TL;DR

On with Advent of Code puzzle 6 from 2022: finding diversity in strings.

This challenge was pretty straightforward but tickling. So many ways of addressing it!

The gist is to find the leftmost sequence of $n$ characters that are all different from one another. Part 1 and 2 only differ for the number of different characters that should be in the sequence.

Some people went on taking substrings and building sets out of them, stopping when the set size is the right one. So clever.

I’m always scared of this kind of solutions because it seems to me that some work is being done over and over again. Which is unfortunate in a challenge like this, which has a speed factor.

Well, unless you don’t mind about the speed factor.

In my case, I decided to keep track of a sliding window, counting stuff with a BagHash. The compact solution is the following:

sub detect-different($string, $n) {
   my ($i, $window) = $n - 1, BagHash.new($string.substr(0, $n - 1).comb);
   loop {
      $window.add($string.substr($i++, 1));
      return $i if $window.elems == $n;
      $window.remove($string.substr($i - $n, 1));
   };
}
put '06.input'.IO.lines.map({detect-different($_, 4)});
put '06.input'.IO.lines.map({detect-different($_, 14)});

The BagHash is perfect in this case, because it allows to implement the sliding window counting in the right way. Getting one character in means increasing one of the counts; getting one out, decreasing the count.

I mean, this is just a regular hash/dictionary which holds a count, but the name is fun!

Full solution.

Stay safe!