<?php

class ImagePrefetcher implements Prefetcher
{
    const ROOT_DIR = 'root';
    /**
     * @var DocumentManager
     */
    private $manager;

    public function __construct(DocumentManager $manager)
    {
        $this->manager = $manager;
    }

    private function fetchDir($dirName, array $files)
    {
        $item = [
            'name' => $dirName,
            'images' => [],
        ];
        /** @var File $file */
        foreach ($files as $file) {
            $item['images'][] = [
                'url' => $file->getUrl(),
                'name' => $file->getName(),
            ];
        }
        
        return $item;
    }

    public function prefetch()
    {
        $collection = $this->manager->getChildren();
        $response = [];
        $root = $this->fetchDir(self::ROOT_DIR, $collection->getFiles());

        foreach ($collection->getDirs() as $dir) {
            $col = $this->manager->getChildren($dir->getId());
            $files = $col->getFiles();
            if (!count($files)) {
                continue;
            }
            $item = $this->fetchDir($dir->getName(), $files);
            $root['images'] = array_merge($root['images'], $item['images']);
            $response[] = $item;
        }
        array_unshift($response, $root);

        return count($root['images']) ? $response : [];
    }

}