ParallelScan.h

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#pragma once

#include <tbb/parallel_for.h>
#include <tbb/parallel_scan.h>

#if TBB_INTERFACE_VERSION >= 10000

// Check if the C++ standard library implements parallel algorithms
// and use this over parallelstl to avoid conflicts.
// Clang does not implement it so far, so checking for C++17 is not sufficient.
#ifdef __cpp_lib_parallel_algorithm
#include <execution>
#include <numeric>
#else
#include <pstl/execution>
#include <pstl/numeric>

// parallelstl incorrectly assumes MSVC to unconditionally implement
// parallel algorithms even if __cpp_lib_parallel_algorithm is not defined.
// So manually include the header which pulls all "pstl::execution" definitions
// into the "std" namespace.
#if __PSTL_CPP17_EXECUTION_POLICIES_PRESENT
#include <pstl/internal/glue_execution_defs.h>
#endif

#endif
#endif

namespace open3d {
namespace utility {

namespace {
template <class Tin, class Tout>
class ScanSumBody {
    Tout sum;
    const Tin* in;
    Tout* const out;

public:
    ScanSumBody(Tout* out_, const Tin* in_) : sum(0), in(in_), out(out_) {}
    Tout get_sum() const { return sum; }

    template <class Tag>
    void operator()(const tbb::blocked_range<size_t>& r, Tag) {
        Tout temp = sum;
        for (size_t i = r.begin(); i < r.end(); ++i) {
            temp = temp + in[i];
            if (Tag::is_final_scan()) out[i] = temp;
        }
        sum = temp;
    }
    ScanSumBody(ScanSumBody& b, tbb::split) : sum(0), in(b.in), out(b.out) {}
    void reverse_join(ScanSumBody& a) { sum = a.sum + sum; }
    void assign(ScanSumBody& b) { sum = b.sum; }
};
}  // namespace

template <class Tin, class Tout>
void InclusivePrefixSum(const Tin* first, const Tin* last, Tout* out) {
#if TBB_INTERFACE_VERSION >= 10000
    // use parallelstl if we have TBB 2018 or later
    std::inclusive_scan(std::execution::par_unseq, first, last, out);
#else
    ScanSumBody<Tin, Tout> body(out, first);
    size_t n = std::distance(first, last);
    tbb::parallel_scan(tbb::blocked_range<size_t>(0, n), body);
#endif
}

}  // namespace utility
}  // namespace open3d