Merge pull request #138 from pankajsingh92533/main

I have implemented the Boyer moore algo in python .

Author: himanshu-03

Searching Techniques/Boyer_Moore_Algorithm.py

@@ -0,0 +1,74 @@
+"""Program for Bad Character Heuristic
+of Boyer Moore String Matching Algorithm""" 
+
+
+NO_OF_CHARS = 256
+
+def badCharHeuristic(string, size):
+    '''
+    The preprocessing function for
+    Boyer Moore's bad character heuristic
+    '''
+    # Initialize all occurrences as -1
+    badChar = [-1] * NO_OF_CHARS
+
+    # Fill the actual value of the last occurrence
+    for i in range(size):
+        badChar[ord(string[i])] = i
+
+    # Return the initialized list
+    return badChar
+
+def search(txt, pat):
+    '''
+    A pattern searching function that uses the Bad Character
+    Heuristic of the Boyer Moore Algorithm
+    '''
+    m = len(pat)
+    n = len(txt)
+
+    # Create the bad character list by calling
+    # the preprocessing function badCharHeuristic()
+    # for the given pattern
+    badChar = badCharHeuristic(pat, m)
+
+    # s is the shift of the pattern with respect to the text
+    s = 0
+    while s <= n - m:
+        j = m - 1
+
+        # Keep reducing index j of the pattern while
+        # characters of the pattern and text are matching
+        # at this shift s
+        while j >= 0 and pat[j] == txt[s + j]:
+            j -= 1
+
+        # If the pattern is present at the current shift,
+        # then index j will become -1 after the above loop
+        if j < 0:
+            print("Pattern occurs at shift =", s)
+
+            ''' 
+                Shift the pattern so that the next character in the text
+                aligns with the last occurrence of it in the pattern.
+                The condition s+m < n is necessary for the case when
+                the pattern occurs at the end of the text
+            '''
+            s += (m - badChar[ord(txt[s + m])] if s + m < n else 1)
+        else:
+            '''
+            Shift the pattern so that the bad character in the text
+            aligns with the last occurrence of it in the pattern. The
+            max function is used to make sure that we get a positive
+            shift. We may get a negative shift if the last occurrence
+            of the bad character in the pattern is on the right side of the
+            current character.
+            '''
+            s += max(1, j - badChar[ord(txt[s + j])])
+
+while True:
+    txt = input('Enter the text (or press Enter to exit): ')
+    if not txt:
+        break
+    pat = input('Enter the pattern to search for: ')
+    search(txt, pat)