Improvements to Vampire Number

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

@@ -1,78 +1,48 @@
 package com.thealgorithms.maths;
 
 import java.util.ArrayList;
-import java.util.Collections;
 
 /**
- * n number theory, a vampire number (or true vampire number) is a composite
+ * In number theory, a vampire number (or true vampire number) is a composite
  * natural number with an even number of digits, that can be factored into two
  * natural numbers each with half as many digits as the original number and not
  * both with trailing zeroes, where the two factors contain precisely all the
  * digits of the original number, in any order, counting multiplicity. The first
- * vampire number is 1260 = 21 × 60. *
+ * vampire number is 1260 = 21 × 60.
  *
- * <p>
- * link: https://en.wikipedia.org/wiki/Vampire_number *
- *
- * <p>
+ * @see <a href='https://en.wikipedia.org/wiki/Vampire_number'>Vampire number on Wikipedia</a>
  */
 public final class VampireNumber {
+    // Forbid instantiation.
     private VampireNumber() {
     }
 
-    public static void main(String[] args) {
-        test(10, 1000);
-    }
-
-    static void test(int startValue, int stopValue) {
-        int countofRes = 1;
-        StringBuilder res = new StringBuilder();
-
-        for (int i = startValue; i <= stopValue; i++) {
-            for (int j = i; j <= stopValue; j++) {
-                // System.out.println(i+ " "+ j);
-                if (isVampireNumber(i, j, true)) {
-                    countofRes++;
-                    res.append("").append(countofRes).append(": = ( ").append(i).append(",").append(j).append(" = ").append(i * j).append(")").append("\n");
-                }
-            }
-        }
-        System.out.println(res);
-    }
-
-    static boolean isVampireNumber(int a, int b, boolean noPseudoVamireNumbers) {
-        // this is for pseudoVampireNumbers  pseudovampire number need not be of length n/2 digits
-        // for example 126 = 6 x 21
-        if (noPseudoVamireNumbers) {
-            if (a * 10 <= b || b * 10 <= a) {
-                return false;
-            }
+    static boolean isVampireNumber(int a, int b, boolean ignorePseudoVampireNumbers) {
+        // Pseudo vampire numbers don't have to be of n/2 digits. E.g., 126 = 6 x 21 is such a number.
+        if (ignorePseudoVampireNumbers && String.valueOf(a).length() != String.valueOf(b).length()) {
+            return false;
         }
 
-        String mulDigits = splitIntoDigits(a * b, 0);
-        String faktorDigits = splitIntoDigits(a, b);
+        String mulDigits = splitIntoSortedDigits(a * b);
+        String factorDigits = splitIntoSortedDigits(a, b);
 
-        return mulDigits.equals(faktorDigits);
+        return mulDigits.equals(factorDigits);
     }
 
-    // methode to Split the numbers to Digits
-    static String splitIntoDigits(int num, int num2) {
-        StringBuilder res = new StringBuilder();
-
+    // Method to split a pair of numbers to digits and sort them in the ascending order.
+    static String splitIntoSortedDigits(int... nums) {
+        // Collect all digits in a list.
         ArrayList<Integer> digits = new ArrayList<>();
-        while (num > 0) {
-            digits.add(num % 10);
-            num /= 10;
-        }
-        while (num2 > 0) {
-            digits.add(num2 % 10);
-            num2 /= 10;
-        }
-        Collections.sort(digits);
-        for (int i : digits) {
-            res.append(i);
+        for (int num : nums) {
+            while (num > 0) {
+                digits.add(num % 10);
+                num /= 10;
+            }
         }
 
+        // Sort all digits and convert to String.
+        StringBuilder res = new StringBuilder();
+        digits.stream().sorted().forEach(res::append);
         return res.toString();
     }
 }

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

@@ -0,0 +1,32 @@
+package com.thealgorithms.maths;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+class VampireNumberTest {
+    @Test
+    void areVampireNumbers() {
+        Assertions.assertTrue(VampireNumber.isVampireNumber(15, 93, true));
+        Assertions.assertTrue(VampireNumber.isVampireNumber(135, 801, true));
+        Assertions.assertTrue(VampireNumber.isVampireNumber(201, 600, true));
+    }
+
+    @Test
+    void arePseudoVampireNumbers() {
+        Assertions.assertTrue(VampireNumber.isVampireNumber(150, 93, false));
+        Assertions.assertTrue(VampireNumber.isVampireNumber(546, 84, false));
+        Assertions.assertTrue(VampireNumber.isVampireNumber(641, 65, false));
+    }
+
+    @Test
+    void areNotVampireNumbers() {
+        Assertions.assertFalse(VampireNumber.isVampireNumber(51, 39, false));
+        Assertions.assertFalse(VampireNumber.isVampireNumber(51, 39, true));
+    }
+
+    @Test
+    void testSplitIntoSortedDigits() {
+        Assertions.assertEquals("123", VampireNumber.splitIntoSortedDigits(321));
+        Assertions.assertEquals("02234", VampireNumber.splitIntoSortedDigits(20, 324));
+    }
+}