Discussion on: AoC Day 12: Subterranean Sustainability

#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

my $state = <>;
$state =~ s/[^#.]//g;

<>;
my %rule;
while (<>) {
    chomp;
    my ($left, $right) = split /\s*=>\s*/;
    $rule{$left} = $right;
}

my $start_at = 0;
for (1 .. 20) {
    substr $state, 0, 0, '...';
    --$start_at;
    $state .= '...';
    my $new;
    while ($state =~ /(?=(.....))/g) {
        my $before = $1;
        if (exists $rule{$before} && '#' eq $rule{$before}) {
            $new .= '#';
        } else {
            $new .= '.';
        }
    }
    $state = $new;
    $state =~ s/\.+$//;
    $state =~ s/^(\.*)//;
    $start_at += length $1;
}

my $count = 0;
$count += $start_at - 1 + pos $state while $state =~ /#/g;
say $count;

20000a + b = 2041377
20002a + b = 2041581

b = 2041377 - 20000a

20002a + 2041377 - 20000a = 2041581
             2a + 2041377 = 2041581
                       2a = 204
                        a = 102

b = 2041377 - 20000 * 102
b = 1377