04.php

<?php
$time_start = microtime(true);
$input = trim(file_get_contents('04.txt'));

// parse as array of strings
// we add some padding so we don't have to bounds check all over the place
$grid = array_map(function($row) {
	return '.' . $row . '.';
}, explode("\n", $input));
$placeholder = str_repeat('.', strlen($grid[0]));
$grid = array_merge([$placeholder], $grid, [$placeholder]);

$dirs = [
	[0, 1], 	// down
	[0, -1], 	// up
	[1, 0], 	// right
	[-1, 0], 	// left
	[1, 1], 	// south-east
	[-1, -1], 	// north-west
	[1, -1], 	// north-east
	[-1, 1], 	// south-west
];

// we use a character that is definitely not in grid as a stop condition
$word = "XMAS!";
$pt1 = 0;
for ($row = 0; $row < count($grid); $row++) {
	for ($col = 0; $col < strlen($grid[$row]); $col++) {
		if ($grid[$row][$col] !== 'X') continue;

		// current grid pos == 'X', search all directions for XMAS
		foreach ($dirs as $d) {
			$char_index = 0;
			$r = $row;
			$c = $col;
			do {
				$r += $d[1];
				$c += $d[0];
				$char_index++;
			} while ($grid[$r][$c] === $word[$char_index]);

			// if we made it to 'S', we found "XMAS"
			if ($char_index === 4) {
				$pt1++;
			}
		}
	}
}

$pt2 = 0;
for ($row = 2; $row < count($grid) - 2; $row++) {
	for ($col = 2; $col < strlen($grid[$row]) - 2; $col++) {
		if ($grid[$row][$col] !== 'A') continue;

		// current grid position === 'A', search diagonals for M S combo
		$nw = $grid[$row - 1][$col - 1];
		$ne = $grid[$row - 1][$col + 1];
		$se = $grid[$row + 1][$col + 1];
		$sw = $grid[$row + 1][$col - 1];
		if (
			(($nw === 'M' && $se === 'S') || ($nw === 'S' && $se === 'M'))
			&& (($ne === 'M' && $sw === 'S') || ($ne === 'S' && $sw === 'M'))
		) {
			$pt2++;
		}
	}
}

echo "--- Day 4: Ceres Search ---", PHP_EOL;
echo "Part 1: ", $pt1, PHP_EOL;
echo "Part 2: ", $pt2, PHP_EOL;
echo "Took ", (microtime(true) - $time_start) * 1000, " ms", PHP_EOL;
echo PHP_EOL;