Boyer Moore algorithm added to PR

Python implementation of Boyer Moore algorithm using Bad Character Heuriristic, Details in comment..

boyer_moore.py

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+# Python  implementations of Bad Character Heuristic
+# of Boyer Moore String Matching Algorithm.
+# Worst case time complexity= O(n*m).
+# Space complexity: O(1)
+
+NO_OF_CHARS = 256
+
+def badCharHeuristic(string, size):
+	'''
+	The preprocessing function for
+	Boyer Moore's bad character heuristic
+	'''
+
+	# Initialize all occurrence as -1
+	badChar = [-1]*NO_OF_CHARS
+
+	# Fill the actual value of last occurrence
+	for i in range(size):
+		badChar[ord(string[i])] = i;
+
+	# return initialized list
+	return badChar
+
+def search(txt, pat):
+	'''
+	A pattern searching function that uses Bad Character
+	Heuristic of Boyer Moore Algorithm
+	'''
+	m = len(pat)
+	n = len(txt)
+
+	# create the bad character list by calling
+	# the preprocessing function badCharHeuristic()
+	# for given pattern
+	badChar = badCharHeuristic(pat, m)
+
+	# s is shift of the pattern with respect to text
+	s = 0
+	while(s <= n-m):
+		j = m-1
+
+		# Keep reducing index j of pattern while
+		# characters of 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 current shift,
+		# then index j will become -1 after the above loop
+		if j<0:
+			print("Pattern occur at shift = {}".format(s))
+
+			'''
+				Shift the pattern so that the next character in text
+					aligns with the last occurrence of it in pattern.
+				The condition s+m < n is necessary for the case when
+				pattern occurs at the end of text
+			'''
+			s += (m-badChar[ord(txt[s+m])] if s+m<n else 1)
+		else:
+			'''
+			Shift the pattern so that the bad character in text
+			aligns with the last occurrence of it in 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 bad character in pattern is on the right side of the
+			current character.
+			'''
+			s += max(1, j-badChar[ord(txt[s+j])])
+
+
+# Driver program to test above function
+
+if __name__ == '__main__':
+    txt = "ABAAABCD"
+    pat= "ABC"
+    search(txt,pat)
+