+#!/usr/bin/perl -w
+
+use v5.36;
+use strict;
+use experimental 'multidimensional';
+
+my @input = <>;
+
+my (@map, $xmax, $ymax, @units);
+
+sub init_map {
+ @map = map { chomp; [ split // ] } @input;
+ $xmax = $#{ $map[0] };
+ $ymax = $#map;
+
+ @units = ();
+ for my $y (0 .. $ymax) {
+ for my $x (0 .. $xmax) {
+ if ($map[$y][$x] eq 'G') {
+ push @units, [$x, $y, 'G', 200];
+ } elsif ($map[$y][$x] eq 'E') {
+ push @units, [$x, $y, 'E', 200];
+ }
+ }
+ }
+}
+
+init_map;
+
+sub enemy_of($unit) { $unit->[2] eq 'G' ? 'E' : 'G' }
+
+my @neighbors = ([0, -1], [-1, 0], [1, 0], [0, 1]);
+
+sub reading_sort {
+ return sort { $a->[1] == $b->[1] ? $a->[0] <=> $b->[0] : $a->[1] <=> $b->[1] } @_;
+}
+
+sub print_map {
+ for my $y (0 .. $ymax) {
+ for my $x (0 .. $xmax) {
+ print $map[$y][$x];
+ }
+ print "\n";
+ }
+ for my $unit (grep { $_->[3] } reading_sort(@units)) {
+ say "$unit->[2] at $unit->[0],$unit->[1] ($unit->[3])"
+ }
+}
+
+sub neigh_squares($unit, $type) {
+ my @rv;
+ for my $n (@neighbors) {
+ my ($dx, $dy) = @$n;
+ $dx += $unit->[0];
+ $dy += $unit->[1];
+ if ($map[$dy][$dx] eq $type) {
+ push @rv, [ $dx, $dy ];
+ }
+ }
+ return @rv;
+}
+
+sub enemy_within_range($unit) {
+ return neigh_squares($unit, enemy_of($unit));
+}
+
+sub loc2unit($loc) {
+ for my $unit (@units) {
+ return $unit
+ if $unit->[0] == $loc->[0] && $unit->[1] == $loc->[1];
+ }
+ die "Can't locate unit at ", join(',', @$loc);
+}
+
+sub attack($unit, $enemies, $power) {
+ say "Attacking unit: ", join(',', @$unit);
+ my $min_hp;
+ my @min_hp;
+ for my $enemy (@$enemies) {
+ say "Enemy: ", join(',', @$enemy);
+ my $enemy_unit = loc2unit($enemy);
+ if (!defined $min_hp || $min_hp > $enemy_unit->[3]) {
+ $min_hp = $enemy_unit->[3];
+ @min_hp = ();
+ }
+ if ($min_hp == $enemy_unit->[3]) {
+ push @min_hp, $enemy_unit;
+ }
+ }
+
+ @min_hp = reading_sort(@min_hp);
+ my $enemy = shift @min_hp;
+ say "attacking";
+ $enemy->[3] -= $power;
+ if ($enemy->[3] <= 0) {
+ die if $enemy->[2] eq 'E';
+ $enemy->[3] = 0;
+ $map[$enemy->[1]][$enemy->[0]] = '.';
+ }
+ $enemy->[3] = 0 if $enemy->[3] < 0;
+ say "attacked ", join(',', @$enemy);
+}
+
+sub dump_path($path) {
+ join(' ', map { "[$_->[0],$_->[1]]" } @$path);
+}
+
+sub move($unit) {
+ my %target_sq;
+ for my $enemy (grep { $_->[2] ne $unit->[2] } @units) {
+ for my $sq (neigh_squares($enemy, '.')) {
+ if (!$target_sq{$sq->[0],$sq->[1]}++) {
+ # say "target square $sq->[0],$sq->[1]";
+ }
+ }
+ }
+
+ my @q = [ [ $unit->[0], $unit->[1] ] ];
+ my %visited;
+ my $found_at_dist;
+ my @reachable;
+ while (@q) {
+ my $path = shift @q;
+ # say "walking path ", dump_path($path);
+ last if $found_at_dist && @$path > $found_at_dist;
+ for my $sq (neigh_squares($path->[-1], '.')) {
+ next if $visited{$sq->[0],$sq->[1]}++;
+
+ if ($target_sq{$sq->[0],$sq->[1]}) {
+ $found_at_dist //= @$path;
+ my $path1 = [ @$path, [ @$sq ] ];
+ # say "Found square $sq->[0],$sq->[1] through ",
+ # dump_path($path1);
+ push @reachable, [ @$sq, @{ $path1->[1] } ];
+ } else {
+ push @q, [ @$path, [ @$sq ] ];
+ }
+ }
+ }
+
+ if (!@reachable) {
+ # say "no reachable squares";
+ return;
+ }
+
+ @reachable = reading_sort(@reachable);
+ my $target = $reachable[0];
+ # say "moving to $target->[0],$target->[1] via $target->[2],$target->[3]";
+ $map[$unit->[1]][$unit->[0]] = '.';
+ $map[$unit->[1] = $target->[3]][$unit->[0] = $target->[2]] = $unit->[2];
+}
+
+sub unit_round($unit, $attack) {
+ return if !$unit->[3]; # are we alive?
+ # say "\nunit_round ", join(',', @$unit);
+
+ my @enemies = enemy_within_range($unit);
+ if (!@enemies) {
+ move($unit);
+ @enemies = enemy_within_range($unit);
+ }
+ if (@enemies) {
+ attack($unit, \@enemies, $attack);
+ }
+}
+
+sub battle($elf_hp) {
+ my $round = 0;
+ init_map;
+ say "elf_hp: $elf_hp =====================================";
+ my %total_hp;
+ ROUND:
+ while (1) {
+ say "After $round rounds:";
+ print_map;
+ $round++;
+ @units = reading_sort(grep { $_->[3] } @units);
+
+ for my $unit (@units) {
+ next if !$unit->[3];
+ %total_hp = ();
+ for my $u (@units) {
+ $total_hp{$u->[2]} += $u->[3]
+ if $u->[3];
+ }
+ if (keys %total_hp < 2) {
+ $round--;
+ last ROUND;
+ }
+ eval { unit_round($unit, $unit->[2] eq 'E' ? $elf_hp : 3); };
+ return 0 if $@;
+ }
+
+ }
+ say "After $round rounds with hp $elf_hp:";
+ print_map;
+
+ say "elf_hp $elf_hp ended after round $round with hp ", join('=>', %total_hp);
+ say $round * (%total_hp)[1];
+ return 1;
+}
+
+my $elf_hp = 4;
+$elf_hp++ while !battle($elf_hp);
+
+# There is a bug out there - on the real puzzle input it returned
+# remaining hp lower by three, which means there had to be one less Goblin
+# attack somewhere.