2_PatternSearching.py

# Python program for Finite Automata
# 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 Automata 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			
def main():
	txt = "An apple a day keeps a doctor away. An apple is good for health and keeps you healthy"
	pat = "apple"
	search(pat, txt)

if __name__ == '__main__':
	main()