Day 21: examining the input

[aoc.git] / 2023 / 42.pl

diff --git a/2023/42.pl b/2023/42.pl
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index 0000000..ae9152b
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+++ b/2023/42.pl
@@ -0,0 +1,69 @@
+#!/usr/bin/perl -w
+
+use v5.38;
+use experimental 'multidimensional';
+$; = ';';
+
+my @map;
+my ($sx, $sy);
+while (<>) {
+       chomp;
+       if (/S/g) {
+               $sx = pos()-1;
+               $sy = $.-1;
+               s/S/./;
+       }
+       push @map, [ split // ];
+}
+my $xmax = $#{ $map[0] };
+my $ymax = $. - 1;
+
+my @dx = (1, 0, -1, 0);
+my @dy = (0, 1, 0, -1);
+
+my @sums;
+my @oddeven;
+my @todo = [ $sx, $sy ];
+my %seen;
+$seen{$sx,$sy}++;
+
+for my $step (0 .. 1000) {
+       my @ntodo;
+       for my $pt (@todo) {
+               my ($x, $y) = @$pt;
+               $oddeven[$step & 1]++;
+               for my $d (0 .. 3) {
+                       my $nx = $x + $dx[$d];
+                       my $ny = $y + $dy[$d];
+                       next if $map[$ny % ($ymax + 1)][$nx % ($xmax + 1)] ne '.';
+                       next if $seen{$nx,$ny}++;
+                       push @ntodo, [$nx, $ny];
+               }
+       }
+       
+       push @sums, $oddeven[$step & 1];
+       @todo = @ntodo;
+}
+
+my $start = 100;
+my $period = 1;
+my $steps = 26501365;
+
+PERIOD:
+while ($period++) {
+       for my $i ($start .. ($#sums - 3*$period)) {
+               my @d;
+               push @d, $sums[$i+($_+1)*$period]
+                       -$sums[$i+$_*$period]
+                       for 0 .. 2;
+               next PERIOD if $d[1]-$d[0] != $d[2]-$d[1];
+       }
+       last;
+}
+
+$start += ($steps - $start) % $period;
+my $a = $sums[$start+$period] - $sums[$start];
+my $d = $sums[$start+2*$period] - $sums[$start+$period] - $a;
+my $n = ($steps - $start) / $period;
+say $sums[$start] + (2*$a + ($n-1)*$d)*$n/2;
+