Added Rabin_karp Algo

Algorithms/String Matching/Rabin_karp.cpp

@@ -0,0 +1,75 @@
+/*Rolling Hash
+1) Calculate Hash for pattern
+2) Calculate Hash for 1st window in text
+3) Repeat the following : (until text ends)
+	a) If Hash(pat)==Hash(Text_Window)
+	then match characters one by one
+	b) Subtract leftmost (msb) from HASH(text_window)
+ 	c) Shift entire hash(text_window) by 1 unit to left
+	d) Add new character to window
+*/
+
+#include<bits/stdc++.h>
+#define ll long long int
+using namespace std;
+
+ll createHashValue (string text, int len, ll prime) {
+
+    ll result = 0;
+    for (int i=0; i<len; i++) {
+      result += (ll)text[i]*(ll)pow(prime,i);
+    }
+
+    return result;
+}
+
+ll reCalculateHash (string str, int oldIndex, int newIndex, ll oldHash, int patLen, ll prime) {
+  ll newhash = oldHash - str[oldIndex];
+  newhash=newhash/prime;
+  newhash+= (ll)str[newIndex]*(ll)pow(prime, patLen-1);
+  return newhash;
+}
+
+bool checkEqual (string str1, string str2, int start1, int end1, int start2, int end2) {
+
+    if(end1-start1!=end2-start2)
+      return false;
+
+    while (start1<=end1 && start2<=end2) {
+      if(str1[start1]!=str2[start2])
+        return false;
+      start1++;
+      start2++;
+    }
+    return true;
+}
+
+
+void solve(){
+
+  string str = "ababcabdabd";
+  string pat  = "abd";
+  ll prime = 119;
+
+  ll patHash = createHashValue(pat,pat.length(),prime);
+  ll strHash = createHashValue(str,pat.length(),prime);
+
+  for (int i=0; i<=str.length()-pat.length(); i++) {
+    if (patHash == strHash && checkEqual(str,pat,i,i+pat.length()-1,0,pat.length()-1)){
+      // found pattern at index i
+      cout<<i<<endl;
+      return ;
+    }
+
+    if (i<str.length()-pat.length()) {
+      strHash = reCalculateHash(str, i, i+pat.length(), strHash, pat.length(), prime);
+    }
+
+  }
+
+}
+
+int main() {
+   solve();
+   return 0;
+}