add ons in string directory - Bayer_Moore_Search (#933)

strings/Boyer_Moore_Search.py

@@ -0,0 +1,88 @@
+"""
+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)
+    
+