched
|
|
|
|
Returns :
|
|
i (int): index of char from last in pattern
|
|
-1 (int): if char is not found in pattern
|
|
"""
|
|
|
|
for i in range(self.patLen-1, -1, -1):
|
|
if char == self.pattern[i]:
|
|
return i
|
|
return -1
|
|
|
|
|
|
def mismatch_in_text(self, currentPos):
|
|
""" finds the index of mis-matched character in text when compared with pattern from last
|
|
|
|
Paremeters :
|
|
currentPos (int): current index position of text
|
|
|
|
Returns :
|
|
i (int): index of mismatched char from last in text
|
|
-1 (int): if there is no mis-match between pattern and text block
|
|
"""
|
|
|
|
for i in range(self.patLen-1, -1, -1):
|
|
if self.pattern[i] != self.text[currentPos + i]:
|
|
return currentPos + i
|
|
return -1
|
|
|
|
|
|
def bad_character_heuristic(self):
|
|
# searches pattern in text and returns index positions
|
|
positions = []
|
|
for i in range(self.textLen - self.patLen + 1):
|
|
mismatch_index = self.mismatch_in_text(i)
|
|
if mismatch_index == -1:
|
|
positions.append(i)
|
|
else:
|
|
match_index = self.match_in_pattern(self.text[mismatch_index])
|
|
i = mismatch_index - match_index #shifting index
|
|
return positions
|
|
|
|
|
|
text = "ABAABA"
|
|
pattern = "AB"
|
|
bms = BoyerMooreSearch(text, pattern)
|
|
positions = bms.bad_character_heuristic()
|
|
|
|
if len(positions) == 0:
|
|
print("No match found")
|
|
else:
|
|
print("Pattern found in following positions: ")
|
|
print(positions)
|
|
|
|
|