Year 2018

[aoc.git] / 2018 / 28.pl

diff --git a/2018/28.pl b/2018/28.pl
new file mode 100755 (executable)
index 0000000..d60cd89
--- /dev/null
+++ b/2018/28.pl
@@ -0,0 +1,27 @@
+#!/usr/bin/perl -w
+
+use v5.20;
+use strict;
+
+my $input = shift @ARGV;
+
+my $r = '37';
+my @p = qw(0 1);
+
+my $l = length $input;
+my $lr = length $r;
+while (1) {
+       my @n = (substr($r, $p[0], 1), substr($r, $p[1], 1));
+       $r .= $n[0]+$n[1];
+       $lr = length $r;
+       for (0 .. 1) {
+               $p[$_] += 1 + $n[$_];
+               $p[$_] -= $lr while $p[$_] >= $lr;
+       }
+       next if $lr < $l;
+       last if substr($r, -$l) eq $input;
+       last if substr($r, -$l-1, -1) eq $input;
+}
+
+$r =~ s/$input.*//;
+say length $r;