FieldsFilter.php

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<?php
namespace Magento\Framework\Webapi\Rest\Response;

use Magento\Framework\Api\AbstractExtensibleObject;
use Magento\Framework\Api\AttributeInterface;
use Magento\Framework\Webapi\Rest\Request as RestRequest;

class FieldsFilter
{
    const FILTER_PARAMETER = 'fields';

    protected $_request;

    public function __construct(RestRequest $request)
    {
        $this->_request = $request;
    }

    public function filter($response)
    {
        $filter = $this->_request->getParam(self::FILTER_PARAMETER);
        if (!is_string($filter)) {
            return [];
        }
        $filterArray = $this->parse($filter);
        if ($filterArray === null) {
            return [];
        }
        $partialResponse = $this->applyFilter($response, $filterArray);
        return $partialResponse;
    }

    protected function parse($filterString)
    {
        $length = strlen($filterString);
        //Permissible characters in filter string: letter, number, underscore, square brackets and comma
        if ($length == 0 || preg_match('/[^\w\[\],]+/', $filterString)) {
            return null;
        }

        $start = null;
        $current = [];
        $stack = [];
        $parent = [];
        $currentElement = null;

        for ($position = 0; $position < $length; $position++) {
            //Extracting field when encountering field separators
            if (in_array($filterString[$position], ['[', ']', ','])) {
                if ($start !== null) {
                    $currentElement = substr($filterString, $start, $position - $start);
                    $current[$currentElement] = 1;
                }
                $start = null;
            }
            switch ($filterString[$position]) {
                case '[':
                    $parent[] = $currentElement;
                    // push current field in stack and initialize current
                    $stack[] = $current;
                    $current = [];
                    break;

                case ']':
                    //cache current
                    $temp = $current;
                    //Initialize with previous
                    $current = array_pop($stack);
                    //Add from cache
                    $current[array_pop($parent)] = $temp;
                    break;

                //Do nothing on comma. On the next iteration field will be extracted
                case ',':
                    break;

                default:
                    //Move position if no field separators found
                    if ($start === null) {
                        $start = $position;
                    }
            }
        }
        //Check for wrongly formatted filter
        if (!empty($stack)) {
            return null;
        }
        //Check if there's any field remaining that's not added to response
        if ($start !== null) {
            $currentElement = substr($filterString, $start, $position - $start);
            $current[$currentElement] = 1;
        }
        return $current;
    }

    protected function applyFilter(array $responseArray, array $filter)
    {
        $arrayIntersect = null;
        //Check if its a sequential array. Presence of sequential arrays mean that the filed is a collection
        //and the filtering will be applied to all the collection items
        if (!(bool)count(array_filter(array_keys($responseArray), 'is_string'))) {
            foreach ($responseArray as $key => &$item) {
                $arrayIntersect[$key] = $this->recursiveArrayIntersectKey($item, $filter);
            }
        } else {
            $arrayIntersect = $this->recursiveArrayIntersectKey($responseArray, $filter);
        }
        return $arrayIntersect;
    }

    protected function recursiveArrayIntersectKey(array $array1, array $array2)
    {
        //If the field in array2 (filter) is not present in array1 (response) it will be removed after intersect
        $arrayIntersect = array_intersect_key($array1, $array2);
        foreach ($arrayIntersect as $key => &$value) {
            if ($key == AbstractExtensibleObject::CUSTOM_ATTRIBUTES_KEY
                && is_array($array2[AbstractExtensibleObject::CUSTOM_ATTRIBUTES_KEY])
            ) {
                $value = $this->filterCustomAttributes(
                    $value,
                    $array2[AbstractExtensibleObject::CUSTOM_ATTRIBUTES_KEY]
                );
                continue;
            }
            if (is_array($value) && is_array($array2[$key])) {
                $value = $this->applyFilter($value, $array2[$key]);
            }
        }
        return $arrayIntersect;
    }

    private function filterCustomAttributes(array $item, array $filter) : array
    {
        $fieldResult = [];
        foreach ($item as $key => $field) {
            $filterKeys = array_keys($filter);
            if (in_array($field[AttributeInterface::ATTRIBUTE_CODE], $filterKeys)) {
                $fieldResult[$key][AttributeInterface::ATTRIBUTE_CODE] = $field[AttributeInterface::ATTRIBUTE_CODE];
                $fieldResult[$key][AttributeInterface::VALUE] = $field[AttributeInterface::VALUE];
            } else {
                if (isset($filter[AttributeInterface::ATTRIBUTE_CODE])) {
                    $fieldResult[$key][AttributeInterface::ATTRIBUTE_CODE] = $field[AttributeInterface::ATTRIBUTE_CODE];
                }
                if (isset($filter[AttributeInterface::VALUE])) {
                    $fieldResult[$key][AttributeInterface::VALUE] = $field[AttributeInterface::VALUE];
                }
            }
        }

        return $fieldResult;
    }
}