Divide and conquer Algorithms Issue#817

strings/Boyer_Moore_Search.py

@@ -0,0 +1,74 @@
+"""
+This algorithm tries to find the pattern in given text using Bad Character Heuristic method. 
+The bad-character rule considers the character in Text at which there is a mis-match. The next occurrence of that character to the left in Pattern is found, and a shift which brings that occurrence in line with the mismatched occurrence in Text is proposed. 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
+
+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)
+
+