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

<?php declare(strict_types=1);

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

class Report {
	public function __construct(
		public array $rawLevels,
		public array $safeLevels = [],
		private bool $allIncreasing = true,
		private bool $allDecreasing = true,
		private bool $safeDifference = true,
	) {
		$this->rawLevels = array_map(fn ($item) => (int)$item, $this->rawLevels);
		$this->calculateLevelSafety();
	}
	
	public function isSafe(): bool
	{
		return $this->safeDifference && ($this->allIncreasing xor $this->allDecreasing);
	}
	
	public function isSafeWithDampener(): bool
	{
		if ($this->isSafe())
		{
			return true;
		}
		
		$unsafe = $this->getUnsafeKeys();
		$unsafeCount = count($unsafe);
		
		foreach ($unsafe as $key)
		{
			$front = array_slice($this->rawLevels, 0, $key);
			$back = array_slice($this->rawLevels, $key + 1);
			$newRawLevels = array_merge($front, $back);
			$newReport = new Report($newRawLevels);
			
			$isNowSafe = $newReport->isSafe();
			if ($isNowSafe)
			{
				// echo json_encode([
				// 	'old' => $this,
				// 	'new' => $newReport
				// ], JSON_PRETTY_PRINT);
				
				return true;
			}
		}
		
		return false;
	}
	
	private function getUnsafeKeys(): array
	{
		$out = [];
		foreach ($this->safeLevels as $key => $value)
		{
			if (!$value)
			{
				$out[] = $key;
			}
		}
		
		return $out;
	}
	
	private function calculateLevelSafety (): void
	{
		$allIncreasing = true;
		$allDecreasing = true;
		$safeDifference = true;
		$safeLevels = [];
		
		$safeDiff = function ($curr, $prev): bool {
			$diff = abs($curr - $prev);
		
			return $diff > 0 && $diff < 4;
		};
		$increasing = fn ($curr, $prev): bool => $curr > $prev;
		$decreasing = fn ($curr, $prev): bool => $curr < $prev;
			
		for ($i = 1; $i < count($this->rawLevels); $i++)
		{
			$curr = $this->rawLevels[$i];
			$prev = $this->rawLevels[$i - 1];
			
			$isIncreasing = $increasing($curr, $prev);
			$isDecreasing = $decreasing($curr, $prev);
			$isSafeDiff = $safeDiff($curr, $prev);
			
			$this->allIncreasing = $this->allIncreasing && $isIncreasing;
			$this->allDecreasing = $this->allDecreasing && $isDecreasing;
			$this->safeDifference = $this->safeDifference && $isSafeDiff;
			
			$safeLevel = $isSafeDiff && (
				$this->allIncreasing xor
				$this->allDecreasing
			);
			$this->safeLevels[] = $safeLevel;
		}
	}
}

// ----------------------------------------------------------------------------

$testReports = array_map(fn (array $line) => new Report($line), parse_reports('./test_input.txt'));

$testSafeReports = array_filter($testReports, fn (Report $r) => $r->isSafe());
$testSafeReportsMap = array_map(fn (Report $r) => $r->isSafe(), $testReports);
assert($testSafeReportsMap == [true, false, false, false, false, true]);
assert(count($testSafeReports)==2, 'First example has 2 safe reports');

$testSafeReportsPart2 = array_filter($testReports, fn (Report $r) => $r->isSafeWithDampener());
$testSafeReportsPart2Map = array_map(fn (Report $r) => $r->isSafeWithDampener(), $testReports);
assert($testSafeReportsPart2Map == [true, false, false, true, true, true]);
assert(count($testSafeReportsPart2)==4);


// ----------------------------------------------------------------------------

$reports = array_map(fn (array $line) => new Report($line), parse_reports());
$safeReports = array_filter($reports, fn (Report $r) => $r->isSafe());
$simpleSafeReports = count($safeReports);
$allSafeReports = array_filter($reports, fn (Report $r) => $r->isSafeWithDampener());

$dampenedSafeReports = count($allSafeReports);

$maybeValid = $dampenedSafeReports > 550 
	&& $dampenedSafeReports != 557
	&& $dampenedSafeReports < 572;

echo "(Part 1) Number of safe reports: $simpleSafeReports\n";
echo "(Part 2) Number of safe reports with dampener: $dampenedSafeReports\n";
echo "Part two may be valid: " . $maybeValid . "\n";