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Day13.php
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Day13.php
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<?php
declare(strict_types=1);
namespace App\Days;
use App\Contracts\Day;
use Illuminate\Support\Collection;
class Day13 extends Day
{
public const EXAMPLE1 = <<<eof
[1,1,3,1,1]
[1,1,5,1,1]
[[1],[2,3,4]]
[[1],4]
[9]
[[8,7,6]]
[[4,4],4,4]
[[4,4],4,4,4]
[7,7,7,7]
[7,7,7]
[]
[3]
[[[]]]
[[]]
[1,[2,[3,[4,[5,6,7]]]],8,9]
[1,[2,[3,[4,[5,6,0]]]],8,9]
eof;
/**
* Solve Part 1 of the day's problem.
*/
public function solvePart1(mixed $input): int|string|null
{
return $this->parseInput($input)
// compare each pair and return the comparison result
->map(fn (array $pair): int => $this->compare($pair[0], $pair[1]))
// filter for the pairs that are in the right order
->filter(fn (int $comparison): bool => -1 === $comparison)
// get the keys of the pairs that are in the right order
->keys()
// map the keys to their position in the array + 1 (since we want 1-indexed positions)
->map(fn (int $key): int => $key + 1)
// sum the keys
->sum();
}
/**
* Solve Part 2 of the day's problem.
*/
public function solvePart2(mixed $input): int|string|null
{
return $this->parseInput($input)
// flattern the array so it's no longer in pairs
->flatten(1)
// add the divider packets
->push([[2]], [[6]])
// sort the packets using the compare function
->sort(fn (array $left, array $right): int => $this->compare($left, $right))
->values()
// filter for the divider packets
->filter(fn (array $item): bool => $item === [[2]] || $item === [[6]])
// get the keys of the divider packets
->keys()
// map the keys to their position in the array + 1 (since we want 1-indexed positions)
->map(fn (int $key): int => $key + 1)
// reduce the keys to a single value by multiplying them together
->reduce(fn (int $carry, int $item): int => $carry * $item, 1);
}
/**
* Recursively compare two arrays and returns 1, -1 or 0 following the <=> spaceship operator rules.
* Using the following rules:
* - If both values are integers, compare them directly.
* - If both values are arrays, recursively compare each element.
* - If one value is an integer and the other is an array, convert the integer to an array and compare each element.
* @return int -1 if the left array is less than the right array, 1 if the left array is greater than the right array, 0 if they are equal.
*/
protected function compare(array $left, array $right): int
{
while (true) {
if (empty($left) && empty($right)) {
return 0;
}
if (empty($left)) {
return -1;
}
if (empty($right)) {
return 1;
}
$leftItem = array_shift($left);
$rightItem = array_shift($right);
// compare two integers, if they are not equal, return the comparison result
if (is_int($leftItem) && is_int($rightItem)) {
if (0 !== ($comparison = $leftItem <=> $rightItem)) {
return $comparison;
}
} elseif (is_array($leftItem) && is_array($rightItem)) {
// compare two arrays, if they are not equal, return the comparison result
if (0 !== ($comparison = $this->compare($leftItem, $rightItem))) {
return $comparison;
}
} else {
// compare an integer to an array
if (is_int($leftItem)) {
$leftItem = [$leftItem];
} elseif (is_int($rightItem)) {
$rightItem = [$rightItem];
}
// compare the two arrays, if they are not equal, return the comparison result
if (0 !== ($comparison = $this->compare($leftItem, $rightItem))) {
return $comparison;
}
}
}
}
/**
* Parse the input data.
*/
protected function parseInput(mixed $input): Collection
{
$input = is_array($input) ? $input : explode("\n", $input);
return collect($input)
->map(fn ($line) => trim($line))
->chunkWhile(fn ($value) => '' !== $value)
->map(fn ($chunk) => $chunk->filter(fn ($value) => '' !== $value))
->map(fn ($chunk) => $chunk->map(fn ($line) => json_decode($line, true))->values()->toArray())
->values();
}
}