Added CountBitsFlip algo (#6603)

* Added CountBitsFlip aldo

* checkstyle fix

* checkstyle fix

---------

Co-authored-by: Alx <alexanderklmn@gmail.com>

src/main/java/com/thealgorithms/bitmanipulation/CountBitsFlip.java

@@ -0,0 +1,63 @@
+package com.thealgorithms.bitmanipulation;
+
+/**
+ * Implementation to count number of bits to be flipped to convert A to B
+ *
+ * Problem: Given two numbers A and B, count the number of bits needed to be
+ * flipped to convert A to B.
+ *
+ * Example:
+ * A = 10 (01010 in binary)
+ * B = 20 (10100 in binary)
+ * XOR = 30 (11110 in binary) - positions where bits differ
+ * Answer: 4 bits need to be flipped
+ *
+ * Time Complexity: O(log n) - where n is the number of set bits
+ * Space Complexity: O(1)
+ *
+ *@author [Yash Rajput](https://github.com/the-yash-rajput)
+ */
+public final class CountBitsFlip {
+
+    private CountBitsFlip() {
+        throw new AssertionError("No instances.");
+    }
+
+    /**
+     * Counts the number of bits that need to be flipped to convert a to b
+     *
+     * Algorithm:
+     * 1. XOR a and b to get positions where bits differ
+     * 2. Count the number of set bits in the XOR result
+     * 3. Use Brian Kernighan's algorithm: n & (n-1) removes rightmost set bit
+     *
+     * @param a the source number
+     * @param b the target number
+     * @return the number of bits to flip to convert A to B
+     */
+    public static long countBitsFlip(long a, long b) {
+        int count = 0;
+
+        // XOR gives us positions where bits differ
+        long xorResult = a ^ b;
+
+        // Count set bits using Brian Kernighan's algorithm
+        while (xorResult != 0) {
+            xorResult = xorResult & (xorResult - 1); // Remove rightmost set bit
+            count++;
+        }
+
+        return count;
+    }
+
+    /**
+     * Alternative implementation using Long.bitCount().
+     *
+     * @param a the source number
+     * @param b the target number
+     * @return the number of bits to flip to convert a to b
+     */
+    public static long countBitsFlipAlternative(long a, long b) {
+        return Long.bitCount(a ^ 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));
+        }
+    }
+}