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fsa_pattern.py

@@ -0,0 +1,77 @@
+# Python program for Finite State Automaton
+# Pattern searching Algorithm
+
+NO_OF_CHARS = 256
+
+def getNextState(pat, M, state, x):
+	'''
+	calculate the next state
+	'''
+
+	# If the character c is same as next character
+	# in pattern, then simply increment state
+
+	if state < M and x == ord(pat[state]):
+		return state+1
+
+	i=0
+	# ns stores the result which is next state
+
+	# ns finally contains the longest prefix
+	# which is also suffix in "pat[0..state-1]c"
+
+	# Start from the largest possible value and
+	# stop when you find a prefix which is also suffix
+	for ns in range(state,0,-1):
+		if ord(pat[ns-1]) == x:
+			while(i<ns-1):
+				if pat[i] != pat[state-ns+1+i]:
+					break
+				i+=1
+			if i == ns-1:
+				return ns
+	return 0
+
+def computeTF(pat, M):
+	'''
+	This function builds the TF table which
+	represents Finite Automaton for a given pattern
+	'''
+	global NO_OF_CHARS
+
+	TF = [[0 for i in range(NO_OF_CHARS)]\
+		for _ in range(M+1)]
+
+	for state in range(M+1):
+		for x in range(NO_OF_CHARS):
+			z = getNextState(pat, M, state, x)
+			TF[state][x] = z
+
+	return TF
+
+def search(pat, txt):
+	'''
+	Prints all occurrences of pat in txt
+	'''
+	global NO_OF_CHARS
+	M = len(pat)
+	N = len(txt)
+	TF = computeTF(pat, M)
+
+	# Process txt over FA.
+	state=0
+	for i in range(N):
+		state = TF[state][ord(txt[i])]
+		if state == M:
+			print("Pattern found at index: {}".\
+				format(i-M+1))
+
+# Driver program to test above function		
+
+
+if __name__ == '__main__':
+    txt = "AABAACAADAABAAABAA"
+    pat = "AABA"
+    search(pat,txt)
+
+