This is a solution to Advent of Code 2023 day 21, written in Raku.
https://adventofcode.com/2023/day/21
Part One
Starting from the garden plot marked S on your map, how many garden plots could the Elf reach in exactly 64 steps?
constant \size = 131;
my @grid[size;size] = '21-input.txt'.IO.lines>>.comb;
my ($start-pos, $) = @grid.kv.grep(-> $k, $v { $v eq 'S' })[0];
my @deltas = (0-1i), (0+1i), (-1+0i), (1+0i);
my $coords = SetHash.new;
$coords.set($start-pos[0] + $start-pos[1]i);
for ^64 {
my $new-coords = SetHash.new;
for $coords.keys -> $c {
for @deltas -> $d {
my $dc = $c + $d;
if @grid[$dc.im % size; $dc.re % size] ne '#' {
$new-coords.set($dc);
}
}
}
$coords = $new-coords;
}
say +$coords.keys;
say "Took " ~ (now - ENTER now).base(10,2) ~ " seconds";3768 Took 1.06 seconds
Part Two
However, the step count the Elf needs is much larger! Starting from the garden plot marked S on your infinite map, how many garden plots could the Elf reach in exactly 26501365 steps?
constant \size = 11;
my @grid[size;size] = '21-test.txt'.IO.lines>>.comb;
my ($start-pos, $) = @grid.kv.grep(-> $k, $v { $v eq 'S' })[0];
my @deltas = (0-1i), (0+1i), (-1+0i), (1+0i);
my $coords = SetHash.new;
$coords.set($start-pos[0] + $start-pos[1]i);
for ^500 {
my $new-coords = SetHash.new;
for $coords.keys -> $c {
for @deltas -> $d {
my $dc = $c + $d;
if @grid[$dc.im % size; $dc.re % size] ne '#' {
$new-coords.set($dc);
}
}
}
$coords = $new-coords;
}
say +$coords.keys;
# say $coords.keys.map(*.re).min;
# say $coords.keys.map(*.re).max;
# say $coords.keys.map(*.im).min;
# say $coords.keys.map(*.im).max;
say "Took " ~ (now - ENTER now).base(10,2) ~ " seconds";