class DataHolder {
    /** @var array<string>  */
    private array $items;

    public function __construct(array $items)
    {
        $this->items = $items;
    }

    public function hasSameElsCallable()
    {
        $this->traverseCallable(fn(?string $prev, string $current, ?string $next) => $next !== null && $current === $next);
    }

    public function hasSameElsType()
    {
        $handler = new class implements ItemsTraverse {
            public function traverse(?string $prev, string $current, ?string $next)
            {
                return $next !== null && $current === $next;
            }
        };

        $this->traverseByType($handler);
    }

    /**
     * @psalm-param callable(null|string, string, null|string) $handler
     */
    public function traverseCallable(callable $handler)
    {
        for($i = 0; $i < count($this->parts); $i++) {
            $prev = $parts[$i - 1] ?? null;
            $current = $parts[$i];
            $next = $parts[$i + 1] ?? null;

            $handler($prev, $current, $next);
        }
    }

    public function traverseByType(ItemsTraverse $handler)
    {
        for($i = 0; $i < count($this->parts); $i++) {
            $prev = $parts[$i - 1] ?? null;
            $current = $parts[$i];
            $next = $parts[$i + 1] ?? null;

            $handler->traverse($prev, $current, $next);
        }
    }
}

interface ItemsTraverse {
    public function traverse(?string $prev, string $current, ?string $next);
}