Added Rabin_karp Algorithm

data_structures/strings/Rabin_karp.py

@@ -0,0 +1,54 @@
+# Following program is the python implementation of Rabin Karp Algo.
+# Time Complexity: O(nm), where m is the length of the pattern and n is the length 
+
+# 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 on by one
+		if p == t:
+			# Check for characters one by one
+			for j in range(M):
+				if txt[i + j] != pat[j]:
+					break
+
+			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
+
+txt = "Abcd bc Abcd"
+pat = "Abcd"
+q = 101             # A prime number
+search(pat, txt, q)