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buffered_multiway_merge.hpp
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1 /*******************************************************************************
2  * thrill/core/buffered_multiway_merge.hpp
3  *
4  * Part of Project Thrill - http://project-thrill.org
5  *
6  * Copyright (C) 2016 Timo Bingmann <[email protected]>
7  *
8  * All rights reserved. Published under the BSD-2 license in the LICENSE file.
9  ******************************************************************************/
10 
11 #pragma once
12 #ifndef THRILL_CORE_BUFFERED_MULTIWAY_MERGE_HEADER
13 #define THRILL_CORE_BUFFERED_MULTIWAY_MERGE_HEADER
14 
16 
17 #include <algorithm>
18 #include <utility>
19 #include <vector>
20 
21 namespace thrill {
22 namespace core {
23 
24 template <typename ValueType, typename ReaderIterator, typename Comparator>
26 {
27 public:
28  using Reader = typename std::iterator_traits<ReaderIterator>::value_type;
29 
30  using LoserTreeType = tlx::LoserTree<
31  /* stable */ false, ValueType, Comparator>;
32 
33  BufferedMultiwayMergeTree(ReaderIterator readers_begin, ReaderIterator readers_end,
34  const Comparator& comp)
35  : readers_(readers_begin),
36  num_inputs_(static_cast<unsigned>(readers_end - readers_begin)),
38  lt_(static_cast<unsigned>(num_inputs_), comp),
40 
41  for (unsigned t = 0; t < num_inputs_; ++t)
42  {
43  if (TLX_LIKELY(readers_[t].HasNext())) {
44  current_[t].first = true;
45  current_[t].second = readers_[t].template Next<ValueType>();
46  lt_.insert_start(&current_[t].second, t, false);
47  }
48  else {
49  current_[t].first = false;
50  lt_.insert_start(nullptr, t, true);
51  assert(remaining_inputs_ > 0);
53  }
54  }
55 
56  lt_.init();
57  }
58 
59  bool HasNext() const {
60  return (remaining_inputs_ != 0);
61  }
62 
63  const ValueType& Top() {
64  return current_[lt_.min_source()].second;
65  }
66 
67  bool Update() {
68  unsigned top = lt_.min_source();
69 
70  if (TLX_LIKELY(readers_[top].HasNext())) {
71  current_[top].first = true;
72  current_[top].second = readers_[top].template Next<ValueType>();
73  lt_.delete_min_insert(&current_[top].second, false);
74  return true;
75  }
76  else {
77  current_[top].first = false;
78  lt_.delete_min_insert(nullptr, true);
79  assert(remaining_inputs_ > 0);
81  return (remaining_inputs_ > 0);
82  }
83  }
84 
85 private:
86  ReaderIterator readers_;
87  unsigned num_inputs_;
89 
91  //! current values in each input (exist flag, value)
92  std::vector<std::pair<bool, ValueType> > current_;
93 };
94 
95 /*!
96  * Sequential multi-way merging switch for a file writer as output
97  *
98  * The decision if based on the branching factor and runtime settings.
99  *
100  * \param seqs_begin Begin iterator of iterator pair input sequence.
101  * \param seqs_end End iterator of iterator pair input sequence.
102  * \param comp Comparator.
103  * \tparam Stable Stable merging incurs a performance penalty.
104  * \tparam Sentinels The sequences have a sentinel element.
105  * \return End iterator of output sequence.
106  */
107 template <typename ValueType, typename ReaderIterator, typename Comparator>
109  ReaderIterator seqs_begin, ReaderIterator seqs_end,
110  const Comparator& comp) {
111 
112  assert(seqs_end - seqs_begin >= 1);
114  ValueType, ReaderIterator,
115  Comparator>(seqs_begin, seqs_end, comp);
116 }
117 
118 } // namespace core
119 } // namespace thrill
120 
121 #endif // !THRILL_CORE_BUFFERED_MULTIWAY_MERGE_HEADER
122 
123 /******************************************************************************/
tlx::LoserTree< false, ValueType, Comparator > LoserTreeType
#define TLX_LIKELY(c)
Definition: likely.hpp:23
BufferedMultiwayMergeTree(ReaderIterator readers_begin, ReaderIterator readers_end, const Comparator &comp)
auto make_buffered_multiway_merge_tree(ReaderIterator seqs_begin, ReaderIterator seqs_end, const Comparator &comp)
Sequential multi-way merging switch for a file writer as output.
std::vector< std::pair< bool, ValueType > > current_
current values in each input (exist flag, value)
typename std::iterator_traits< ReaderIterator >::value_type Reader