Reverse bytes of a Hexadecimal Number

Given an unsigned integer N. The task is to reverse all bytes of N without using a temporary variable and print the reversed number. Examples:

Input: N = 0xaabbccdd Output: 0xddccbbaa Input: N = 0xa912cbd4 Output: 0xd4cb12a9

The naive approach is to extract the appropriate byte is to use mask(&) with shift operators.

#define REV(x) ( ((x&0xff000000)>>24) | (((x&0x00ff0000)<<8)>>16) | (((x&0x0000ff00)>>8)<<16) | ((x&0x000000ff) << 24) )

Efficient approach: The idea is to use shift operators only.

• Move the position of the last byte to the first byte using left shift operator(<<).
• Move the position of the first byte to the last byte using right shift operator(>>).
• Move the middle bytes using the combination of left shift and right shift operator.
• Apply logical OR (|) to the output of all the above expression, to get the desired output.

Below is the implementation of the above approach : 

C

// C program to reverse bytes of a hexadecimal number
#include <stdio.h>
// macro which reverse the hexadecimal integer
#define REV(n) ((n << 24) | (((n>>16)<<24)>>16) |  (((n<<16)>>24)<<16) | (n>>24))
// Driver code
int main()
{ unsigned int n = 0xa912cbd4;
// n = 0xaabbccdd
// (n >> 24) - 0x000000aa
// (n << 24) - 0xdd000000
// (((n >> 16) << 24) >> 16) - 0xbb00
// (((n >> 8) << 24) >> 8) - 0xcc0000
// If output of all the above expression is
// OR'ed then it results in 0xddccbbaa
printf("%x is reversed to %x", n, REV(n));
return 0; }

Java

// Java program to reverse bytes of a hexadecimal number

class GFG {
    public static int REV(int n)
    {
        return ((n >> 24) & 0xff)
            | // (n >> 24) - 0x000000aa
            ((n << 8) & 0xff0000)
            | // (n << 24) - 0xdd000000
            ((n >> 8) & 0xff00)
            | // (((n >> 16) << 24) >> 16) - 0xbb00
            ((n << 24) & 0xff000000); // (((n >> 8) << 24)
                                      // >> 8) - 0xcc0000
        // If output of all the above expression is
        // OR'ed then it results in 0xddccbbaa
    }

    public static void main(String[] args)
    {
        int n = 0xa912cbd4;
        System.out.println(Integer.toHexString(n) + " is reversed to " + Integer.toHexString(REV(n)));
    }
}

//this code is contributed by phasing17

C#

// C# program to reverse bytes of a hexadecimal number
using System;

class GFG {
  public static ulong REV(uint n)
  {
    return (ulong)((n >> 24) & 0xff)
      | // (n >> 24) - 0x000000aa
      (ulong)((n << 8) & 0xff0000)
      | // (n << 24) - 0xdd000000
      (ulong)((n >> 8) & 0xff00)
      | // (((n >> 16) << 24) >> 16) - 0xbb00
      (ulong)((n << 24)
              & 0xff000000); // (((n >> 8) << 24)
    // >> 8) - 0xcc0000
    // If output of all the above expression is
    // OR'ed then it results in 0xddccbbaa
  }

  // Driver code
  public static void Main(string[] args)
  {
    uint n = 0xa912cbd4;

    // Function call
    Console.WriteLine(n.ToString("X")
                      + " is reversed to "
                      + REV(n).ToString("X"));
  }
}

// This code is contributed by phasing17

Output:

a912cbd4 is reversed to d4cb12a9