Added C implementation of Rabin Karp Algorithm for Pattern Searching

Author: kaustumbh7

String/String Matching/rabin_karp.c

@@ -0,0 +1,76 @@
+
+#include<stdio.h> 
+#include<string.h> 
+  
+// 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 on 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) 
+                printf("Pattern found at index %d \n", i); 
+        } 
+  
+        // 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 program to test above function */
+int main() 
+{ 
+    char txt[] = "HELLO GITHUB HELLO"; 
+    char pat[] = "HELLO"; 
+    int q = 101; // A prime number 
+    search(pat, txt, q); 
+    return 0; 
+}