adventofcode19.pl

#!/usr/bin/env perl

use v5.18;
use strict;

our %rules;

sub compile_rule {
	state %compiled;

	my $idx = shift;
	return $compiled{$idx} if exists $compiled{$idx};

	my $rule = $rules{$idx};
	if ($rule =~ /^\"(.)\"$/) {
		return $1;
	}

	my @parts = split /\s*\|\s*/, $rule;

	my @compiled_parts;
	foreach my $part (@parts) {
		my @deps = split /\s+/, $part;
		push @compiled_parts, (join '', map { compile_rule($_) } @deps);
	}
	my $res = join '|', @compiled_parts;
	$res = "(?:$res)";
	$compiled{$idx} = $res;
	$res
}

while (my $rule = <>) {
	chomp $rule;
	last unless $rule;

	my ($idx, $body) = split /: /, $rule;
	$rules{$idx} = $body;
}

my $reg = compile_rule 0;
say STDERR "Generated regex (@{[length $reg]} characters)";
$reg = qr/^$reg$/;

my $count;
while (<>) {
	$count++ if /$reg/;
}

say $count;