this post was submitted on 11 Dec 2023
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Advent Of Code

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☃️ - 2023 DAY 11 SOLUTIONS - ☃️ (programming.dev)
submitted 9 months ago* (last edited 9 months ago) by [email protected] to c/[email protected]
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Day 11: Cosmic Expansion

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[–] [email protected] 3 points 9 months ago* (last edited 9 months ago)

Raku

Today I'm thankful that I have the combinations() method available. It's not hard to implement combinations(), but it's not particularly interesting. This code is a bit anachronistic because I solved part 1 by expanding the universe instead of contracting it, but this way makes the calculations for part 1 and part 2 symmetric. I was worried for a bit there that I'd have to do some "here are all the places where expansion happens, check this list when calculating distances" bookkeeping, and I was quite relieved when I realized that I could just use arithmetic.

edit: Also, first time using the Slip class in Raku, which is mind bending, but very useful for expanding/contracting the universe and generating the lists of galaxy coordinates. And I learned a neat way to transpose 2D arrays using [Z].

View the code on Github

Code

use v6;

sub MAIN($input) {
    my $file = open $input;

    my @map = $file.lines».comb».Array;
    my @galaxies-original = @map».grep("#", :k).grep(*.elems > 0, :kv).rotor(2).map({($_[0] X $_[1]).Slip});
    my $distances-original = @galaxies-original.List.combinations(2).map({($_[0] Z- $_[1])».abs.sum}).sum;

    # contract the universe
    @map = @map.map: {$_.all eq '.' ?? slip() !! $_};
    @map = [Z] @map;
    @map = @map.map: {$_.all eq '.' ?? slip() !! $_};
    @map = [Z] @map;

    my @galaxies = @map».grep("#", :k).grep(*.elems > 0, :kv).rotor(2).map({($_[0] X $_[1]).Slip});
    my $distances-contracted = @galaxies.List.combinations(2).map({($_[0] Z- $_[1])».abs.sum}).sum;

    my $distances-twice-expanded = ($distances-original - $distances-contracted) * 2 + $distances-contracted;
    say "part 1: $distances-twice-expanded";
    my $distances-many-expanded = ($distances-original - $distances-contracted) * 1000000 + $distances-contracted;
    say "part 2: $distances-many-expanded";
}