OpenVDB  9.1.1
Reduce.h
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1 // Copyright Contributors to the OpenVDB Project
2 // SPDX-License-Identifier: MPL-2.0
3 
4 /*!
5  \file Reduce.h
6 
7  \author Ken Museth
8 
9  \date March 4, 2021
10 
11  \brief A unified wrapper for tbb::parallel_reduce and a naive std::future analog
12 */
13 
14 #ifndef NANOVDB_REDUCE_H_HAS_BEEN_INCLUDED
15 #define NANOVDB_REDUCE_H_HAS_BEEN_INCLUDED
16 
17 #include "Range.h"// for Range1D
18 
19 #ifdef NANOVDB_USE_TBB
20 #include <tbb/parallel_reduce.h>
21 #else
22 #include <thread>
23 #include <future>
24 #include <vector>
25 #endif
26 
27 namespace nanovdb {
28 
29 /// @return reduction
30 ///
31 /// @param range RangeT can be Range<dim,T>, CoordBBox, tbb::blocked_range, blocked_range2D, or blocked_range3D.
32 /// @param identity initial value
33 /// @param func functor with signature T FuncT::operator()(const RangeT& range, const T& a) const
34 /// @param join functor with the signature T JoinT::operator()(const T& a, const T& b) const
35 /// @code
36 /// std::vector<int> array(100, 1);
37 /// auto func = [&array](auto &r, int a){for (auto i=r.begin(); i!=r.end(); ++i) a+=array[i]; return a;};
38 /// int sum = reduce(array, 0, func, [](int a, int b){return a + b;});
39 /// @endcode
40 
41 template <typename RangeT, typename T, typename FuncT, typename JoinT>
42 inline T reduce(RangeT range, const T& identity, const FuncT &func, const JoinT &join)
43 {
44  if (range.empty()) return identity;
45 #ifdef NANOVDB_USE_TBB
46  return tbb::parallel_reduce(range, identity, func, join);
47 #else// naive and likely slow alternative based on std::future
48  if (const size_t threadCount = std::thread::hardware_concurrency()>>1) {
49  std::vector<RangeT> rangePool{ range };
50  while(rangePool.size() < threadCount) {
51  const size_t oldSize = rangePool.size();
52  for (size_t i = 0; i < oldSize && rangePool.size() < threadCount; ++i) {
53  auto &r = rangePool[i];
54  if (r.is_divisible()) rangePool.push_back(RangeT(r, Split()));
55  }
56  if (rangePool.size() == oldSize) break;// none of the ranges were divided so stop
57  }
58  std::vector< std::future<T> > futurePool;
59  for (auto &r : rangePool) {
60  auto task = std::async(std::launch::async, [&](){return func(r, identity);});
61  futurePool.push_back( std::move(task) );// launch tasks
62  }
63  T result = identity;
64  for (auto &f : futurePool) {
65  result = join(result, f.get());// join results
66  }
67  return result;
68  } else {// serial
69  return static_cast<T>(func(range, identity));
70  }
71 #endif
72  return identity;// should never happen
73 }
74 
75 /// @brief Simple wrapper to the function defined above
76 template <typename T, typename FuncT, typename JoinT >
77 inline T reduce(size_t begin, size_t end, size_t grainSize, const T& identity, const FuncT& func, const JoinT& join)
78 {
79  Range1D range(begin, end, grainSize);
80  return reduce( range, identity, func, join );
81 }
82 
83 /// @brief Simple wrapper that works with std::containers
84 template <template<typename...> class ContainerT, typename... ArgT, typename T, typename FuncT, typename JoinT >
85 inline T reduce(const ContainerT<ArgT...> &c, const T& identity, const FuncT& func, const JoinT& join)
86 {
87  Range1D range(0, c.size(), 1);
88  return reduce( range, identity, func, join );
89 
90 }
91 
92 /// @brief Simple wrapper that works with std::containers
93 template <template<typename...> class ContainerT, typename... ArgT, typename T, typename FuncT, typename JoinT >
94 inline T reduce(const ContainerT<ArgT...> &c, size_t grainSize, const T& identity, const FuncT& func, const JoinT& join)
95 {
96  Range1D range(0, c.size(), grainSize);
97  return reduce( range, identity, func, join );
98 }
99 
100 }// namespace nanovdb
101 
102 #endif // NANOVDB_REDUCE_H_HAS_BEEN_INCLUDED
Definition: NanoVDB.h:184
Custom Range class that is compatible with the tbb::blocked_range classes.
T reduce(RangeT range, const T &identity, const FuncT &func, const JoinT &join)
Definition: Reduce.h:42
Definition: Range.h:28
Definition: Range.h:25