Merge pull request #731 from DDullahan/Development

Added FastPower

src/main/java/com/Others/FastPower.java

@@ -0,0 +1,37 @@
+package src.main.java.com.Others;
+
+import java.math.BigInteger;
+
+/**
+ * We may calculate power with loops, but what if the index is too large ?
+ * FastPower aims to calculate quickly in this circumstances with time complexity O(log k),
+ * where k is the index.
+ *
+ * @author DDullahan
+ */
+public class FastPower {
+    public static BigInteger calculate(BigInteger n, BigInteger k, BigInteger mod) {
+        BigInteger ans = BigInteger.ONE;
+        while (!k.equals(BigInteger.ZERO)) {
+            int odd = k.mod(BigInteger.valueOf(2)).compareTo(BigInteger.ZERO);
+            if(odd > 0){
+                ans = ans.multiply(n).mod(mod);
+            }
+            k = k.divide(BigInteger.valueOf(2));
+            n = n.multiply(n).mod(mod);
+        }
+        return ans.mod(mod);
+    }
+
+    public static long calculate(long n, long k, long mod) {
+        long ans = 1;
+        while (k != 0) {
+            if (k % 2 == 1) {
+                ans = (ans * n) % mod;
+            }
+            k >>= 1;
+            n = (n * n) % mod;
+        }
+        return ans % mod;
+    }
+}

src/main/java/com/Others/FastPowerTest.java

@@ -0,0 +1,32 @@
+package src.main.java.com.Others;
+
+import org.junit.*;
+
+import java.math.BigInteger;
+
+import static org.junit.Assert.*;
+
+public class FastPowerTest {
+
+    void testLong(long n, long k, long m){
+        long result = FastPower.calculate(n,k,m);
+        assertEquals(result, BigInteger.valueOf(n).modPow(BigInteger.valueOf(k), BigInteger.valueOf(m)).longValue());
+    }
+
+    void testBigInteger(BigInteger n, BigInteger k, BigInteger m){
+        BigInteger result = FastPower.calculate(n,k,m);
+        assertEquals(result, n.modPow(k,m));
+    }
+
+    @Test
+    public void test() {
+        testLong(2,2,10);
+        testLong(100,1000,20);
+        testLong(123456,123456789,234);
+
+        testBigInteger(BigInteger.TEN,BigInteger.TEN, BigInteger.valueOf(4));
+        testBigInteger(new BigInteger("123456"), new BigInteger("123456789"), new BigInteger("234"));
+        testBigInteger(new BigInteger("123456789101112"), new BigInteger("12345678910111213"), new BigInteger("567890"));
+
+    }
+}