Advent of Code 2023 - Day 8

day8.php

<?php

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

$input = "LR

11A = (11B, XXX)
11B = (XXX, 11Z)
11Z = (11B, XXX)
22A = (22B, XXX)
22B = (22C, 22C)
22C = (22Z, 22Z)
22Z = (22B, 22B)
XXX = (XXX, XXX)";

final readonly class Node
{
    public function __construct(
        public string $current,
        public string $left,
        public string $right,
    ) {
    }

    public function isStart(): bool
    {
        return str_ends_with($this->current, 'A');
    }

    public function isDestination(): bool
    {
        return str_ends_with($this->current, 'Z');
    }

    public function next(string $instruction): string
    {
        return match ($instruction) {
            'L' => $this->left,
            'R' => $this->right,
            default => throw new LogicException(),
        };
    }
}

/** @var Collection<array-key, Stringable> $lines */
$lines = Str::of($input)
    ->split("/\n/")
    ->filter()
    ->mapInto(Stringable::class);

$instructions = new InfiniteIterator(
    $lines
        ->shift()
        ->matchAll("/L|R/")
        ->getIterator(),
);

/** @var Collection<string, Node> $nodes */
$nodes = $lines
    ->map(static fn (Stringable $node) => new Node(...$node->matchAll('/[0-9A-Z]+/')))
    ->keyBy(static fn (Node $node): string => $node->current);

$startNodes = $nodes
    ->filter(static fn (Node $node): bool => $node->isStart())
    ->map(static function (Node $node) use ($instructions, $nodes): int {
        $instructions->rewind();
        $steps = 0;

        do {
            $position = $node->next($instructions->current());
            $node = $nodes->get($position);
            $instructions->next();
            $steps++;
        } while ($instructions->valid() && ! $node->isDestination());

        return $steps;
    })
    ->reduce(static fn (int $lcm, int $steps): int => intval(gmp_lcm($lcm, $steps)), 1);