Day 11 part 2: rewrite

A simple recursive cached/memoized solution,
runs under 1 second. Oh well.

diff --git a/2024/22.pl b/2024/22.pl
index 5819be5b8872e141b2e82ec5297a55346ca9616c..298dfd74664ab9c63a082f4a10c0bba5045a55ed 100755 (executable)
--- a/2024/22.pl
+++ b/2024/22.pl
@@ -4,57 +4,29 @@ use v5.40;
 
 my @st = <> =~ /\d+/g;
 
-sub step {
-       my (@st) = @_;
-       my @new;
-       for my $s (@st) {
-               my $l = length $s;
-               if (!$s) {
-                       push @new, 1;
-               } elsif ($l % 2 == 0) {
-                       push @new, 0+substr($s, 0, $l/2),
-                               0+substr($s, $l/2);
-               } else {
-                       push @new, 2024*$s;
-               }
-       }
-       @new;
-}
-
-my %seen;
-my %cache5;
-my @q = @st;
-while (defined (my $s = shift @q)) {
-       next if $seen{$s}++;
-       my @rv = ($s);
-       @rv = step(@rv) for 1 .. 5;
-       for my $r (@rv) {
-               if (!$cache5{$s}{$r}++) {
-                       if (!defined $cache5{$r}) {
-                               $cache5{$r} = {};
-                               push @q, $r;
-                       }
-               }
-       }
-}
+my %cache;
+sub stones {
+       my ($s, $steps) = @_;
+       my $k = "$s,$steps";
+       return $cache{$k} if defined $cache{$k};
+       return $cache{$k} = 1 if !$steps;
 
-my %cache = %cache5;
-for (1 .. 14) {
-       my %ncache;
-       for my $s (keys %cache) {
-               for my $d (keys %{ $cache{$s} }){
-                       for my $e (keys %{ $cache5{$d} }) {
-                               $ncache{$s}{$e} += $cache{$s}{$d} * $cache5{$d}{$e};
-                       }
-               }
+       $steps--;
+       my $l = length $s;
+       my $res;
+       if (!$s) {
+               $res = stones(1, $steps);
+       } elsif ($l % 2 == 0) {
+               $res = stones(0+substr($s, 0, $l/2), $steps)
+                    + stones(0+substr($s, $l/2),    $steps);
+       } else {
+               $res = stones(2024*$s, $steps);
        }
-       %cache = %ncache;
+       return $cache{$k} = $res;
 }
 
 my $sum;
 for my $s (@st) {
-       for my $d (keys %{ $cache{$s} }) {
-               $sum += $cache{$s}{$d};
-       }
+       $sum += stones($s, 75);
 }
 say $sum;