Merge pull request #352 from hmahajan99/rabinKarp

Added Rabin Karp algorithm for Pattern Matching

Author: amritansh22

Rabin_Karp_Algorithm.cpp

@@ -0,0 +1,66 @@
+#include <iostream>
+using namespace std;
+
+
+// Using Rabin Karp Algorithm for Pattern Searching
+// Rabin-Karp algorithm slides the pattern one by one. 
+// But unlike the Naive algorithm, Rabin Karp algorithm matches the hash value of the pattern with the hash value of current substring of text, 
+// and if the hash values match then only it starts matching individual characters. 
+
+// The average and best case running time of the Rabin-Karp algorithm is O(n+m), but its worst-case time is O(nm). 
+// Worst case of Rabin-Karp algorithm occurs when all characters of pattern and text are same 
+// as the hash values of all the substrings of txt[] match with hash value of pat[]. For example pat[] = “AAA” and txt[] = “AAAAAAA”.
+
+#define lli long long int
+#define prime 101
+#define M (lli)(1009)
+
+// To compute x^y under modulo m (y,m>0)
+lli fastPower(int x,int y,int m){ 
+    if(y==0) return 1; 
+    lli smallAns = fastPower(x,y/2,m); 
+    smallAns*=smallAns;
+    smallAns%=m;
+    return (y%2==0)?(smallAns):(x*smallAns)%m; 
+} 
+
+lli createHash(string str, int len){
+    // p is base used for conversion, it is a prime (101)
+    // hash = ( str[0]*(p^(len-1)) + str[1]*(p^(len-2)) + .... ) % M
+    lli hash = 0;
+    for(int i=0;i<len;i++) {
+        hash = (prime*hash + str[i]) % M; 
+    }
+    return hash;
+}
+
+int rabinKarp(const string text, const string pattern) {
+    int m = pattern.size();
+    int n = text.size();
+    lli patternHash = createHash(pattern,m);
+    lli textHash = createHash(text,m);
+    lli h = fastPower(prime,m-1,M);
+    for(int i=0;i<=n-m;i++) {
+        if(patternHash==textHash) {
+            bool isSubstring=true;
+            for(int j=i,k=0;k<m;j++,k++){
+                if(pattern[k]!=text[j]){
+                    isSubstring=false;
+                    break;
+                }
+            }
+            if(isSubstring) return i;
+        }
+        if(i<n-m) {
+            // recalculate hash
+            textHash = (prime*(textHash-text[i]*h) + text[i+m])%M;
+            if(textHash<0) textHash = textHash + M; 
+        }
+    }
+    return -1;
+}
+
+int main() {
+    cout<<rabinKarp("RabinKarp","Karp")<<endl; // Output:5
+    cout<<rabinKarp("RabinKarp","karp")<<endl; // Output:-1
+}