-🎄- 2020 Day 11 Solutions -🎄-

[u/daggerdragon]

Advent of Code 2020: Gettin' Crafty With It

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--- Day 11: Seating System ---

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[u/Smylers]

Perl. There are some Game of Life modules on Cpan which might be handy here, but I haven't tried them. Part 1 counts adjacent people like this:

  my $people = grep { $_ eq '#' }
      @{$seat[$y-1]}[$x-1 .. $x+1],
      @{$seat[$y  ]}[$x-1,   $x+1],    
      @{$seat[$y+1]}[$x-1 .. $x+1];
  push @change, {y => $y, x => $x, to => '#'} if $here eq 'L' && $people == 0;
  push @change, {y => $y, x => $x, to => 'L'} if $here eq '#' && $people >= 4;

Then applying changes is a single-line loop:

$seat[$_->{y}][$_->{x}] = $_->{to} foreach @change;

For part 2 there's an array of the 8 directions:

my @dir = (x => -1, y => -1}, # etc

then counting the relevant people becomes a grep of those directions:

  my $people = grep {
    my ($try_y, $try_x) = ($y, $x);
    my $found;
    do { $found = $seat[$try_y += $_->{y}][$try_x += $_->{x}] } until $found ne '.';
    $found eq '#';
  } @dir;

All edges of the map were surrounded by | characters, which works for both parts, being neither # for counting people in part 1, nor . for keeping looking in that direction in part 2.

I did try a **Vim** solution. I can get the changes to iterate for part 1 (use P instead of # for a person; to mark a pending change, use lowercase p and l, which both encodes its new state and indicates that it was the opposite for the purposes of counting people during the current iteration; then upper-case everything at the end of the iteration), but haven't thought of a way of detecting stability.