PWC114 - Next Palindrome Number


Here we are with TASK #1 from the Perl Weekly Challenge #114. Enjoy!

The challenge

You are given a positive integer $N.

Write a script to find out the next Palindrome Number higher than the given integer $N.


Input: $N = 1234 Output: 1331

Input: $N = 999 Output: 1001

The questions

I guess there are almost no question this time, assuming that:

  • a Palindrome Number is an integer whose representation in base 10 is a palindrome string
  • we trust our inputs to contain a valid sequence of digits.

The solution

As it often happens, this challenge can be addressed with a simple brute force attack or something a bit more aimed.

The brute force in this case would be to start counting from $N + 1 on, checking for palindrome values and stopping at the first.

The aimed approach requires dealing with a few corner cases…

How many digits?

First of all, there is a corner case that manwar very gently reminds us of: a sequence of all 9s. This is the only case where the next palindrome actually contains one more digit than $N, e.g. for 999 we go to 1001.

How do I know that this is the only case, you might be asking? Well, if the starting $N contains any digit that is not a 9, then $N will surely be less than a sequence of 9s that is long the same as $N. Considering that a sequence of all 9s is palindrome, surely any number below it has a greater palindrome that has the same number of digits.

OK, so we will treat a sequence of all 9s as a special case, what to do next?

Even number of digits

If we have an even number of digits, then we only need to count on the first half of the number, because the second half is constrained by the palindrome constraint. To get the lowest number, it will be just regular counting.

Whatever candidate we get for the first half, we can easily form the full number and check it against $N. If it’s greater than $N we’re done! Otherwise, just increasing the first half by one unit will suffice.

Odd number of digits

This case is pretty much similar to the previous, but not the same because it’s difficult to cut the string representation of $N in half. So we will take just half-a-character less of one half and put it into $n, and treat the middle character $mid specially.

As before, our first attempt is to work with what we have, i.e. merging $n, $mid and the reverse of $n. If it’s greater than $N we’re done.

Next, to get the closest greater one, we try to increase $mid. This will work… unless $mid is equal to 9, which forces us to increase $n instead.

Putting even and odd cases together

The two cases for even and odd alternatives are pretty similar, except for some special handling of the middle character. Hence, with a few special cases… we should be able to address both cases in a mostly unified way.

Perl solution

Let’s start with the Perl solution:

#!/usr/bin/env perl
use 5.024;
use warnings;
use experimental qw< postderef signatures >;
no warnings qw< experimental::postderef experimental::signatures >;

sub next_palindrome_number ($N) {
   my $l = length $N;
   return '1' . ('0' x ($l - 1)) . '1' unless $N =~ m{[0-8]}mxs;
   my $n = substr $N, 0, $l / 2;
   my $mid = $l % 2 ? substr($N, $l / 2, 1) : '';

   # just try to build straight from the inputs...
   if ((my $candidate = $n . $mid . reverse($n)) > $N) {
      return $candidate;

   # if there's a "$mid", try increasing that
   if ($l % 2) {
      return $n . ($mid + 1) . reverse($n) if $mid != 9;
      $mid = 0;

   return $n . $mid . reverse($n);

@ARGV = (1234) unless @ARGV;
say next_palindrome_number($_) for @ARGV;

I hope it has no surprises after the explanations above!

Raku solution

So here’s the translation of the solution in Raku:

#!/usr/bin/env raku
use v6;

sub next-palindrome-number (IntStr $N) {
   my $l = $N.chars;
   return '1' ~ ('0' x ($l - 1)) ~ '1' unless $N ~~ m{<[0 .. 8]>};
   my $n = $N.substr(0, $l / 2);
   my $mid = $l % 2 ?? $N.substr($l / 2, 1) !! '';

   # just try to build straight from the inputs...
   if (my $candidate = $n ~ $mid ~ $n.flip) > $N {
      return $candidate;

   # if there's a "$mid", try increasing that
   if ($l % 2) {
      return $n ~ ($mid + 1) ~ $n.flip if $mid < 9;
      $mid = 0;

   return $n ~ $mid ~ $n.flip;

sub MAIN (*@inputs is copy) {
   @inputs.push(1234) unless @inputs.elems;
   next-palindrome-number($_).say for @inputs;

This is basically my second (well… third) venture in Raku, and so far it’s been a bit frustrating.

Ok, ok, it was an occasion to break with the past and fix a few things. Was it really necessary to move from reverse to flip? Change the ternary operator that we all love from C into ?? !!? Turn character classes to something different from the rest of the world?

Probably yes, but it’s frustrating. For me, at least.

Another nit-pick is about the signatures, especially for MAIN. I think that there’s been a substantial loss in whipuptitude, because it took me a bit to figure how to replicate the simple input arguments passing that we have in Perl.

I mean, I usually resort to Getopt::Long for getting input parameters, and I understand that the signature for MAIN() can help with this. But, again… whipuptitude lost in my opinion.

Or, maybe… I didn’t read enough 🙄

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