Answer ⏱ 7ms
Part 1 :
7173
Part 2 :
291
$row = strpos($input, "\n");
$grid = str_split(str_replace("\n", "", $input));
$pipes = [
"|" => [-$row, $row], "-" => [-1, 1],
"L" => [-$row, 1], "F" => [$row, 1],
"7" => [$row, -1], "J" => [-$row, -1],
"S" => [$row, -$row, -1, 1],
];
function get_next($pos, $grid, $pipes, $seen) {
$next = [];
foreach ($pipes[$grid[$pos]] as $move) $next[] = $pos + $move;
if ($grid[$pos] == "S") foreach ($next as $n)
foreach ($pipes[$grid[$n]] as $move)
if ($n + $move == $pos) return $n;
foreach ($next as $n) if (!isset($seen[$n])) return $n;
return null;
}
// ==================================================
// > PART 1
// ==================================================
$pos = strpos($input, "S") - 1;
$seen = [$pos => ($d = 0)];
while (($pos = get_next($pos, $grid, $pipes, $seen)) != null) {
$seen[$pos] = ++$d;
}
$solution_1 = (count($seen)) / 2;
// ==================================================
// > PART 2
// ==================================================
$solution_2 = 0;
foreach ($grid as $i=>$p) {
if ($i % $row == 0) $o = 0;
if (!isset($seen[$i])) {
if ($o % 2) $solution_2++;
} else {
if ($p == "|") $o += 1;
if ($p == "J" || $p == "F") $o += .5;
if ($p == "L" || $p == "7") $o -= .5;
}
}