2022/18.pl

   1 #!/usr/bin/perl -w
   2
   3 use v5.36;
   4 use strict;
   5 use experimental 'multidimensional';
   6
   7 my %seen = ("0,0" => 1);
   8 my @knots;
   9 push @knots, [0, 0] for 1 .. 10;
  10
  11 sub follow ($x, $y, $tx, $ty) {
  12         if ($x == $tx) {
  13                 if ($y == $ty - 2) {
  14                         $ty--;
  15                 } elsif ($y == $ty + 2) {
  16                         $ty++;
  17                 }
  18         } elsif ($y == $ty) {
  19                 if ($x == $tx - 2) {
  20                         $tx--;
  21                 } elsif ($x == $tx + 2) {
  22                         $tx++;
  23                 }
  24         } elsif ($x == $tx - 2) {
  25                 $tx--;
  26                 if ($y > $ty) {
  27                         $ty++;
  28                 } else {
  29                         $ty--
  30                 }
  31         } elsif ($x == $tx + 2) {
  32                 $tx++;
  33                 if ($y > $ty) {
  34                         $ty++;
  35                 } else {
  36                         $ty--;
  37                 }
  38         } elsif ($y == $ty - 2) {
  39                 $ty--;
  40                 if ($x > $tx) {
  41                         $tx++;
  42                 } else {
  43                         $tx--;
  44                 }
  45         } elsif ($y == $ty + 2) {
  46                 $ty++;
  47                 if ($x > $tx) {
  48                         $tx++;
  49                 } else {
  50                         $tx--;
  51                 }
  52         }
  53         ($tx, $ty);
  54 }
  55
  56 while (<>) {
  57         my ($dir, $len) = /^(.) (\d+)/;
  58
  59         while ($len--) {
  60                 if ($dir eq 'U') {
  61                         $knots[0][1]--;
  62                 } elsif ($dir eq 'D') {
  63                         $knots[0][1]++;
  64                 } elsif ($dir eq 'L') {
  65                         $knots[0][0]--;
  66                 } elsif ($dir eq 'R') {
  67                         $knots[0][0]++;
  68                 }
  69
  70                 for (1 .. $#knots) {
  71                         ($knots[$_][0], $knots[$_][1]) = follow(
  72                                 @{ $knots[$_-1] },
  73                                 @{ $knots[$_] }
  74                         );
  75                 }
  76
  77                 $seen{"$knots[-1][0],$knots[-1][1]"}++;
  78         }
  79 }
  80
  81 say scalar keys %seen;
  82