Added CountBitsFlip algo (#6603)

* Added CountBitsFlip aldo * checkstyle fix * checkstyle fix --------- Co-authored-by: Alx <alexanderklmn@gmail.com>
2026-03-13 08:40:43 +08:00 · 2025-10-03 15:22:53 +05:30
parent 506b6d1ab2
commit 8d14b492fd
2 changed files with 157 additions and 0 deletions
--- a/src/test/java/com/thealgorithms/bitmanipulation/CountBitsFlipTest.java
+++ b/src/test/java/com/thealgorithms/bitmanipulation/CountBitsFlipTest.java
@@ -0,0 +1,94 @@
+package com.thealgorithms.bitmanipulation;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.Random;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Unit tests for CountBitsFlip.
+ * Covers:
+ *  - simple examples
+ *  - zeros and identical values
+ *  - negative numbers and two's complement edge cases
+ *  - Long.MIN_VALUE / Long.MAX_VALUE
+ *  - randomized consistency checks between two implementations
+ */
+@DisplayName("CountBitsFlip Tests")
+class CountBitsFlipTest {
+
+    @Test
+    @DisplayName("Example: A=10, B=20 => 4 bits")
+    void exampleTenTwenty() {
+        long a = 10L;
+        long b = 20L;
+        long expected = 4L;
+        assertEquals(expected, CountBitsFlip.countBitsFlip(a, b), "Brian Kernighan implementation should return 4");
+        assertEquals(expected, CountBitsFlip.countBitsFlipAlternative(a, b), "Long.bitCount implementation should return 4");
+    }
+
+    @Test
+    @DisplayName("Identical values => 0 bits")
+    void identicalValues() {
+        long a = 123456789L;
+        long b = 123456789L;
+        long expected = 0L;
+        assertEquals(expected, CountBitsFlip.countBitsFlip(a, b));
+        assertEquals(expected, CountBitsFlip.countBitsFlipAlternative(a, b));
+    }
+
+    @Test
+    @DisplayName("Both zeros => 0 bits")
+    void bothZeros() {
+        assertEquals(0L, CountBitsFlip.countBitsFlip(0L, 0L));
+        assertEquals(0L, CountBitsFlip.countBitsFlipAlternative(0L, 0L));
+    }
+
+    @Test
+    @DisplayName("Small example: A=15 (1111), B=8 (1000) => 3 bits")
+    void smallExample() {
+        long a = 15L; // 1111
+        long b = 8L; // 1000
+        long expected = 3L; // differs in three low bits
+        assertEquals(expected, CountBitsFlip.countBitsFlip(a, b));
+        assertEquals(expected, CountBitsFlip.countBitsFlipAlternative(a, b));
+    }
+
+    @Test
+    @DisplayName("Negative values: -1 vs 0 => 64 bits (two's complement all ones)")
+    void negativeVsZero() {
+        long a = -1L;
+        long b = 0L;
+        long expected = 64L; // all 64 bits differ
+        assertEquals(expected, CountBitsFlip.countBitsFlip(a, b));
+        assertEquals(expected, CountBitsFlip.countBitsFlipAlternative(a, b));
+    }
+
+    @Test
+    @DisplayName("Long.MIN_VALUE vs Long.MAX_VALUE => 64 bits")
+    void minMaxLongs() {
+        long a = Long.MIN_VALUE;
+        long b = Long.MAX_VALUE;
+        long expected = 64L; // MAX ^ MIN yields all ones on 64-bit long
+        assertEquals(expected, CountBitsFlip.countBitsFlip(a, b));
+        assertEquals(expected, CountBitsFlip.countBitsFlipAlternative(a, b));
+    }
+
+    @Test
+    @DisplayName("Randomized consistency: both implementations agree across many pairs")
+    void randomizedConsistency() {
+        final int iterations = 1000;
+        final Random rnd = new Random(12345L); // deterministic seed for reproducibility
+
+        for (int i = 0; i < iterations; i++) {
+            long a = rnd.nextLong();
+            long b = rnd.nextLong();
+
+            long res1 = CountBitsFlip.countBitsFlip(a, b);
+            long res2 = CountBitsFlip.countBitsFlipAlternative(a, b);
+
+            assertEquals(res2, res1, () -> String.format("Mismatch for a=%d, b=%d: impl1=%d, impl2=%d", a, b, res1, res2));
+        }
+    }
+}