Bit swap done

Author: rampatra

src/main/java/com/ctci/bitmanipulation/PairwiseSwap.java

@@ -0,0 +1,40 @@
+package com.ctci.bitmanipulation;
+
+/**
+ * @author rampatra
+ * @since 2019-03-18
+ */
+public class PairwiseSwap {
+
+    /**
+     * Write a program to swap odd and even bits in an integer with as few instructions as 
+     * possible (e.g., bit O and bit 1 are swapped, bit 2 and bit 3 are swapped, and so on).
+     * 
+     * Approach:
+     * Shift the odd bits to the left, shift the even bits to the right, and finally, OR both the results. 
+     * Note: You can operate on only odd bits or only even bits by using the appropriate masks, for e.g., 
+     * 0x55555555 for odd bits and 0xaaaaaaaa for even bits.
+     * 
+     * @param n an input integer.
+     * @return an integer with even and odd bits swapped.
+     */
+    private static int swapBits(int n) {
+        return ((n & 0x55555555) << 1) | ((n & 0xaaaaaaaa) >>> 1);
+    }
+
+    public static void main(String[] args) {
+        System.out.println("Input:  " + Integer.toBinaryString(1569) +
+                "\nOutput: " + Integer.toBinaryString(swapBits(1569)));
+        
+        assert Integer.toBinaryString(swapBits(1569)).equals("100100010010");
+
+        System.out.println("Input:  " + Integer.toBinaryString(2680) +
+                "\nOutput: " + Integer.toBinaryString(swapBits(2680)));
+
+        assert Integer.toBinaryString(swapBits(2680)).equals("10110110100");
+        
+        // fyi
+        System.out.println(Integer.toBinaryString(0xaaaaaaaa));
+        System.out.println(Integer.toBinaryString(0x99999999));
+    }
+}