# 2023 AoC Day 18 – Lavaduct Lagoon

This is a solution to Advent of Code 2023 day 18, written in Raku.

### Part One

The digger starts in a 1 meter cube hole in the ground. They then dig the specified number of meters up (U), down (D), left (L), or right (R), clearing full 1 meter cubes as they go. The directions are given as seen from above, so if "up" were north, then "right" would be east, and so on. Each trench is also listed with the color that the edge of the trench should be painted as an RGB hexadecimal color code …

The Elves are concerned the lagoon won't be large enough; if they follow their dig plan, how many cubic meters of lava could it hold?

Strategy:

• Build a list of the vertices for the trench polygon
• Calculate the area inside the polygon
• Add half the trench area (which is outside the polygon)
my @plan = '18-input.txt'.IO.lines.map(
,*.match(/^(\w) ' ' (\d+) /).list>>.Str
);

my @coords;
my @cur-pos = 0, 0;
@coords.push(@cur-pos);
my $trench = 0; for @plan -> @line { my$dir = @line[0];
my $distance = @line[1].Int;$trench += $distance; my @new-pos = @cur-pos; given$dir {
when 'L' { @new-pos[0] -= $distance; } when 'R' { @new-pos[0] +=$distance; }
when 'U' { @new-pos[1] -= $distance; } when 'D' { @new-pos[1] +=$distance; }
}
@coords.push(@new-pos);
@cur-pos = @new-pos;
}

my $total = 0; for ^ (+@coords - 1) ->$i {
$total += @coords[$i][0] * @coords[$i + 1][1];$total -= @coords[$i + 1][0] * @coords[$i][1];
}

my $area = (abs($total) + $trench) / 2 + 1; say "Total {$area}";
say "Took " ~ (now - ENTER now).base(10,2) ~ " seconds";
Total 46334
Took 0.07 seconds


### Part Two

Each hexadecimal code is six hexadecimal digits long. The first five hexadecimal digits encode the distance in meters as a five-digit hexadecimal number. The last hexadecimal digit encodes the direction to dig: 0 means R, 1 means D, 2 means L, and 3 means U …

Convert the hexadecimal color codes into the correct instructions; if the Elves follow this new dig plan, how many cubic meters of lava could the lagoon hold?

Part 2 uses a copy of the solution from part 1 with a modified regex and the data converted to the same format as part 1.

my @plan = '18-input.txt'.IO.lines.map(
,*.match(/ '(#' (.....) (.) ')' /).list>>.Str
);

my @coords;
my @cur-pos = 0, 0;
@coords.push(@cur-pos);
my $trench = 0; for @plan -> @line { my$dir = @line[1].trans('0123' => 'RDLU');
my $distance = @line[0].parse-base(16);$trench += $distance; my @new-pos = @cur-pos; given$dir {
when 'L' { @new-pos[0] -= $distance; } when 'R' { @new-pos[0] +=$distance; }
when 'U' { @new-pos[1] -= $distance; } when 'D' { @new-pos[1] +=$distance; }
}
@coords.push(@new-pos);
@cur-pos = @new-pos;
}

my $total = 0; for ^ (+@coords - 1) ->$i {
$total += @coords[$i][0] * @coords[$i + 1][1];$total -= @coords[$i + 1][0] * @coords[$i][1];
}

my $area = (abs($total) + $trench) / 2 + 1; say "Total {$area}";
say "Took " ~ (now - ENTER now).base(10,2) ~ " seconds";
Total 102000662718092
Took 0.08 seconds