Advent of Code 2023 - Day 9

day9.php

<?php

use Illuminate\Support\Collection;
use Illuminate\Support\Str;
use Illuminate\Support\Stringable;

$input = "0 3 6 9 12 15
1 3 6 10 15 21
10 13 16 21 30 45";

$histories = Str::of($input)
    ->explode(PHP_EOL)
    ->mapInto(Stringable::class)
    ->map(static function (Stringable $report) {
        return $report
            ->explode(' ')
            ->map(static fn (string $measurement): int => intval($measurement));
    })
    ->mapInto(History::class);

final readonly class History
{
    public function __construct(
        private Collection $sequence,
    ) {
    }

    public function predictFirst(): int
    {
        return $this->process(new Collection([$this->sequence->reverse()]));
    }

    public function predictLast(): int
    {
        return $this->process(new Collection([$this->sequence]));
    }

    /**
     * @param Collection<array-key, Collection<array-key, int>> $groups
     */
    private function process(Collection $groups): int
    {
        if ($groups->last()->every(static fn (int $difference): bool => $difference === 0)) {
            return $groups
                ->reduce(static fn (int $prediction, Collection $sequence): int => $sequence->last() + $prediction, 0);
        }

        return $this->process(
            $groups->push(
                $groups
                    ->last()
                    ->eachCons(2)
                    ->map(static fn (Collection $window): int => $window->last() - $window->first()),
            ),
        );
    }
}

$part1 = $histories->map(static fn (History $history): int => $history->predictLast())->sum();
$part2 = $histories->map(static fn (History $history): int => $history->predictFirst())->sum();

$result = [$part1, $part2];