text: /**
   *    Copyright (C) 2018-present MongoDB, Inc.
   *
   *    This program is free software: you can redistribute it and/or modify
   *    it under the terms of the Server Side Public License, version 1,
   *    as published by MongoDB, Inc.
   *
   *    This program is distributed in the hope that it will be useful,
   *    but WITHOUT ANY WARRANTY; without even the implied warranty of
   *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   *    Server Side Public License for more details.
   *
   *    You should have received a copy of the Server Side Public License
   *    along with this program. If not, see
   *    <http://www.mongodb.com/licensing/server-side-public-license>.
   *
   *    As a special exception, the copyright holders give permission to link the
   *    code of portions of this program with the OpenSSL library under certain
   *    conditions as described in each individual source file and distribute
   *    linked combinations including the program with the OpenSSL library. You
   *    must comply with the Server Side Public License in all respects for
   *    all of the code used other than as permitted herein. If you modify file(s)
   *    with this exception, you may extend this exception to your version of the
   *    file(s), but you are not obligated to do so. If you do not wish to do so,
   *    delete this exception statement from your version. If you delete this
   *    exception statement from all source files in the program, then also delete
   *    it in the license file.
   */
  
  #pragma once
  
  #include "mongo/bson/bsonobj.h"
  #include "mongo/bson/bsonobjbuilder.h"
  #include "mongo/db/field_ref.h"
  #include "mongo/db/matcher/path.h"
  
  namespace mongo {
  
  class MatchableDocument {
  public:
      // Inlining to allow subclasses to see that this is a no-op and avoid a function call.
      // Speeds up query execution measurably.
      virtual ~MatchableDocument() {}
  
      virtual BSONObj toBSON() const = 0;
  
      /**
       * The neewly returned ElementIterator is allowed to keep a pointer to path.
       * So the caller of this function should make sure path is in scope until
       * the ElementIterator is deallocated
       */
      virtual ElementIterator* allocateIterator(const ElementPath* path) const = 0;
  
      virtual void releaseIterator(ElementIterator* iterator) const = 0;
  
      class IteratorHolder {
      public:
          IteratorHolder(const MatchableDocument* doc, const ElementPath* path) {
              _doc = doc;
              _iterator = _doc->allocateIterator(path);
          }
  
          ~IteratorHolder() {
              _doc->releaseIterator(_iterator);
          }
  
          ElementIterator* operator->() const {
              return _iterator;
          }
  
      private:
          const MatchableDocument* _doc;
          ElementIterator* _iterator;
      };
  };
  
  class BSONMatchableDocument : public MatchableDocument {
  public:
      BSONMatchableDocument(const BSONObj& obj);
      virtual ~BSONMatchableDocument();
  
      virtual BSONObj toBSON() const {
          return _obj;
      }
  
      virtual ElementIterator* allocateIterator(const ElementPath* path) const {
          if (_iteratorUsed)
              return new BSONElementIterator(path, _obj);
          _iteratorUsed = true;
          _iterator.reset(path, _obj);
          return &_iterator;
      }
  
      virtual void releaseIterator(ElementIterator* iterator) const {
          if (iterator == &_iterator) {
              _iteratorUsed = false;
          } else {
              delete iterator;
          }
      }
  
  private:
      BSONObj _obj;
      mutable BSONElementIterator _iterator;
      mutable bool _iteratorUsed;
  };
  
  /**
   * A MatchableDocument interface for viewing a BSONElement as if it were wrapped in the top-level
   * field of any given path. For example, given the object obj={a: [5]}, we can create a view over
   * the element obj["a"]["0"]. An iterator over this view with path "i" would behave identically to
   * an iterator over {i: 5} with path "i".
   */
  class BSONElementViewMatchableDocument : public MatchableDocument {
  public:
      BSONElementViewMatchableDocument(BSONElement elem) : _elem(elem), _iteratorUsed(false) {}
  
      BSONObj toBSON() const override {
          return BSON("" << _elem);
      }
  
      /**
       * Creates an iterator over '_elem' as if '_elem' were wrapped in the first field of 'path'.
       * 'path' must have at least one field.
       */
      ElementIterator* allocateIterator(const ElementPath* path) const override {
          // Skip the first field in the path when traversing the document.
          const size_t suffixIndex = 1;
  
          if (_iteratorUsed) {
              return new BSONElementIterator(path, suffixIndex, _elem);
          }
          _iteratorUsed = true;
          _iterator.reset(path, suffixIndex, _elem);
          return &_iterator;
      }
  
      void releaseIterator(ElementIterator* iterator) const override {
          if (iterator == &_iterator) {
              _iteratorUsed = false;
          } else {
              delete iterator;
          }
      }
  
  private:
      BSONElement _elem;
      mutable BSONElementIterator _iterator;
      mutable bool _iteratorUsed;
  };
  }  // namespace mongo
  