advent-of-code/2024/day2/index.php

<?php declare(strict_types=1);

function parse_reports(): array
{
	$raw = file_get_contents('./input.txt');
	$lines = explode("\n", trim($raw));
	
	return array_map(fn (string $line) => explode(' ', $line), $lines);
}

class Report {
	public function __construct(
		public bool $allIncreasing,
		public bool $allDecreasing,
		public bool $safeDifference,
	) {}

	public function isSafe(): bool
	{
		return $this->safeDifference && ($this->allIncreasing || $this->allDecreasing);
	}
}

function get_report_safety (array $report): Report {
	$allIncreasing = true;
	$allDecreasing = true;
	$safeDifference = true;

	$safeDiff = function ($curr, $prev): bool {
		$diff = abs((int)$curr - (int)$prev);

		return $diff > 0 && $diff <= 3;
	};
	$increasing = fn ($curr, $prev): bool => (int)$curr > (int)$prev;
	$decreasing = fn ($curr, $prev): bool => (int)$curr < (int)$prev;

	for ($i = 1;$i < count($report); $i++)
	{
		$curr = $report[$i];
		$prev = $report[$i - 1];

		$allIncreasing = $allIncreasing && $increasing($curr, $prev);
		$allDecreasing = $allDecreasing && $decreasing($curr, $prev);
		$safeDifference = $safeDifference && $safeDiff($curr, $prev);
	}

	return new Report($allIncreasing, $allDecreasing, $safeDifference);
}

$reports = parse_reports();
$reports = array_map('get_report_safety', $reports);
$safeReports = count(array_filter($reports, fn (Report $r) => $r->isSafe()));

echo "Number of safe reports: $safeReports\n";