Enhance docs, add more tests in SwapAdjacentBits (#5861)

Author: Hardvan

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

@@ -1,14 +1,45 @@
 package com.thealgorithms.bitmanipulation;
 
 /**
- * Swap every pair of adjacent bits of a given number.
- * @author Lakshyajeet Singh Goyal (https://github.com/DarkMatter-999)
+ * A utility class to swap every pair of adjacent bits in a given integer.
+ * This operation shifts the even-positioned bits to odd positions and vice versa.
+ *
+ * Example:
+ * - Input: 2 (binary: `10`) → Output: 1 (binary: `01`)
+ * - Input: 43 (binary: `101011`) → Output: 23 (binary: `010111`)
+ *
+ * **Explanation of the Algorithm:**
+ * 1. Mask even-positioned bits: Using `0xAAAAAAAA` (binary: `101010...`),
+ *    which selects bits in even positions.
+ * 2. Mask odd-positioned bits: Using `0x55555555` (binary: `010101...`),
+ *    which selects bits in odd positions.
+ * 3. Shift bits:
+ *    - Right-shift even-positioned bits by 1 to move them to odd positions.
+ *    - Left-shift odd-positioned bits by 1 to move them to even positions.
+ * 4. Combine both shifted results using bitwise OR (`|`) to produce the final result.
+ *
+ * Use Case: This algorithm can be useful in applications involving low-level bit manipulation,
+ * such as encoding, data compression, or cryptographic transformations.
+ *
+ * Time Complexity: O(1) (constant time, since operations are bitwise).
+ *
+ * Author: Lakshyajeet Singh Goyal (https://github.com/DarkMatter-999)
  */
-
 public final class SwapAdjacentBits {
     private SwapAdjacentBits() {
     }
 
+    /**
+     * Swaps every pair of adjacent bits of a given integer.
+     * Steps:
+     * 1. Mask the even-positioned bits.
+     * 2. Mask the odd-positioned bits.
+     * 3. Shift the even bits to the right and the odd bits to the left.
+     * 4. Combine the shifted bits.
+     *
+     * @param num the integer whose bits are to be swapped
+     * @return the integer after swapping every pair of adjacent bits
+     */
     public static int swapAdjacentBits(int num) {
         // mask the even bits (0xAAAAAAAA => 10101010...)
         int evenBits = num & 0xAAAAAAAA;

src/test/java/com/thealgorithms/bitmanipulation/SwapAdjacentBitsTest.java

@@ -8,13 +8,20 @@
 class SwapAdjacentBitsTest {
 
     @ParameterizedTest
-    @CsvSource({
-        "2, 1", // 2 (10 in binary) should become 1 (01 in binary)
-        "43, 23", // 43 should become 23
-        "153, 102", // 153 should become 102
-        "15, 15", // 15 (1111) remains 15 (1111)
-        "0, 0" // 0 (0000) remains 0 (0000)
-    })
+    @CsvSource({"2, 1", // 2 (binary: 10) -> 1 (binary: 01)
+        "43, 23", // 43 (binary: 101011) -> 23 (binary: 010111)
+        "153, 102", // 153 (binary: 10011001) -> 102 (binary: 01100110)
+        "15, 15", // 15 (binary: 1111) -> 15 (binary: 1111) (no change)
+        "0, 0", // 0 (binary: 0000) -> 0 (binary: 0000) (no change)
+        "1, 2", // 1 (binary: 01) -> 2 (binary: 10)
+        "170, 85", // 170 (binary: 10101010) -> 85 (binary: 01010101)
+        "85, 170", // 85 (binary: 01010101) -> 170 (binary: 10101010)
+        "255, 255", // 255 (binary: 11111111) -> 255 (binary: 11111111) (no change)
+        "128, 64", // 128 (binary: 10000000) -> 64 (binary: 01000000)
+        "1024, 2048",
+        "-1, -1", // -1 (all bits 1) remains -1 (no change due to two's complement)
+        "-2, -3", // -2 (binary: ...1110) -> -3 (binary: ...1101)
+        "2147483647, -1073741825", "-2147483648, -1073741824"})
     void
     testSwapAdjacentBits(int input, int expected) {
         assertEquals(expected, SwapAdjacentBits.swapAdjacentBits(input));