Merge pull request #1 from rajbodhak/rajbodhak-patch-1

Implement Robin-Karp String Matching Algorithm in C++

robinKarpStringMatching.cpp

@@ -0,0 +1,65 @@
+#include <iostream>
+#include <string>
+#include <vector>
+
+using namespace std;
+
+#define d 256 // Number of characters in the input alphabet
+
+// Function to implement Robin-Karp algorithm
+void RobinKarp(string pat, string txt, int q) {
+    int M = pat.size();
+    int N = txt.size();
+    int i, j;
+    int p = 0; // hash value for pattern
+    int t = 0; // hash value for text
+    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 the current window of text and pattern
+        // If the hash values match, then check characters one by one
+        if (p == t) {
+            bool flag = true;
+            for (j = 0; j < M; j++) {
+                if (txt[i + j] != pat[j]) {
+                    flag = false;
+                    break;
+                }
+            }
+
+            // If p == t and pat[0...M-1] == txt[i, i+1, ...i+M-1]
+            if (flag)
+                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 values of t, convert it to positive
+            if (t < 0)
+                t = (t + q);
+        }
+    }
+}
+
+int main() {
+    string txt = "GEEKS FOR GEEKS";
+    string pat = "GEEK";
+    int q = 101; // A prime number to mod hash values
+
+    RobinKarp(pat, txt, q);
+
+    return 0;
+}