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Reduce.h
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// Copyright Contributors to the OpenVDB Project
// SPDX-License-Identifier: MPL-2.0
/*!
\file Reduce.h
\author Ken Museth
\date March 4, 2021
\brief A unified wrapper for tbb::parallel_reduce and a naive std::future analog
*/
#ifndef NANOVDB_REDUCE_H_HAS_BEEN_INCLUDED
#define NANOVDB_REDUCE_H_HAS_BEEN_INCLUDED
#include "Range.h"// for Range1D
#ifdef NANOVDB_USE_TBB
#include <tbb/parallel_reduce.h>
#else
#include <thread>
#include <future>
#include <vector>
#endif
namespace nanovdb {
/// @return reduction
///
/// @param range RangeT can be Range<dim,T>, CoordBBox, tbb::blocked_range, blocked_range2D, or blocked_range3D.
/// @param identity initial value
/// @param func functor with signature T FuncT::operator()(const RangeT& range, const T& a) const
/// @param join functor with the signature T JoinT::operator()(const T& a, const T& b) const
/// @code
/// std::vector<int> array(100, 1);
/// auto func = [&array](auto &r, int a){for (auto i=r.begin(); i!=r.end(); ++i) a+=array[i]; return a;};
/// int sum = reduce(array, 0, func, [](int a, int b){return a + b;});
/// @endcode
template <typename RangeT, typename T, typename FuncT, typename JoinT>
inline T reduce(RangeT range, const T& identity, const FuncT &func, const JoinT &join)
{
if (range.empty()) return identity;
#ifdef NANOVDB_USE_TBB
return tbb::parallel_reduce(range, identity, func, join);
#else// naive and likely slow alternative based on std::future
if (const size_t threadCount = std::thread::hardware_concurrency()>>1) {
std::vector<RangeT> rangePool{ range };
while(rangePool.size() < threadCount) {
const size_t oldSize = rangePool.size();
for (size_t i = 0; i < oldSize && rangePool.size() < threadCount; ++i) {
auto &r = rangePool[i];
if (r.is_divisible()) rangePool.push_back(RangeT(r, Split()));
}
if (rangePool.size() == oldSize) break;// none of the ranges were divided so stop
}
std::vector< std::future<T> > futurePool;
for (auto &r : rangePool) {
auto task = std::async(std::launch::async, [&](){return func(r, identity);});
futurePool.push_back( std::move(task) );// launch tasks
}
T result = identity;
for (auto &f : futurePool) {
result = join(result, f.get());// join results
}
return result;
} else {// serial
return static_cast<T>(func(range, identity));
}
#endif
return identity;// should never happen
}
/// @brief Simple wrapper to the function defined above
template <typename T, typename FuncT, typename JoinT >
inline T reduce(size_t begin, size_t end, size_t grainSize, const T& identity, const FuncT& func, const JoinT& join)
{
Range1D range(begin, end, grainSize);
return reduce( range, identity, func, join );
}
/// @brief Simple wrapper that works with std::containers
template <template<typename...> class ContainerT, typename... ArgT, typename T, typename FuncT, typename JoinT >
inline T reduce(const ContainerT<ArgT...> &c, const T& identity, const FuncT& func, const JoinT& join)
{
Range1D range(0, c.size(), 1);
return reduce( range, identity, func, join );
}
/// @brief Simple wrapper that works with std::containers
template <template<typename...> class ContainerT, typename... ArgT, typename T, typename FuncT, typename JoinT >
inline T reduce(const ContainerT<ArgT...> &c, size_t grainSize, const T& identity, const FuncT& func, const JoinT& join)
{
Range1D range(0, c.size(), grainSize);
return reduce( range, identity, func, join );
}
}// namespace nanovdb
#endif // NANOVDB_REDUCE_H_HAS_BEEN_INCLUDED
Range.h
Custom Range class that is compatible with the tbb::blocked_range classes.
nanovdb::Split
Definition Range.h:25
nanovdb
Definition NanoVDB.h:208
nanovdb::reduce
T reduce(RangeT range, const T &identity, const FuncT &func, const JoinT &join)
Definition Reduce.h:42
nanovdb::Range1D
Range< 1, size_t > Range1D
Definition Range.h:30