Get closer to finishing day2 part 2

2024/day2/README.md

@@ -38,3 +38,22 @@ So, in this example, `2` reports are **safe**.
 Analyze the unusual data from the engineers. **How many reports are safe?**
 
 ## Part 2
+
+The engineers are surprised by the low number of safe reports until they realize they forgot to tell you about the Problem Dampener.
+
+The Problem Dampener is a reactor-mounted module that lets the reactor safety systems **tolerate a single bad level** in what would otherwise be a safe report. It's like the bad level never happened!
+
+Now, the same rules apply as before, except if removing a single level from an unsafe report would make it safe, the report instead counts as safe.
+
+More of the above example's reports are now safe:
+
+- `7 6 4 2 1`: **Safe** without removing any level.
+- `1 2 7 8 9`: **Unsafe** regardless of which level is removed.
+- `9 7 6 2 1`: **Unsafe** regardless of which level is removed.
+- `1 3 2 4 5`: **Safe** by removing the second level, `3`.
+- `8 6 4 4 1`: **Safe** by removing the third level, `4`.
+- `1 3 6 7 9`: **Safe** without removing any level.
+
+Thanks to the Problem Dampener, `4` reports are actually **safe**!
+
+Update your analysis by handling situations where the Problem Dampener can remove a single level from unsafe reports. **How many reports are now safe?**

2024/day2/index.php

@@ -1,8 +1,8 @@
 <?php declare(strict_types=1);
 
-function parse_reports(): array
+function parse_reports(string $file = './input.txt'): array
 {
-	$raw = file_get_contents('./input.txt');
+	$raw = file_get_contents($file);
 	$lines = explode("\n", trim($raw));
 	
 	return array_map(fn (string $line) => explode(' ', $line), $lines);
@@ -10,45 +10,132 @@ function parse_reports(): array
 
 class Report {
 	public function __construct(
-		public bool $allIncreasing,
+		public array $rawLevels,
-		public bool $allDecreasing,
+		public array $safeLevels = [],
-		public bool $safeDifference,
+		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 || $this->allDecreasing);
+		return $this->safeDifference && ($this->allIncreasing xor $this->allDecreasing);
 	}
-}
 	
-function get_report_safety (array $report): Report {
+	public function isSafeWithDampener(): bool
+	{
+		if ($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 {
-		$diff = abs((int)$curr - (int)$prev);
+			$diff = abs($curr - $prev);
 		
-		return $diff > 0 && $diff <= 3;
+			return $diff > 0 && $diff < 4;
 		};
-	$increasing = fn ($curr, $prev): bool => (int)$curr > (int)$prev;
+		$increasing = fn ($curr, $prev): bool => $curr > $prev;
-	$decreasing = fn ($curr, $prev): bool => (int)$curr < (int)$prev;
+		$decreasing = fn ($curr, $prev): bool => $curr < $prev;
 			
-	for ($i = 1;$i < count($report); $i++)
+		for ($i = 1; $i < count($this->rawLevels); $i++)
 		{
-		$curr = $report[$i];
+			$curr = $this->rawLevels[$i];
-		$prev = $report[$i - 1];
+			$prev = $this->rawLevels[$i - 1];
 			
-		$allIncreasing = $allIncreasing && $increasing($curr, $prev);
+			$isIncreasing = $increasing($curr, $prev);
-		$allDecreasing = $allDecreasing && $decreasing($curr, $prev);
+			$isDecreasing = $decreasing($curr, $prev);
-		$safeDifference = $safeDifference && $safeDiff($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[] = $safeLevel;
+		}
 	}
-
-	return new Report($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: $safeReports\n";
+$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";