Rabin-Karp Pattern Matching

2017/hashing.cpp

+#include <cstdio>
+#include <algorithm>
+#include <cstring>
+
+#define MAXN 100100
+#define BASE 256
+#define MOD 1000000007LL
+
+using namespace std;
+
+typedef long long ll;
+
+char T[ MAXN + 1 ], P[ MAXN + 1 ];
+
+int hashing() {
+    int n = strlen( T ), k = strlen( P );
+    ll hash = 0LL, rhash = 0LL;
+    for( int i = 0; i < k; i++ ) {
+        hash = hash * BASE % MOD;
+        hash = ( hash + P[ i ] ) % MOD;
+        rhash = ( rhash * BASE + T[ i ] ) % MOD;
+    }
+    if( rhash == hash ) return 0;
+    ll po = 1LL;
+    for( int i = 0; i < k - 1; i++ ) {
+        po = po * BASE % MOD;
+    }
+    for( int i = 1; i < n - k + 1; i++ ) {
+        rhash = ( rhash - po * T[ i - 1 ] );
+        if( rhash < 0 ) rhash += MOD;
+        rhash = rhash * BASE % MOD;
+        rhash = ( rhash + T[ i + k - 1 ] ) % MOD;
+        if( rhash == hash ) return i;
+    }
+    return -1;
+}
+
+int main( void ) {
+    scanf("%s%s", T, P );
+    int val = hashing();
+    if( val == -1 ) {
+        printf("Not found!\n");
+    } else {
+        printf("First occurence found in pos: %d\n", val );
+    }
+    return 0;
+}