Create rabin_karp.cpp

rabin_karp.cpp

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+/* Following program is a C++ implementation of Rabin Karp
+Algorithm given in the CLRS book */
+#include <bits/stdc++.h>
+using namespace std;
+
+// d is the number of characters in the input alphabet
+#define d 256
+
+/* pat -> pattern
+	txt -> text
+	q -> A prime number
+*/
+void search(char pat[], char txt[], int q)
+{
+	int M = strlen(pat);
+	int N = strlen(txt);
+	int i, j;
+	int p = 0; // hash value for pattern
+	int t = 0; // hash value for txt
+	int h = 1;
+
+	// The value of h would be "pow(d, M-1)%q"
+	for (i = 0; i < M - 1; i++)
+		h = (h * d) % q;
+
+	// Calculate the hash value of pattern and first
+	// window of text
+	for (i = 0; i < M; i++) {
+		p = (d * p + pat[i]) % q;
+		t = (d * t + txt[i]) % q;
+	}
+
+	// Slide the pattern over text one by one
+	for (i = 0; i <= N - M; i++) {
+
+		// 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 = 0; j < M; j++) {
+				if (txt[i + j] != pat[j]) {
+					break;
+				}
+			}
+
+			// if p == t and pat[0...M-1] = txt[i, i+1,
+			// ...i+M-1]
+
+			if (j == M)
+				cout << "Pattern found at index " << i
+					<< endl;
+		}
+
+		// Calculate hash value for next window of text:
+		// Remove leading digit, add trailing digit
+		if (i < N - M) {
+			t = (d * (t - txt[i] * h) + txt[i + M]) % q;
+
+			// We might get negative value of t, converting
+			// it to positive
+			if (t < 0)
+				t = (t + q);
+		}
+	}
+}
+
+/* Driver code */
+int main()
+{
+	char txt[] = "GEEKS FOR GEEKS";
+	char pat[] = "GEEK";
+
+	// we mod to avoid overflowing of value but we should
+	// take as big q as possible to avoid the collison
+	int q = INT_MAX;
+
+	// Function Call
+	search(pat, txt, q);
+	return 0;
+}
+
+// This is code is contributed by rathbhupendra