Create Boyer_Moore.py

chapter12/Boyer_Moore.py

@@ -0,0 +1,46 @@
+text = "acbaacacababacacac"
+pattern = "acacac"
+
+
+matched_indexes = []
+
+i=0
+flag = True
+while i<=len(text)-len(pattern):
+    for j in range(len(pattern)-1, -1, -1):     #reverse searching
+        if pattern[j] != text[i+j]:
+            flag = False    #indicates there is a mismatch
+            if j == len(pattern)-1:     #if good-suffix is not present, we test bad character 
+                if text[i+j] in pattern[0:j]:
+                    i=i+j-pattern[0:j].rfind(text[i+j])   
+                    #i+j is index of bad character, this line is used for jumping pattern to match 
+                    #bad character of text with same character in pattern
+                else:
+                    i=i+j+1     #if bad character is not present, jump pattern next to it
+            else:
+                k=1
+                while text[i+j+k:i+len(pattern)] not in pattern[0:len(pattern)-1]:     
+                    #used for finding sub part of a good-suffix
+                    k=k+1
+                if len(text[i+j+k:i+len(pattern)]) != 1:    #good-suffix should not be of one character
+                    gsshift=i+j+k-pattern[0:len(pattern)-1].rfind(text[i+j+k:i+len(pattern)])    
+                    #jumps pattern to a position where good-suffix of pattern matches with good-suffix of text
+                else:
+                    #gsshift=i+len(pattern)
+                    gsshift=0   #when good-suffix heuristic is not applicable, 
+                                #we prefer bad character heuristic
+                if text[i+j] in pattern[0:j]:
+                    bcshift=i+j-pattern[0:j].rfind(text[i+j])  
+                    #i+j is index of bad character, this line is used for jumping pattern to match bad character 
+                    #of text with same character in pattern
+                else:
+                    bcshift=i+j+1
+                i=max((bcshift, gsshift))
+            break
+    if flag:    #if pattern is found then normal iteration
+        matched_indexes.append(i)
+        i = i+1
+    else:   #again set flag to True so new string in text can be examined
+        flag = True
+
+print ("Pattern found at", matched_indexes)