TL;DR

On with Advent of Code puzzle 14 from 2022: making sand fall and time pass.

This was a nice puzzle, and I feel like I would be spoilering the fun writing about it.

Well, let’s be honest. I’m a lot tired, because I spent most of the last night trying to solve puzzle 19, to no avail. So this year I failed at solving each puzzle in its own day.

This failures are a goldmine. I don’t think I’ll go looking for solutions until I have one by myself, but I actually was pissed way more than I anticipated. So well, yeah… who knows if I’ll make it in time for when I’ll write about that puzzle?

Anyway, back to puzzle 14. To be honest, there’s nothing tricky about it. I went for a brute-forcish approach where I rasterized all segments, and it worked fine.

Reading the inputs is a little exercise in getting array and maps and hyperoperators right:

sub get-inputs ($filename) {
   [
      $filename.IO.lines.map(
         {
            [ .split(/\s*\-\>\s*/).map({[.comb(/\d+/)».Int]}) ]
         }
      )
   ];
}

As we will be dealing with .min and .max later, we have to make sure that they will treat numbers as numbers. Method .comb() gives strings back, hence the explicit casting with ».Int.

Incidentally, I find that I’m always going for Arrays instead of using plain sequences. I’m surely not getting something in the right way.

Part 1 is about finding how many units of sand will get stuck in the different levels until sands starts going off into the pit. The main insight here is that as soon as a unit of sand goes outside the levels bounding box (either downwards or on a side) then everything after that will go for good as well.

In the following function the first loop builds the rasterization of all polygonals, using a hash to represent the field in a sparse way. Raster, but with style!

The second loop is infinite, but not too much (because we will return as soon as the right conditions show up).

This is just the outer loop, though, where new sand units are dropped from the entrance on the top. The inner loop, still undefined, is about making this unit of sand drop down as much as possible, according to the rules. Here is where we check if the unit is still inside the bounds or is gone for good, making the whole search stop.

If the unit stops somewhere, we fix that position with a O and proceed with a new one, and so on.

sub part1 ($inputs) { return 'part 1';
   my $min_x = $inputs.map({$_»[0]}).flat.min;
   my $max_x = $inputs.map({$_»[0]}).flat.max;
   my $min_y = $inputs.map({$_»[1]}).flat.min;
   my $max_y = $inputs.map({$_»[1]}).flat.max;
   my %field;
   for @$inputs -> $poly {
      my ($fx, $fy) = $poly[0].Slip;
      for 1 .. $poly.end -> $i {
         my ($tx, $ty) = $poly[$i].Slip;
         my @xrange = min($fx, $tx) .. max($fx, $tx);
         my @yrange = min($fy, $ty) .. max($fy, $ty);
         for @xrange X @yrange -> ($x, $y) { %field{"$x,$y"} = '#' }
         ($fx, $fy) = $tx, $ty;
      }
   }

   for 1 .. * -> $i {
      my $x = 500;
      my $y = 0;
      loop {
         my $in-bounds = $min_x <= $x <= $max_x && $y <= $max_y;
         return $i - 1 unless $in-bounds;

         my $ny = $y + 1;
         if    %field{"$x,$ny"}:!exists       {      }
         elsif %field{"{$x - 1},$ny"}:!exists { --$x }
         elsif %field{"{$x + 1},$ny"}:!exists { ++$x }
         else                                 { last }
         $y = $ny;
      }
      %field{"$x,$y"} = 'O';
   }
}

Part 2 is a variation on the theme, only this time there’s no falling in a pit but the sand might accumulate “pyramidally” up to closing the entrance. As a matter of fact, we just have to run the same simulation with different boundary and exit conditions:

sub part2 ($inputs) {
   my $min_y = $inputs.map({$_»[1]}).flat.min;
   my $max_y = $inputs.map({$_»[1]}).flat.max;
   $max_y += 2;

   my $min_x = min(500 - $max_y, $inputs.map({$_»[0]}).flat.min);
   my $max_x = max(500 + $max_y, $inputs.map({$_»[0]}).flat.max);

   my %field;
   for @$inputs -> $poly {
      my ($fx, $fy) = $poly[0].Slip;
      for 1 .. $poly.end -> $i {
         my ($tx, $ty) = $poly[$i].Slip;
         my @xrange = min($fx, $tx) .. max($fx, $tx);
         my @yrange = min($fy, $ty) .. max($fy, $ty);
         for @xrange X @yrange -> ($x, $y) { %field{"$x,$y"} = '#' }
         ($fx, $fy) = $tx, $ty;
      }
   }

   %field{"$_,$max_y"} = '#' for $min_x .. $max_x;

   for 1 .. * -> $i {
      my $x = 500;
      my $y = 0;
      loop {
         return $i - 1 if %field{'500,0'}:exists;

         my $ny = $y + 1;
         if    %field{"$x,$ny"}:!exists       {      }
         elsif %field{"{$x - 1},$ny"}:!exists { --$x }
         elsif %field{"{$x + 1},$ny"}:!exists { ++$x }
         else                                 { last }
         $y = $ny;
      }
      %field{"$x,$y"} = 'O';
   }
}

We still have the sort-of rasterization at the beginning, with the addition of the bottom floor that can be represented as an additional segment. It’s potentially infinite in size, but to meet the top entrance closing condition we just need the right amount of floor, i.e. the strict necessary to accomodate a triangular pattern of fallen sand.

The rest… is left as a simple exercise for the reader!

Full solution (with some visualization!).

Stay safe!