Added NumberUtils problem

src/test/java/io/github/sandeeplakka/codewars/kyu6/NumberUtils.java

@@ -0,0 +1,79 @@
+package io.github.sandeeplakka.codewars.kyu6;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.Arrays;
+import java.util.concurrent.ThreadLocalRandom;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+
+/*
+    A Narcissistic Number (or Armstrong Number) is a positive number which is the sum of its own digits, each raised to the power of the number of digits in a given base. In this Kata, we will restrict ourselves to decimal (base 10).
+    For example, take 153 (3 digits), which is narcissistic:
+        1^3 + 5^3 + 3^3 = 1 + 125 + 27 = 153
+    and 1652 (4 digits), which isn't:
+        1^4 + 6^4 + 5^4 + 2^4 = 1 + 1296 + 625 + 16 = 1938
+
+    The Challenge:
+    Your code must return true or false (not 'true' and 'false') depending upon whether the given number is a Narcissistic number in base 10.
+
+    Error checking for text strings or other invalid inputs is not required, only valid positive non-zero integers will be passed into the function.
+
+    Categories: Algorithms
+ */
+
+public class NumberUtils {
+
+    private static final int[] NARCISSISTIC_INTS = {
+            1, 2, 3, 4, 5, 6, 7, 8, 9, 153, 370, 371, 407, 1634,
+            8208, 9474, 54748, 92727, 93084, 548834, 1741725, 4210818,
+            9800817, 9926315, 24678050, 24678051, 88593477, 146511208,
+            472335975, 534494836, 912985153
+    };
+
+    @Test
+    void fixedTests() {
+        assertTrue(NumberUtils.isNarcissistic(153), "153 is narcissistic");
+        assertTrue(NumberUtils.isNarcissistic(1634), "1634 is narcissistic");
+        assertFalse(NumberUtils.isNarcissistic(112), "112 is not narcissistic");
+        assertFalse(NumberUtils.isNarcissistic(10), "10 is not narcissistic");
+        assertFalse(NumberUtils.isNarcissistic(Integer.MAX_VALUE), Integer.MAX_VALUE + " is not narcissistic");
+    }
+
+    @Test
+    void randomTests() {
+        ThreadLocalRandom rnd = ThreadLocalRandom.current();
+        int nrTests = 100_000;
+        int[] testCases = Arrays.copyOf(NARCISSISTIC_INTS, nrTests);
+        for (int i = NARCISSISTIC_INTS.length; i < nrTests; i++)
+            testCases[i] = rnd.nextInt(Integer.MAX_VALUE);
+        for (int i = 0; i < nrTests; i++) {
+            int idx = rnd.nextInt(nrTests);
+            int tmp = testCases[i];
+            testCases[i] = testCases[idx];
+            testCases[idx] = tmp;
+        }
+
+        for (int n : testCases) {
+            boolean expected = Arrays.binarySearch(NARCISSISTIC_INTS, n) >= 0;
+            assertEquals(expected, isNarcissistic(n), String.format("%d is %snarcissistic", n, expected ? "" : "not "));
+        }
+    }
+
+    //-----------------------------------------------------
+
+    public static boolean isNarcissistic(int number) {
+        return Arrays.stream(("" + number).split("")).mapToInt(Integer::parseInt).map(v -> pow(v, number)).sum() == number;
+    }
+
+    private static int pow(int value, int number) {
+        int len = ("" + number).length();
+        int result = 1;
+        for (int i = 0; i < len; i++) {
+            result *= value;
+        }
+        return result;
+    }
+
+}