Get closer to finishing day2 part 2
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Timothy Warren
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@ -38,3 +38,22 @@ So, in this example, `2` reports are **safe**.
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Analyze the unusual data from the engineers. **How many reports are safe?**
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## Part 2
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The engineers are surprised by the low number of safe reports until they realize they forgot to tell you about the Problem Dampener.
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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!
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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.
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More of the above example's reports are now safe:
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- `7 6 4 2 1`: **Safe** without removing any level.
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- `1 2 7 8 9`: **Unsafe** regardless of which level is removed.
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- `9 7 6 2 1`: **Unsafe** regardless of which level is removed.
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- `1 3 2 4 5`: **Safe** by removing the second level, `3`.
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- `8 6 4 4 1`: **Safe** by removing the third level, `4`.
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- `1 3 6 7 9`: **Safe** without removing any level.
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Thanks to the Problem Dampener, `4` reports are actually **safe**!
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Update your analysis by handling situations where the Problem Dampener can remove a single level from unsafe reports. **How many reports are now safe?**
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@ -1,8 +1,8 @@
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<?php declare(strict_types=1);
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function parse_reports(): array
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function parse_reports(string $file = './input.txt'): array
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{
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$raw = file_get_contents('./input.txt');
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$raw = file_get_contents($file);
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$lines = explode("\n", trim($raw));
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return array_map(fn (string $line) => explode(' ', $line), $lines);
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@ -10,45 +10,132 @@ function parse_reports(): array
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class Report {
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public function __construct(
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public bool $allIncreasing,
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public bool $allDecreasing,
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public bool $safeDifference,
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) {}
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public array $rawLevels,
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public array $safeLevels = [],
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private bool $allIncreasing = true,
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private bool $allDecreasing = true,
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private bool $safeDifference = true,
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) {
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$this->rawLevels = array_map(fn ($item) => (int)$item, $this->rawLevels);
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$this->calculateLevelSafety();
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}
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public function isSafe(): bool
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{
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return $this->safeDifference && ($this->allIncreasing || $this->allDecreasing);
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return $this->safeDifference && ($this->allIncreasing xor $this->allDecreasing);
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}
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}
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function get_report_safety (array $report): Report {
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$allIncreasing = true;
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$allDecreasing = true;
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$safeDifference = true;
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$safeDiff = function ($curr, $prev): bool {
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$diff = abs((int)$curr - (int)$prev);
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return $diff > 0 && $diff <= 3;
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};
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$increasing = fn ($curr, $prev): bool => (int)$curr > (int)$prev;
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$decreasing = fn ($curr, $prev): bool => (int)$curr < (int)$prev;
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for ($i = 1;$i < count($report); $i++)
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public function isSafeWithDampener(): bool
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{
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$curr = $report[$i];
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$prev = $report[$i - 1];
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$allIncreasing = $allIncreasing && $increasing($curr, $prev);
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$allDecreasing = $allDecreasing && $decreasing($curr, $prev);
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$safeDifference = $safeDifference && $safeDiff($curr, $prev);
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if ($this->isSafe())
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{
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return true;
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}
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$unsafe = $this->getUnsafeKeys();
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$unsafeCount = count($unsafe);
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foreach ($unsafe as $key)
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{
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$front = array_slice($this->rawLevels, 0, $key);
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$back = array_slice($this->rawLevels, $key + 1);
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$newRawLevels = array_merge($front, $back);
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$newReport = new Report($newRawLevels);
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$isNowSafe = $newReport->isSafe();
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if ($isNowSafe)
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{
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// echo json_encode([
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// 'old' => $this,
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// 'new' => $newReport
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// ], JSON_PRETTY_PRINT);
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return true;
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}
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}
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return false;
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}
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private function getUnsafeKeys(): array
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{
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$out = [];
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foreach ($this->safeLevels as $key => $value)
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{
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if (!$value)
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{
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$out[] = $key;
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}
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}
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return $out;
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}
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private function calculateLevelSafety (): void
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{
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$allIncreasing = true;
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$allDecreasing = true;
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$safeDifference = true;
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$safeLevels = [];
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$safeDiff = function ($curr, $prev): bool {
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$diff = abs($curr - $prev);
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return $diff > 0 && $diff < 4;
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};
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$increasing = fn ($curr, $prev): bool => $curr > $prev;
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$decreasing = fn ($curr, $prev): bool => $curr < $prev;
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for ($i = 1; $i < count($this->rawLevels); $i++)
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{
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$curr = $this->rawLevels[$i];
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$prev = $this->rawLevels[$i - 1];
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$isIncreasing = $increasing($curr, $prev);
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$isDecreasing = $decreasing($curr, $prev);
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$isSafeDiff = $safeDiff($curr, $prev);
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$this->allIncreasing = $this->allIncreasing && $isIncreasing;
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$this->allDecreasing = $this->allDecreasing && $isDecreasing;
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$this->safeDifference = $this->safeDifference && $isSafeDiff;
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$safeLevel = $isSafeDiff && (
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$this->allIncreasing xor
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$this->allDecreasing
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);
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$this->safeLevels[] = $safeLevel;
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}
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}
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return new Report($allIncreasing, $allDecreasing, $safeDifference);
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}
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$reports = parse_reports();
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$reports = array_map('get_report_safety', $reports);
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$safeReports = count(array_filter($reports, fn (Report $r) => $r->isSafe()));
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// ----------------------------------------------------------------------------
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echo "Number of safe reports: $safeReports\n";
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$testReports = array_map(fn (array $line) => new Report($line), parse_reports('./test_input.txt'));
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$testSafeReports = array_filter($testReports, fn (Report $r) => $r->isSafe());
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$testSafeReportsMap = array_map(fn (Report $r) => $r->isSafe(), $testReports);
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assert($testSafeReportsMap == [true, false, false, false, false, true]);
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assert(count($testSafeReports)==2, 'First example has 2 safe reports');
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$testSafeReportsPart2 = array_filter($testReports, fn (Report $r) => $r->isSafeWithDampener());
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$testSafeReportsPart2Map = array_map(fn (Report $r) => $r->isSafeWithDampener(), $testReports);
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assert($testSafeReportsPart2Map == [true, false, false, true, true, true]);
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assert(count($testSafeReportsPart2)==4);
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// ----------------------------------------------------------------------------
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$reports = array_map(fn (array $line) => new Report($line), parse_reports());
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$safeReports = array_filter($reports, fn (Report $r) => $r->isSafe());
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$simpleSafeReports = count($safeReports);
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$allSafeReports = array_filter($reports, fn (Report $r) => $r->isSafeWithDampener());
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$dampenedSafeReports = count($allSafeReports);
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$maybeValid = $dampenedSafeReports > 550
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&& $dampenedSafeReports != 557
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&& $dampenedSafeReports < 572;
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echo "(Part 1) Number of safe reports: $simpleSafeReports\n";
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echo "(Part 2) Number of safe reports with dampener: $dampenedSafeReports\n";
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echo "Part two may be valid: " . $maybeValid . "\n";
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