TL;DR

On with Advent of Code puzzle 3 from 2022: we’re definitely gone the abstract way here.

I like the background story of Advent of Code, even though I understand that sometimes it can be difficult to frame a challenge within it. My feeling is that this… was one of those times.

The final byproduct in puzzle 1 and, as it is eventually revealed, puzzle 2 is a sum of priorities, which are assigned statically based on the character. So going bottom-up here’s the relevant function:

sub char-to-value ($char) { return 1 +$char.ord - 'a'.ord if $char ~~ /<[ a .. z ]>/; return 27 +$char.ord - 'A'.ord;
}


OK, with that out of the way, let’s look at part 1. Here we have to consider each input line/string as composed of two halves; our task is to find the common character between them, turn it into a value with char-to-value above and sum everything:

sub part1 ($inputs) { return$inputs.map({
my $hlen = ($_.chars / 2).Int;
my ($h,$l) =
($_.substr(0,$hlen), $_.substr($hlen)).map({ $_.comb.Set }); char-to-value(($h ∩ $l).keys); }).sum; }  Raku comes with both batteries and a lot of nice tools tailored for a lot of different tasks. In this case: • we get each character using comb • we don’t care about them individually, but treat each half as a set of characters. • with two sets available, finding the common character means finding the intersection of the two sets, which is what the ∩ operator is for. Part 2 goes on a similar tune, only we have to find the common character across triplets taken from the input. Raku comes to help us with a way to take the intersection of many sets at once with hyperoperator [∩]: sub part2 ($inputs) {
gather {
for @$inputs ->$a, $b,$c {
take char-to-value(([∩] ($a,$b, \$c)».comb».Set).keys)
}
}.sum;
}


I could not think of some clever/idiomatic way of expressing the concept of taking inputs three at a time, so I just did that with the for loop specification.

This somehow “breaks the flow” for composition, which would make adding a final .sum complicated. So I thought of using gather/take. I like it. Maybe one day it will be efficient, if it’s not already.

Now, before leaving, a hint from mschaap:

But you don’t have to .Set the lists in order to be able to [∩] them, that is done automatically, see my solution.

I love that there’s always something to learn!

All in all, a nice one… stay safe!