RabbitFarm

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

Part 1

You are given a positive integer $N. Write a script to swap the two nibbles of the binary representation of the given number and print the decimal number of the new binary representation.

Solution

use strict;
use warnings;
sub swap_nibbles{
    my($n) = @_;
    my $bits = substr(unpack("B32", pack("N", shift)), 24, 8);
    my $swapped_bits = substr($bits, 4) . substr($bits, 0, 4);
    my $swapped_decimal = unpack("N", pack("B32", substr("0" x 32 . $swapped_bits, -32)));
    print $swapped_decimal . "\n";
}

MAIN:{
    swap_nibbles(101);
    swap_nibbles(18);
}

Sample Run

$ perl perl/ch-1.pl
86
33

Notes

I was on vacation recently and did not have time for the last couple of Weekly Challenges, but as I posted a meme about it is hard to take a break!

(The Perl Programmers Facebook group is a lof of fun. It is kept Private by the group owner but joining is easy, anyone is allowed provided they are interested in Perl.)

I was able to get through the first part of this week's challenge with the time I had after getting back from vacation. As I was unpacking my suitcase, co-incidentally enough, I noticed that the first task is a great use of pack and unpack!

I have used these functions several times in the past, for example this writeup from Challenge 020 has an example and some links to others. I must admit that from the earliest days of my Perl experience I have been fascinated by pack! At first it seemed like a bit of black magic and due to its versatility, in some ways it still retains this mystique.

In the swap_nibbles function the number is packed into Network Byte Order and that representation is that unpacked bitwise to get the expected binary representation. After that the two nibbles are swapped using substr to get each 4 bit slice. The process is then reversed on the swapped bits to get the result we want.

References

Challenge 119

Network Byte Order

Perl Programmers Facebook Group