Moved 2021 to a subdir

[aoc.git] / 2021 / 28.pl

diff --git a/2021/28.pl b/2021/28.pl
new file mode 100755 (executable)
index 0000000..d07c23c
--- /dev/null
+++ b/2021/28.pl
@@ -0,0 +1,35 @@
+#!/usr/bin/perl -w
+
+use v5.16;
+
+chomp (my $tmpl = <>);
+scalar <>;
+
+my %rules;
+while (<>) {
+       chomp;
+       my ($s1, $s2, $d) = /[A-Z]/g;
+       $rules{"$s1$s2"} = [ "$s1$d", "$d$s2", $d ];
+}
+
+my (%pairs, %count);
+for my $i (0 .. length($tmpl) - 2) {
+       $pairs{substr($tmpl, $i, 2)}++;
+       $count{substr($tmpl, $i, 1)}++;
+}
+$count{substr($tmpl, -1, 1)}++;
+
+for (1 .. 40) {
+       my %newp;
+       for my $pair (grep { $rules{$_} } keys %pairs) {
+               my $rule = $rules{$pair};
+                $newp{ $rule->[0] } += $pairs{$pair};
+                $newp{ $rule->[1] } += $pairs{$pair};
+               $count{ $rule->[2] } += $pairs{$pair};
+       }
+       %pairs = %newp;
+}
+
+my @count = sort { $a <=> $b } values %count;
+say $count[-1] - $count[0];
+