14.php

<?php

$time_start = microtime(true);
$input = trim(file_get_contents('14.txt'));

$robots = array_map(function ($line) {
	preg_match_all('/(-?\d+)/', $line, $matches);
	return new Robot((int) $matches[1][0], (int) $matches[1][1], (int) $matches[1][2], (int) $matches[1][3]);
}, explode("\n", $input));


define('WIDTH', 101);
define('HEIGHT', 103);

class Robot {
	public function __construct(
		public int $x,
		public int $y,
		public int $vx,
		public int $vy,
	) {}

	public function quadrant_after_seconds(int $seconds): int
	{
		$x = ($this->x + $seconds * $this->vx) % WIDTH;
		if ($x < 0) $x += WIDTH;

		$y = ($this->y + $seconds * $this->vy) % HEIGHT;
		if ($y < 0) $y += HEIGHT;

		$y_mid = (int) (HEIGHT / 2);
		$x_mid = (int) (WIDTH / 2);

		// ignore robots in exact middle
		if ($x === $x_mid || $y === $y_mid) return 0;
		if ($x < $x_mid) {
			return $y < $y_mid ? 1 : 3;
		}

		return $y < $y_mid ? 2 : 4;
	}

	public function move(int $seconds): void {
		$this->x = ($this->x + $seconds * $this->vx) % WIDTH;
		if ($this->x < 0) $this->x += WIDTH;

		$this->y = ($this->y + $seconds * $this->vy) % HEIGHT;
		if ($this->y < 0) $this->y += HEIGHT;
	}
}


// count robots in each quadrant after 100 seconds
$quadrants = [0, 0, 0, 0, 0];
foreach ($robots as $r) {
	$q = $r->quadrant_after_seconds(100);
	$quadrants[$q]++;
}
$pt1 = $quadrants[1] * $quadrants[2] * $quadrants[3] * $quadrants[4];

// find 1/4 of width adjacent robots
$empty_grid = str_repeat('.', WIDTH * HEIGHT);
$needle = str_repeat('#', WIDTH / 4);
$tree = false;
for ($pt2 = 0; $tree === false; $pt2++) {
	$grid = $empty_grid;
	foreach ($robots as $r) {
		$r->move(1);
		$grid[$r->y * WIDTH + $r->x] = '#';
	}

	$tree = strpos($grid, $needle);
}

echo "--- Day 14 ---", PHP_EOL;
echo "Part 1: ", $pt1, PHP_EOL;
echo "Part 2: ", $pt2, PHP_EOL;

for ($i = 0; $i < WIDTH * HEIGHT; $i++) {
	echo $grid[$i] === "#" ? "\033[32m▓\033[0m" : ' ';
	if ($i % WIDTH === 0) echo PHP_EOL;
}
echo PHP_EOL;

echo "Took ", (microtime(true) - $time_start) * 1000, " ms", PHP_EOL;
echo PHP_EOL;

assert($pt1 === 225648864);
assert($pt2 === 7847);