class Sort

class Sort {
    
    /**
     * The JSON:API sort key name.
     *
     * @var string
     */
    const KEY_NAME = 'sort';
    
    /**
     * The field key in the sort parameter: sort[lorem][<field>].
     *
     * @var string
     */
    const PATH_KEY = 'path';
    
    /**
     * The direction key in the sort parameter: sort[lorem][<direction>].
     *
     * @var string
     */
    const DIRECTION_KEY = 'direction';
    
    /**
     * The langcode key in the sort parameter: sort[lorem][<langcode>].
     *
     * @var string
     */
    const LANGUAGE_KEY = 'langcode';
    
    /**
     * The fields on which to sort.
     *
     * @var array
     */
    protected $fields;
    
    /**
     * Constructs a new Sort object.
     *
     * Takes an array of sort fields. Example:
     *   [
     *     [
     *       'path' => 'changed',
     *       'direction' => 'DESC',
     *     ],
     *     [
     *       'path' => 'title',
     *       'direction' => 'ASC',
     *       'langcode' => 'en-US',
     *     ],
     *   ]
     *
     * @param array $fields
     *   The entity query sort fields.
     */
    public function __construct(array $fields) {
        $this->fields = $fields;
    }
    
    /**
     * Gets the root condition group.
     */
    public function fields() {
        return $this->fields;
    }
    
    /**
     * Creates a Sort object from a query parameter.
     *
     * @param mixed $parameter
     *   The `sort` query parameter from the Symfony request object.
     *
     * @return self
     *   A Sort object with defaults.
     */
    public static function createFromQueryParameter($parameter) {
        if (empty($parameter)) {
            $cacheability = (new CacheableMetadata())->addCacheContexts([
                'url.query_args:sort',
            ]);
            throw new CacheableBadRequestHttpException($cacheability, 'You need to provide a value for the sort parameter.');
        }
        // Expand a JSON:API compliant sort into a more expressive sort parameter.
        if (is_string($parameter)) {
            $parameter = static::expandFieldString($parameter);
        }
        // Expand any defaults into the sort array.
        $expanded = [];
        foreach ($parameter as $sort_index => $sort_item) {
            $expanded[$sort_index] = static::expandItem($sort_item);
        }
        return new static($expanded);
    }
    
    /**
     * Expands a simple string sort into a more expressive sort that we can use.
     *
     * @param string $fields
     *   The comma separated list of fields to expand into an array.
     *
     * @return array
     *   The expanded sort.
     */
    protected static function expandFieldString($fields) {
        return array_map(function ($field) {
            $sort = [];
            if ($field[0] == '-') {
                $sort[static::DIRECTION_KEY] = 'DESC';
                $sort[static::PATH_KEY] = substr($field, 1);
            }
            else {
                $sort[static::DIRECTION_KEY] = 'ASC';
                $sort[static::PATH_KEY] = $field;
            }
            return $sort;
        }, explode(',', $fields));
    }
    
    /**
     * Expands a sort item in case a shortcut was used.
     *
     * @param array $sort_item
     *   The raw sort item.
     *
     * @return array
     *   The expanded sort item.
     */
    protected static function expandItem(array $sort_item) {
        $cacheability = (new CacheableMetadata())->addCacheContexts([
            'url.query_args:sort',
        ]);
        $defaults = [
            static::DIRECTION_KEY => 'ASC',
            static::LANGUAGE_KEY => NULL,
        ];
        if (!isset($sort_item[static::PATH_KEY])) {
            throw new CacheableBadRequestHttpException($cacheability, 'You need to provide a field name for the sort parameter.');
        }
        $expected_keys = [
            static::PATH_KEY,
            static::DIRECTION_KEY,
            static::LANGUAGE_KEY,
        ];
        $expanded = array_merge($defaults, $sort_item);
        // Verify correct sort keys.
        if (count(array_diff($expected_keys, array_keys($expanded))) > 0) {
            throw new CacheableBadRequestHttpException($cacheability, 'You have provided an invalid set of sort keys.');
        }
        return $expanded;
    }

}