parallel_for_each.h

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// MIT License
//
// Copyright (c) 2019 Ruhr University Bochum, Chair for Embedded Security. All Rights reserved.
// Copyright (c) 2019 Marc Fyrbiak, Sebastian Wallat, Max Hoffmann ("ORIGINAL AUTHORS"). All rights reserved.
// Copyright (c) 2021 Max Planck Institute for Security and Privacy. All Rights reserved.
// Copyright (c) 2021 Jörn Langheinrich, Julian Speith, Nils Albartus, René Walendy, Simon Klix ("ORIGINAL AUTHORS"). All Rights reserved.
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.
 
#pragma once
 
#include "hal_core/defines.h"
 
#include <cmath>
#include <functional>
#include <set>
#include <thread>
 
namespace hal
{
    namespace dataflow
    {
        namespace utils
        {
            template<typename R>
            void parallel_for_each(u32 begin, u32 end, R func)
            {
                if (end - begin == 0)
                {
                    return;
                }
 
                u32 num_threads      = std::min(end - begin, std::thread::hardware_concurrency());
                u32 steps_per_thread = std::ceil((float)(end - begin) / num_threads);
 
                std::vector<std::thread> threads;
                threads.reserve(num_threads);
 
                // spawn threads, start at index 1 since index 0 is the main thread
                for (u32 thread_idx = 1; thread_idx < num_threads; ++thread_idx)
                {
                    threads.emplace_back([&func, end, thread_idx, steps_per_thread]() {
                        u32 local_end = std::min(end, steps_per_thread * (thread_idx + 1));
                        for (u32 i = steps_per_thread * thread_idx; i < local_end; ++i)
                        {
                            func(i);
                        }
                    });
                }
 
                // also do work on main thread
                {
                    u32 local_end = std::min(end, steps_per_thread);
                    for (u32 i = 0; i < local_end; ++i)
                    {
                        func(i);
                    }
                }
 
                // wait until all threads are done
                for (auto& t : threads)
                {
                    t.join();
                }
            }
 
            template<typename T, typename R>
            void parallel_for_each(const std::vector<T>& elements, R func)
            {
                parallel_for_each(0, elements.size(), [&elements, &func](u32 i) { func(elements[i]); });
            }
 
            template<typename T, typename R>
            void parallel_for_each(const std::set<T>& elements, R func)
            {
                std::vector<T> vec;
                vec.reserve(elements.size());
                vec.insert(vec.end(), elements.begin(), elements.end());
                parallel_for_each(vec, func);
            }
 
            template<typename T, typename R>
            void parallel_for_each(const std::unordered_set<T>& elements, R func)
            {
                std::vector<T> vec;
                vec.reserve(elements.size());
                vec.insert(vec.end(), elements.begin(), elements.end());
                parallel_for_each(vec, func);
            }
        }    // namespace utils
    }    // namespace dataflow
}    // namespace hal