refactor: Enhance docs, code, add tests in KrishnaMurthyNumber

src/main/java/com/thealgorithms/maths/KrishnamurthyNumber.java

@@ -3,51 +3,69 @@
 /**
  * Utility class for checking if a number is a Krishnamurthy number.
  *
- * A Krishnamurthy number (also known as a Strong number) is a number whose sum of the factorials of its digits is equal to the number itself.
+ * <p>
+ * A Krishnamurthy number (also known as a Strong number or Factorion) is a
+ * number
+ * whose sum of the factorials of its digits is equal to the number itself.
+ * </p>
  *
- * For example, 145 is a Krishnamurthy number because 1! + 4! + 5! = 1 + 24 + 120 = 145.
+ * <p>
+ * For example, 145 is a Krishnamurthy number because 1! + 4! + 5! = 1 + 24 +
+ * 120 = 145.
+ * </p>
+ *
+ * <p>
+ * The only Krishnamurthy numbers in base 10 are: 1, 2, 145, and 40585.
+ * </p>
+ *
+ * <p>
  * <b>Example usage:</b>
+ * </p>
+ *
  * <pre>
  * boolean isKrishnamurthy = KrishnamurthyNumber.isKrishnamurthy(145);
  * System.out.println(isKrishnamurthy); // Output: true
  *
  * isKrishnamurthy = KrishnamurthyNumber.isKrishnamurthy(123);
  * System.out.println(isKrishnamurthy); // Output: false
  * </pre>
+ *
+ * @see <a href="https://en.wikipedia.org/wiki/Factorion">Factorion
+ *      (Wikipedia)</a>
  */
 public final class KrishnamurthyNumber {
 
+    // Pre-computed factorials for digits 0-9 to improve performance
+    private static final int[] FACTORIALS = {1, 1, 2, 6, 24, 120, 720, 5040, 40320, 362880};
+
     private KrishnamurthyNumber() {
     }
 
     /**
      * Checks if a number is a Krishnamurthy number.
      *
-     * @param n The number to check
+     * <p>
+     * A number is a Krishnamurthy number if the sum of the factorials of its digits
+     * equals the number itself.
+     * </p>
+     *
+     * @param n the number to check
      * @return true if the number is a Krishnamurthy number, false otherwise
      */
     public static boolean isKrishnamurthy(int n) {
-        int tmp = n;
-        int s = 0;
-
         if (n <= 0) {
             return false;
-        } else {
-            while (n != 0) {
-                // initialising the variable fact that will store the factorials of the digits
-                int fact = 1;
-                // computing factorial of each digit
-                for (int i = 1; i <= n % 10; i++) {
-                    fact = fact * i;
-                }
-                // computing the sum of the factorials
-                s = s + fact;
-                // discarding the digit for which factorial has been calculated
-                n = n / 10;
-            }
+        }
 
-            // evaluating if sum of the factorials of the digits equals the number itself
-            return tmp == s;
+        int original = n;
+        int sum = 0;
+
+        while (n != 0) {
+            int digit = n % 10;
+            sum = sum + FACTORIALS[digit];
+            n = n / 10;
         }
+
+        return sum == original;
     }
 }

src/test/java/com/thealgorithms/maths/KrishnamurthyNumberTest.java

@@ -6,57 +6,115 @@
 import org.junit.jupiter.api.Test;
 
 /**
- * Unit tests for the KrishnamurthyNumber class.
+ * Unit tests for the {@link KrishnamurthyNumber} class.
  */
-public class KrishnamurthyNumberTest {
+class KrishnamurthyNumberTest {
 
     /**
-     * Test the isKrishnamurthy method with a known Krishnamurthy number.
+     * Test with known Krishnamurthy number 145.
+     * 1! + 4! + 5! = 1 + 24 + 120 = 145
      */
     @Test
-    public void testIsKrishnamurthyTrue() {
+    void testIsKrishnamurthyWith145() {
         assertTrue(KrishnamurthyNumber.isKrishnamurthy(145));
     }
 
     /**
-     * Test the isKrishnamurthy method with a number that is not a Krishnamurthy number.
+     * Test with a number that is not a Krishnamurthy number.
      */
     @Test
-    public void testIsKrishnamurthyFalse() {
+    void testIsKrishnamurthyWithNonKrishnamurthyNumber() {
         assertFalse(KrishnamurthyNumber.isKrishnamurthy(123));
     }
 
     /**
-     * Test the isKrishnamurthy method with zero.
+     * Test with zero, which is not a Krishnamurthy number.
      */
     @Test
-    public void testIsKrishnamurthyZero() {
+    void testIsKrishnamurthyWithZero() {
         assertFalse(KrishnamurthyNumber.isKrishnamurthy(0));
     }
 
