MAGETWO-30299: [Code Quality] Reduce Complexity of Web API Classes

 - CR changes
 - Reverted changes made for \Magento\Framework\Webapi\Rest\Response\FieldsFilter

Author: marcozheng

lib/internal/Magento/Framework/Webapi/Rest/Response/FieldsFilter.php

@@ -40,10 +40,7 @@ public function filter($response)
         if (!is_string($filter)) {
             return [];
         }
-        $length = strlen($filter);
-        //Permissible characters in filter string: letter, number, underscore, square brackets and comma
-        //Set $filterArray to be null if $filter is empty or has invalid characters
-        $filterArray = ($length == 0 || preg_match('/[^\w\[\],]+/', $filter)) ? null : $this->parse($filter, $length);
+        $filterArray = $this->parse($filter);
         if (is_null($filterArray)) {
             return [];
         }
@@ -55,7 +52,6 @@ public function filter($response)
      * Parse filter string into associative array. Field names are returned as keys with values for scalar fields as 1.
      *
      * @param string $filterString
-     * @param int $length
      * <pre>
      *  ex. customer[id,email],addresses[city,postcode,region[region_code,region]]
      * </pre>
@@ -83,8 +79,14 @@ public function filter($response)
      *
      * @SuppressWarnings(PHPMD.CyclomaticComplexity)
      */
-    protected function parse($filterString, $length)
+    protected function parse($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 = [];