test: add white-box unit tests for Luhn.luhnCheck

Cover the loop (zero, one and many iterations), both outcomes of the
`i % 2 == 0` and `temp > 9` decisions, boundary values (8 / 10 after
doubling), valid and invalid checksums, input immutability and null input.

Author: datpp

src/test/java/com/thealgorithms/others/LuhnTest.java

@@ -0,0 +1,111 @@
+package com.thealgorithms.others;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * White-box tests for {@link Luhn#luhnCheck(int[])}.
+ *
+ * <p>The method has one loop and two decisions:
+ * <ul>
+ * <li>D1: {@code i % 2 == 0} - whether the digit at index i is doubled</li>
+ * <li>D2: {@code temp > 9} - whether 9 is subtracted from the doubled digit</li>
+ * </ul>
+ * The tests below cover the loop executed zero, one and many times, both
+ * outcomes of each decision, and both outcomes of the final {@code sum % 10 == 0}.
+ */
+class LuhnTest {
+
+    @Test
+    void testEmptyArraySkipsLoopAndIsValid() {
+        // Loop body never executes, sum stays 0
+        assertTrue(Luhn.luhnCheck(new int[] {}));
+    }
+
+    @Test
+    void testSingleZeroDigitIsValid() {
+        // One iteration: D1 true, D2 false (0 * 2 = 0)
+        assertTrue(Luhn.luhnCheck(new int[] {0}));
+    }
+
+    @Test
+    void testSingleDigitDoubledWithoutSubtractionIsInvalid() {
+        // One iteration: D1 true, D2 false (4 * 2 = 8, upper boundary of temp <= 9)
+        assertFalse(Luhn.luhnCheck(new int[] {4}));
+    }
+
+    @Test
+    void testSingleDigitDoubledWithSubtractionIsInvalid() {
+        // One iteration: D1 true, D2 true (5 * 2 = 10, lower boundary of temp > 9 -> 1)
+        assertFalse(Luhn.luhnCheck(new int[] {5}));
+    }
+
+    @Test
+    void testTwoDigitsWithoutSubtraction() {
+        // i = 1: D1 false -> 2; i = 0: D1 true, D2 false -> 8; sum = 10
+        assertTrue(Luhn.luhnCheck(new int[] {4, 2}));
+    }
+
+    @Test
+    void testTwoDigitsWithSubtraction() {
+        // i = 1: D1 false -> 9; i = 0: D1 true, D2 true -> 10 - 9 = 1; sum = 10
+        assertTrue(Luhn.luhnCheck(new int[] {5, 9}));
+    }
+
+    @Test
+    void testTwoDigitsInvalidChecksum() {
+        // i = 1: D1 false -> 3; i = 0: D1 true, D2 false -> 8; sum = 11
+        assertFalse(Luhn.luhnCheck(new int[] {4, 3}));
+    }
+
+    @Test
+    void testMaxDigitDoubled() {
+        // i = 1: D1 false -> 0; i = 0: D1 true, D2 true -> 18 - 9 = 9; sum = 9
+        assertFalse(Luhn.luhnCheck(new int[] {9, 0}));
+        // i = 1: D1 false -> 1; i = 0: D1 true, D2 true -> 9; sum = 10
+        assertTrue(Luhn.luhnCheck(new int[] {9, 1}));
+    }
+
+    @Test
+    void testValidCardNumbers() {
+        // Many iterations, all branch combinations exercised
+        assertTrue(Luhn.luhnCheck(new int[] {4, 5, 6, 1, 2, 6, 1, 2, 1, 2, 3, 4, 5, 4, 6, 7}));
+        assertTrue(Luhn.luhnCheck(new int[] {5, 2, 6, 5, 9, 2, 5, 1, 6, 1, 5, 1, 1, 4, 1, 2}));
+        assertTrue(Luhn.luhnCheck(new int[] {4, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1}));
+    }
+
+    @Test
+    void testCardNumberWithTypoInLastDigitIsInvalid() {
+        assertFalse(Luhn.luhnCheck(new int[] {4, 5, 6, 1, 2, 6, 1, 2, 1, 2, 3, 4, 5, 4, 6, 4}));
+    }
+
+    @Test
+    void testCardNumberWithSingleDigitErrorIsInvalid() {
+        // Error in a doubled position (index 0)
+        assertFalse(Luhn.luhnCheck(new int[] {3, 5, 6, 1, 2, 6, 1, 2, 1, 2, 3, 4, 5, 4, 6, 7}));
+        // Error in a non-doubled position (index 1)
+        assertFalse(Luhn.luhnCheck(new int[] {4, 6, 6, 1, 2, 6, 1, 2, 1, 2, 3, 4, 5, 4, 6, 7}));
+    }
+
+    @Test
+    void testAllZerosIsValid() {
+        assertTrue(Luhn.luhnCheck(new int[16]));
+    }
+
+    @Test
+    void testInputArrayIsNotModified() {
+        int[] digits = {5, 2, 6, 5, 9, 2, 5, 1, 6, 1, 5, 1, 1, 4, 1, 2};
+        int[] copy = digits.clone();
+        Luhn.luhnCheck(digits);
+        assertArrayEquals(copy, digits);
+    }
+
+    @Test
+    void testNullInputThrowsNullPointerException() {
+        assertThrows(NullPointerException.class, () -> Luhn.luhnCheck(null));
+    }
+}