text: //  Copyright (c) 2013, Facebook, Inc.  All rights reserved.
  //  This source code is licensed under the BSD-style license found in the
  //  LICENSE file in the root directory of this source tree. An additional grant
  //  of patent rights can be found in the PATENTS file in the same directory.
  //
  // Copyright (c) 2011 The LevelDB Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style license that can be
  // found in the LICENSE file. See the AUTHORS file for names of contributors.
  
  #pragma once
  
  #include "rocksdb/types.h"
  
  namespace rocksdb {
  
  class Comparator;
  class Iterator;
  class Env;
  class Arena;
  
  // Return an iterator that provided the union of the data in
  // children[0,n-1].  Takes ownership of the child iterators and
  // will delete them when the result iterator is deleted.
  //
  // The result does no duplicate suppression.  I.e., if a particular
  // key is present in K child iterators, it will be yielded K times.
  //
  // REQUIRES: n >= 0
  extern Iterator* NewMergingIterator(const Comparator* comparator,
                                      Iterator** children, int n,
                                      Arena* arena = nullptr);
  
  class MergingIterator;
  
  // A builder class to build a merging iterator by adding iterators one by one.
  class MergeIteratorBuilder {
   public:
    // comparator: the comparator used in merging comparator
    // arena: where the merging iterator needs to be allocated from.
    explicit MergeIteratorBuilder(const Comparator* comparator, Arena* arena);
    ~MergeIteratorBuilder() {}
  
    // Add iter to the merging iterator.
    void AddIterator(Iterator* iter);
  
    // Get arena used to build the merging iterator. It is called one a child
    // iterator needs to be allocated.
    Arena* GetArena() { return arena; }
  
    // Return the result merging iterator.
    Iterator* Finish();
  
   private:
    MergingIterator* merge_iter;
    Iterator* first_iter;
    bool use_merging_iter;
    Arena* arena;
  };
  
  }  // namespace rocksdb
  