class Filter

class Filter {
    
    /**
     * The JSON:API filter key name.
     *
     * @var string
     */
    const KEY_NAME = 'filter';
    
    /**
     * The key for the implicit root group.
     */
    const ROOT_ID = '@root';
    
    /**
     * Key in the filter[<key>] parameter for conditions.
     *
     * @var string
     */
    const CONDITION_KEY = 'condition';
    
    /**
     * Key in the filter[<key>] parameter for groups.
     *
     * @var string
     */
    const GROUP_KEY = 'group';
    
    /**
     * Key in the filter[<id>][<key>] parameter for group membership.
     *
     * @var string
     */
    const MEMBER_KEY = 'memberOf';
    
    /**
     * The root condition group.
     *
     * @var string
     */
    protected $root;
    
    /**
     * Constructs a new Filter object.
     *
     * @param \Drupal\jsonapi\Query\EntityConditionGroup $root
     *   An entity condition group which can be applied to an entity query.
     */
    public function __construct(EntityConditionGroup $root) {
        $this->root = $root;
    }
    
    /**
     * Gets the root condition group.
     */
    public function root() {
        return $this->root;
    }
    
    /**
     * Applies the root condition to the given query.
     *
     * @param \Drupal\Core\Entity\Query\QueryInterface $query
     *   The query for which the condition should be constructed.
     *
     * @return \Drupal\Core\Entity\Query\ConditionInterface
     *   The compiled entity query condition.
     */
    public function queryCondition(QueryInterface $query) {
        $condition = $this->buildGroup($query, $this->root());
        return $condition;
    }
    
    /**
     * Applies the root condition to the given query.
     *
     * @param \Drupal\Core\Entity\Query\QueryInterface $query
     *   The query to which the filter should be applied.
     * @param \Drupal\jsonapi\Query\EntityConditionGroup $condition_group
     *   The condition group to build.
     *
     * @return \Drupal\Core\Entity\Query\ConditionInterface
     *   The query with the filter applied.
     */
    protected function buildGroup(QueryInterface $query, EntityConditionGroup $condition_group) {
        // Create a condition group using the original query.
        switch ($condition_group->conjunction()) {
            case 'AND':
                $group = $query->andConditionGroup();
                break;
            case 'OR':
                $group = $query->orConditionGroup();
                break;
        }
        // Get all children of the group.
        $members = $condition_group->members();
        foreach ($members as $member) {
            // If the child is simply a condition, add it to the new group.
            if ($member instanceof EntityCondition) {
                if ($member->operator() == 'IS NULL') {
                    $group->notExists($member->field());
                }
                elseif ($member->operator() == 'IS NOT NULL') {
                    $group->exists($member->field());
                }
                else {
                    $group->condition($member->field(), $member->value(), $member->operator());
                }
            }
            elseif ($member instanceof EntityConditionGroup) {
                // Add the subgroup to this new group.
                $subgroup = $this->buildGroup($query, $member);
                $group->condition($subgroup);
            }
        }
        // Return the constructed group so that it can be added to the query.
        return $group;
    }
    
    /**
     * Creates a Sort object from a query parameter.
     *
     * @param mixed $parameter
     *   The `filter` query parameter from the Symfony request object.
     * @param \Drupal\jsonapi\ResourceType\ResourceType $resource_type
     *   The JSON:API resource type.
     * @param \Drupal\jsonapi\Context\FieldResolver $field_resolver
     *   The JSON:API field resolver.
     *
     * @return self
     *   A Sort object with defaults.
     */
    public static function createFromQueryParameter($parameter, ResourceType $resource_type, FieldResolver $field_resolver) {
        $expanded = static::expand($parameter);
        foreach ($expanded as &$filter_item) {
            if (isset($filter_item[static::CONDITION_KEY][EntityCondition::PATH_KEY])) {
                $unresolved = $filter_item[static::CONDITION_KEY][EntityCondition::PATH_KEY];
                $operator = $filter_item[static::CONDITION_KEY][EntityCondition::OPERATOR_KEY];
                $filter_item[static::CONDITION_KEY][EntityCondition::PATH_KEY] = $field_resolver->resolveInternalEntityQueryPath($resource_type, $unresolved, $operator);
            }
        }
        return new static(static::buildEntityConditionGroup($expanded));
    }
    
