feat: added rabin_karp algorithm for string match (#779)

* feat: added rabin_karp algorithm for string match

* feat: added rabin_karp algorithm for string match

* feat: added rabin_karp algorithm for string match

* Update rabin_karp.cpp
Co-authored-by: NChristian Clauss <cclauss@me.com>

strings/rabin_karp.cpp

+/*
+ * file name : rabin_karp.cpp
+ * author    : Amit Kumar 
+ * Copyright : 2020 , Amit Kumar 
+ * version   : 1.0
+ */
+#include<cassert>
+#include<cmath>
+#include<iostream>
+#include<string>
+
+using std::string;
+using std::pow;
+
+#define PRIME 5
+
+int64_t create_hash(string s , int n) {
+    int64_t result = 0;
+    for ( int i = 0; i < n; ++i ) {
+        result += (int64_t)(s[i] * (int64_t)pow(PRIME , i));
+    }
+    return result;
+}
+
+int64_t recalculate_hash(string s , int old_index ,
+        int new_index , int64_t old_hash , int patLength) {
+    int64_t new_hash = old_hash - s[old_index];
+    new_hash /= PRIME;
+    new_hash += (int64_t)(s[new_index]*(int64_t)pow(PRIME, patLength-1));
+    return new_hash;
+}
+
+bool check_if_equal(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;
+}
+
+/*
+ *  @description : search pattern in the given text 
+ *  @param : string str
+ *  @param : string pat
+ *  @return index of first occurrence of pattern or -1 if pattern not found
+ */
+
+int rabin_karp(const string &str , const string& pat) {
+    int64_t pat_hash = create_hash(pat , pat.size());
+    int64_t str_hash = create_hash(str , pat.size());
+    for (int i=0;  i <= str.size()-pat.size(); ++i) {
+        if (pat_hash == str_hash &&
+            check_if_equal(str , pat , i , i+pat.size()-1 , 0 , pat.size()-1)) {
+                return i;
+        }
+        if (i < str.size()-pat.size()) {
+            str_hash =
+                recalculate_hash(str, i, i+pat.size(), str_hash, pat.size());
+        }
+    }
+    return -1;  // return -1 if given pattern not found
+}
+
+int main(void) {
+    assert(rabin_karp("helloWorld", "world")    == -1);
+    assert(rabin_karp("helloWorld", "World")    ==  5);
+    assert(rabin_karp("this_is_c++" , "c++")   ==  8);
+    assert(rabin_karp("happy_coding", "happy") ==  0);
+    return 0;
+}