# AoC Day 24 – Lobby Layout

This is a solution to Advent of Code 2020 day 24, written in Raku.

### Part One

Go through the renovation crew's list and determine which tiles they need to flip. After all of the instructions have been followed, how many tiles are left with the black side up?

For this problem, I chose to use cube coordinates for the hexagonal grid. Each directional move can be translated into cartesian coordinates and each target tile is the sum of the coordinates. I keep a set of black tiles, adding to the set when a tile is first flipped and removing if it flips again, etc.

  my @input = '24-input.txt'.IO.lines;

my %movements =
e => (1, -1, 0), se => (0, -1, 1), sw => (-1, 0, 1),
w => (-1, 1, 0), nw => (0, 1, -1), ne => (1, 0, -1);

my $black = SetHash.new; for @input { my @tile = 0,0,0; .comb(/'se' | 'sw' | 'ne' | 'nw' | 'e' | 'w'/).map( ->$dir { %movements{$dir} } ).map( -> @delta { @tile = @tile >>+>> @delta }); my$t = @tile.join(',');
$black{$t} ?? $black.unset($t) !! $black.set($t);
}

say "Part one";
say +$black; Part one 391  ### Part Two How many tiles will be black after 100 days? Part two takes the output from part one and runs a Game of Life for 100 iterations. The solution from Day 17 can be reused, with just 6 neighbours for each grid location instead of 26 neighbours in the original Conway Cube.  my @input = '24-input.txt'.IO.lines; my %movements = e => (1, -1, 0), se => (0, -1, 1), sw => (-1, 0, 1), w => (-1, 1, 0), nw => (0, 1, -1), ne => (1, 0, -1); my @rel-coords = %movements.values; my$black = SetHash.new;

for @input {
my @tile = 0,0,0;
.comb(/'se' | 'sw' | 'ne' | 'nw' | 'e' | 'w'/).map(
-> $dir { %movements{$dir} }
).map( -> @delta { @tile = @tile >>+>> @delta });
my $t = @tile.join(',');$black{$t} ??$black.unset($t) !!$black.set($t); } for ^100 { my$neighbour-counts = $black.keys.map( ->$tuple-str {
my @coords = $tuple-str.split(','); @rel-coords.map( ->$rel { @coords >>+>> $rel } ).map(*.join(',')) }).flat.Bag; my %active =$neighbour-counts.kv.grep(
-> $k,$v {
!$black{$k} && $v == 2 or$black{$k} &&$v == (1|2)
}).flat;
$black = %active.keys.SetHash; } say "Part two"; say +$black;
Part two
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