     /**
-     * Test the isKrishnamurthy method with a negative number.
+     * Test with negative numbers, which cannot be Krishnamurthy numbers.
      */
     @Test
-    public void testIsKrishnamurthyNegative() {
+    void testIsKrishnamurthyWithNegativeNumbers() {
+        assertFalse(KrishnamurthyNumber.isKrishnamurthy(-1));
         assertFalse(KrishnamurthyNumber.isKrishnamurthy(-145));
+        assertFalse(KrishnamurthyNumber.isKrishnamurthy(-100));
     }
 
     /**
-     * Test the isKrishnamurthy method with a single-digit Krishnamurthy number.
+     * Test with single-digit Krishnamurthy numbers.
+     * 1! = 1 and 2! = 2, so both 1 and 2 are Krishnamurthy numbers.
      */
     @Test
-    public void testIsKrishnamurthySingleDigitTrue() {
+    void testIsKrishnamurthyWithSingleDigitKrishnamurthyNumbers() {
         assertTrue(KrishnamurthyNumber.isKrishnamurthy(1));
         assertTrue(KrishnamurthyNumber.isKrishnamurthy(2));
     }
 
     /**
-     * Test the isKrishnamurthy method with a single-digit number that is not a Krishnamurthy number.
+     * Test with single-digit numbers that are not Krishnamurthy numbers.
      */
     @Test
-    public void testIsKrishnamurthySingleDigitFalse() {
+    void testIsKrishnamurthyWithSingleDigitNonKrishnamurthyNumbers() {
         assertFalse(KrishnamurthyNumber.isKrishnamurthy(3));
         assertFalse(KrishnamurthyNumber.isKrishnamurthy(4));
+        assertFalse(KrishnamurthyNumber.isKrishnamurthy(5));
+        assertFalse(KrishnamurthyNumber.isKrishnamurthy(6));
+        assertFalse(KrishnamurthyNumber.isKrishnamurthy(7));
+        assertFalse(KrishnamurthyNumber.isKrishnamurthy(8));
+        assertFalse(KrishnamurthyNumber.isKrishnamurthy(9));
+    }
+
+    /**
+     * Test with the largest Krishnamurthy number: 40585.
+     * 4! + 0! + 5! + 8! + 5! = 24 + 1 + 120 + 40320 + 120 = 40585
+     */
+    @Test
+    void testIsKrishnamurthyWithLargestKrishnamurthyNumber() {
+        assertTrue(KrishnamurthyNumber.isKrishnamurthy(40585));
+    }
+
+    /**
+     * Test with various non-Krishnamurthy numbers.
+     */
+    @Test
+    void testIsKrishnamurthyWithVariousNonKrishnamurthyNumbers() {
+        assertFalse(KrishnamurthyNumber.isKrishnamurthy(10));
+        assertFalse(KrishnamurthyNumber.isKrishnamurthy(50));
+        assertFalse(KrishnamurthyNumber.isKrishnamurthy(100));
+        assertFalse(KrishnamurthyNumber.isKrishnamurthy(144));
+        assertFalse(KrishnamurthyNumber.isKrishnamurthy(146));
+        assertFalse(KrishnamurthyNumber.isKrishnamurthy(150));
+        assertFalse(KrishnamurthyNumber.isKrishnamurthy(200));
+        assertFalse(KrishnamurthyNumber.isKrishnamurthy(999));
+        assertFalse(KrishnamurthyNumber.isKrishnamurthy(1000));
+        assertFalse(KrishnamurthyNumber.isKrishnamurthy(40584));
+        assertFalse(KrishnamurthyNumber.isKrishnamurthy(40586));
+    }
+
+    /**
+     * Test with numbers close to known Krishnamurthy numbers.
+     */
+    @Test
+    void testIsKrishnamurthyWithNumbersCloseToKrishnamurthy() {
+        assertFalse(KrishnamurthyNumber.isKrishnamurthy(144));
+        assertFalse(KrishnamurthyNumber.isKrishnamurthy(146));
+        assertFalse(KrishnamurthyNumber.isKrishnamurthy(40584));
+        assertFalse(KrishnamurthyNumber.isKrishnamurthy(40586));
+    }
+
+    /**
+     * Test with all known Krishnamurthy numbers in base 10.
+     */
+    @Test
+    void testAllKnownKrishnamurthyNumbers() {
+        assertTrue(KrishnamurthyNumber.isKrishnamurthy(1));
+        assertTrue(KrishnamurthyNumber.isKrishnamurthy(2));
+        assertTrue(KrishnamurthyNumber.isKrishnamurthy(145));
+        assertTrue(KrishnamurthyNumber.isKrishnamurthy(40585));
     }
 }