added Rabin-Karp pattern mathinc algorithm

Details in the comments in the .py file.

rabin_karp.py

@@ -0,0 +1,73 @@
+# Following program is the python implementation of
+# Rabin Karp Algorithm given in CLRS book.
+# Time complexity in the average and best cases is
+# O(n + m) where n is the text and m is the pattern to
+# be searched. Worst case is O(n*m). 
+
+# d is the number of characters in the input alphabet
+d = 256
+
+# pat -> pattern
+# txt -> text
+# q -> A prime number
+
+
+def search(pat, txt, q):
+	M = len(pat)
+	N = len(txt)
+	i = 0
+	j = 0
+	p = 0 # hash value for pattern
+	t = 0 # hash value for txt
+	h = 1
+
+	# The value of h would be "pow(d, M-1)%q"
+	for i in range(M-1):
+		h = (h*d) % q
+
+	# Calculate the hash value of pattern and first window
+	# of text
+	for i in range(M):
+		p = (d*p + ord(pat[i])) % q
+		t = (d*t + ord(txt[i])) % q
+
+	# Slide the pattern over text one by one
+	for i in range(N-M+1):
+		# Check the hash values of current window of text and
+		# pattern if the hash values match then only check
+		# for characters one by one
+		if p == t:
+			# Check for characters one by one
+			for j in range(M):
+				if txt[i+j] != pat[j]:
+					break
+				else:
+					j += 1
+
+			# if p == t and pat[0...M-1] = txt[i, i+1, ...i+M-1]
+			if j == M:
+				print("Pattern found at index " + str(i))
+
+		# Calculate hash value for next window of text: Remove
+		# leading digit, add trailing digit
+		if i < N-M:
+			t = (d*(t-ord(txt[i])*h) + ord(txt[i+M])) % q
+
+			# We might get negative values of t, converting it to
+			# positive
+			if t < 0:
+				t = t+q
+
+
+# Driver Code
+if __name__ == '__main__':
+	txt = "My ocean lies over the ocean"
+	pat = "ocean"
+
+	# A prime number
+	q = 101
+
+	# Function Call
+	search(pat, txt, q)
+
+