Add RabinKarp implementation

Author: jcazevedo

src/main/java/com/packt/datastructuresandalg/lesson5/rabinkarp/RabinKarp.java

@@ -0,0 +1,57 @@
+package com.packt.datastructuresandalg.lesson5.rabinkarp;
+
+import java.math.BigInteger;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Random;
+
+public class RabinKarp {
+    public List<Integer> match(String P, String T) {
+        // Good enough for ascii characters.
+        int d = 256;
+        int m = P.length();
+        int n = T.length();
+        long q = BigInteger.probablePrime(31, new Random()).longValue();
+
+        // Precompute d^(m-1) % q for use when removing leading digit
+        long dm = 1;
+        for (int i = 1; i <= m - 1; i++)
+            dm = (d * dm) % q;
+
+        // Precompute p and t0
+        long ph = 0;
+        long th = 0;
+        for (int i = 0; i < m; i++) {
+            ph = (d * ph + P.charAt(i)) % q;
+            th = (d * th + T.charAt(i)) % q;
+        }
+
+        List<Integer> shifts = new ArrayList<>();
+        for (int i = 0; i < n - m + 1; i++) {
+            if (ph == th) {
+                boolean hasMatch = true;
+                for (int j = 0; j < m; j++) {
+                    if (P.charAt(j) != T.charAt(i + j)) {
+                        hasMatch = false;
+                        break;
+                    }
+                }
+                if (hasMatch)
+                    shifts.add(i);
+            }
+
+            if (i + m < n) {
+                th = (th + q - dm * T.charAt(i) % q) % q;
+                th = (th * d + T.charAt(i + m)) % q;
+            }
+        }
+        return shifts;
+    }
+
+    public static void main(String[] args) {
+        RabinKarp rk = new RabinKarp();
+        List<Integer> matches = rk.match("rabrabracad", "abacadabrabracabracadabrabrabracad");
+        for (Integer i : matches)
+            System.out.println(i);
+    }
+}