Day 14 part 2: pure regex-based solution

diff --git a/2023/28.pl b/2023/28.pl
index 0699eb466166399800d4de012419d68b218bd3c8..2d147f4829c4d98ef73fae13860894cf93a4745d 100755 (executable)
--- a/2023/28.pl
+++ b/2023/28.pl
@@ -2,48 +2,24 @@
 
 use v5.38;
 
-my @map = map { chomp; [ split // ] } <>;
-my $xmax = $#{ $map[0] };
-my $ymax = $#map;
-
-my %seen;
-my @score;
-my $round;
-my $key;
+my $map; { local $/; $map = <>; }
 
+my (@score, %seen, $round);
 while (++$round) {
        for (1 .. 4) {
-               for my $y (0 .. $ymax) {
-                       for my $x (0 .. $xmax) {
-                               next if $map[$y][$x] ne 'O';
-                               my $y1 = $y;
-                               while ($y1 && $map[$y1-1][$x] eq '.') {
-                                       $map[$y1][$x] = '.';
-                                       $map[--$y1][$x] = 'O';
-                               }
-                       }
-               }
-
-               @map = map { # rotate clockwise
-                       my $x = $_;
-                       [ map { $map[$_][$x] } reverse 0 .. $ymax ]
-               } 0 .. $xmax;
-               ($xmax, $ymax) = ($ymax, $xmax);
-       }
-
-       $key = join("\n", map { join('', @$_) } @map);
-       last if $seen{$key};
-
-       my $sum = 0;
-       for my $y (0 .. $ymax) {
-               for my $x (0 .. $xmax) {
-                       $sum += $ymax + 1 - $y if $map[$y][$x] eq 'O';
-               }
+               my $map1;
+               do { # rotate the map:
+                       $map1 .= join('', reverse $map =~ /^./gm) . "\n"
+               } while $map =~ s/^.//gm;
+               chomp ($map = $map1);
+               1 while $map =~ s/O(\.+)/$1O/g; # move O's to the right
        }
+       $score[$round] +=()= substr($map, pos($map)) =~ /\n/g
+               while $map =~ /O/g;
 
-       $seen{$key} = $round;
-       $score[$round] = $sum;
+       last if $seen{$map};
+       $seen{$map} = $round;
 }
 
-my $remains = (1000000000 - $round) % ($round - $seen{$key});
-say $score[$seen{$key} + $remains];
+my $remains = (1000000000 - $round) % ($round - $seen{$map});
+say $score[$seen{$map} + $remains];