    /**
     * Expands any filter parameters using shorthand notation.
     *
     * @param array $original
     *   The unexpanded filter data.
     *
     * @return array
     *   The expanded filter data.
     */
    protected static function expand(array $original) {
        $expanded = [];
        foreach ($original as $key => $item) {
            // Allow extreme shorthand filters, f.e. `?filter[promote]=1`.
            if (!is_array($item)) {
                $item = [
                    EntityCondition::VALUE_KEY => $item,
                ];
            }
            // Throw an exception if the query uses the reserved filter id for the
            // root group.
            if ($key == static::ROOT_ID) {
                $msg = sprintf("'%s' is a reserved filter id.", static::ROOT_ID);
                throw new \UnexpectedValueException($msg);
            }
            // Add a memberOf key to all items.
            if (isset($item[static::CONDITION_KEY][static::MEMBER_KEY])) {
                $item[static::MEMBER_KEY] = $item[static::CONDITION_KEY][static::MEMBER_KEY];
                unset($item[static::CONDITION_KEY][static::MEMBER_KEY]);
            }
            elseif (isset($item[static::GROUP_KEY][static::MEMBER_KEY])) {
                $item[static::MEMBER_KEY] = $item[static::GROUP_KEY][static::MEMBER_KEY];
                unset($item[static::GROUP_KEY][static::MEMBER_KEY]);
            }
            else {
                $item[static::MEMBER_KEY] = static::ROOT_ID;
            }
            // Add the filter id to all items.
            $item['id'] = $key;
            // Expands shorthand filters.
            $expanded[$key] = static::expandItem($key, $item);
        }
        return $expanded;
    }
    
    /**
     * Expands a filter item in case a shortcut was used.
     *
     * Possible cases for the conditions:
     *   1. filter[uuid][value]=1234.
     *   2. filter[0][condition][field]=uuid&filter[0][condition][value]=1234.
     *   3. filter[uuid][condition][value]=1234.
     *   4. filter[uuid][value]=1234&filter[uuid][group]=my_group.
     *
     * @param string $filter_index
     *   The index.
     * @param array $filter_item
     *   The raw filter item.
     *
     * @return array
     *   The expanded filter item.
     */
    protected static function expandItem($filter_index, array $filter_item) {
        if (isset($filter_item[EntityCondition::VALUE_KEY])) {
            if (!isset($filter_item[EntityCondition::PATH_KEY])) {
                $filter_item[EntityCondition::PATH_KEY] = $filter_index;
            }
            $filter_item = [
                static::CONDITION_KEY => $filter_item,
                static::MEMBER_KEY => $filter_item[static::MEMBER_KEY],
            ];
        }
        if (!isset($filter_item[static::CONDITION_KEY][EntityCondition::OPERATOR_KEY])) {
            $filter_item[static::CONDITION_KEY][EntityCondition::OPERATOR_KEY] = '=';
        }
        return $filter_item;
    }
    
    /**
     * Denormalizes the given filter items into a single EntityConditionGroup.
     *
     * @param array $items
     *   The normalized entity conditions and groups.
     *
     * @return \Drupal\jsonapi\Query\EntityConditionGroup
     *   A root group containing all the denormalized conditions and groups.
     */
    protected static function buildEntityConditionGroup(array $items) {
        $root = [
            'id' => static::ROOT_ID,
            static::GROUP_KEY => [
                'conjunction' => 'AND',
            ],
        ];
        return static::buildTree($root, $items);
    }
    
    /**
     * Organizes the flat, normalized filter items into a tree structure.
     *
     * @param array $root
     *   The root of the tree to build.
     * @param array $items
     *   The normalized entity conditions and groups.
     *
     * @return \Drupal\jsonapi\Query\EntityConditionGroup
     *   The entity condition group
     */
    protected static function buildTree(array $root, array $items) {
        $id = $root['id'];
        // Recursively build a tree of denormalized conditions and condition groups.
        $members = [];
        foreach ($items as $item) {
            if ($item[static::MEMBER_KEY] == $id) {
                if (isset($item[static::GROUP_KEY])) {
                    array_push($members, static::buildTree($item, $items));
                }
                elseif (isset($item[static::CONDITION_KEY])) {
                    $condition = EntityCondition::createFromQueryParameter($item[static::CONDITION_KEY]);
                    array_push($members, $condition);
                }
            }
        }
        $root[static::GROUP_KEY]['members'] = $members;
        // Denormalize the root into a condition group.
        return new EntityConditionGroup($root[static::GROUP_KEY]['conjunction'], $root[static::GROUP_KEY]['members']);
    }

}