Solve day 5.

day05.pl

@@ -0,0 +1,61 @@
+#!/usr/bin/env perl
+
+use strict;
+use warnings;
+use List::Util qw(first reduce);
+
+my @rules = ();
+while (<>) {
+	last if /^\s*$/;
+	chomp;
+	push @rules, [split /\|/];
+}
+
+my @correct_order = ();
+my @incorrect_order = ();
+while (<>) {
+	next if /^\s*$/;
+	chomp;
+	my @pages = split /,/;
+	
+	my $hit = 1;
+	for my $rule (@rules) {
+		my $a = first { $pages[$_] eq $rule->[0] } 0 .. $#pages;
+		my $b = first { $pages[$_] eq $rule->[1] } 0 .. $#pages;
+		
+		if (defined($a) && defined($b) && $a > $b) {
+			$hit = 0;
+			last;
+		}
+	}
+
+	if ($hit) {
+		push @correct_order, \@pages;
+	} else {
+		push @incorrect_order, \@pages;
+	}
+}
+
+print "Part 1: ", reduce { $a + $b } map { $_->[@$_/2] } @correct_order; print "\n";
+my @fixed_incorrect_order = map { fix_order(@$_) } @incorrect_order;
+print "Part 2: ", reduce { $a + $b } map { $_->[@$_/2] } @fixed_incorrect_order; print "\n";
+
+sub fix_order {
+	my @pages = @_;
+
+	my $rule_violated = 1;
+	while ($rule_violated) {
+		$rule_violated = 0;
+		for my $rule (@rules) {
+			my $a = first { $pages[$_] eq $rule->[0] } 0 .. $#pages;
+			my $b = first { $pages[$_] eq $rule->[1] } 0 .. $#pages;
+			
+			if (defined($a) && defined($b) && $a > $b) {
+				@pages[$a, $b] = @pages[$b, $a];
+				$rule_violated = 1;
+			}
+		}
+	}
+
+	return \@pages;
+}