--- /dev/null
+#!/usr/bin/perl -w
+
+use v5.38;
+use experimental 'multidimensional';
+$; = ';';
+
+my @map;
+my ($sx, $sy);
+
+while (<>) {
+ chomp;
+ if (/S/g) {
+ $sx = pos;
+ $sy = $.;
+ s/S/./;
+ }
+ push @map, [ split //, "#$_#" ];
+}
+my $xmax = $#{ $map[0] };
+my $ymax = $. + 1;
+
+push @map, [ ('#') x ($xmax+1) ];
+unshift @map, [ ('#') x ($xmax+1) ];
+
+my %seen;
+
+my @dx = (1, 0, -1, 0);
+my @dy = (0, 1, 0, -1);
+
+my $steps = 64;
+my @q = ([ $sx, $sy, 0 ]);
+while (@q) {
+ my ($x, $y, $st) = @{ shift @q };
+ # say "$x $y";
+ next if $st >= $steps;
+ for my $d (0 .. 3) {
+ my $nx = $x + $dx[$d];
+ my $ny = $y + $dy[$d];
+ if ($map[$ny][$nx] eq '.') {
+ next if $seen{$nx,$ny,$st+1}++;
+ push @q, [ $nx, $ny, $st+1 ];
+ }
+ }
+}
+
+my $sum;
+for my $y (0 .. $ymax) {
+ for my $x (0 .. $xmax) {
+ print $seen{$x,$y,$steps} ? 'O' : $map[$y][$x];
+ $sum++ if $seen{$x,$y,$steps};
+ }
+ print "\n";
+}
+say $sum;
--- /dev/null
+#!/usr/bin/perl -w
+
+use v5.38;
+use experimental 'multidimensional';
+$; = ';';
+
+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;
+