/**
 * The ThreadPool class.
 * Keeps a set of threads constantly waiting to execute incoming jobs.
 * source: http://roar11.com/2016/01/a-platform-independent-thread-pool-using-c14/
 */
#pragma once

#ifndef THREADPOOL_HPP
#define THREADPOOL_HPP

#include "ThreadSafeQueue.hpp"

#include <algorithm>
#include <atomic>
#include <cstdint>
#include <functional>
#include <future>
#include <memory>
#include <thread>
#include <type_traits>
#include <utility>
#include <vector>

namespace TP
{
    class ThreadPool
    {
    private:
        class IThreadTask
        {
        public:
            IThreadTask(void) = default;
            virtual ~IThreadTask(void) = default;
            IThreadTask(const IThreadTask& rhs) = delete;
            IThreadTask& operator=(const IThreadTask& rhs) = delete;
            IThreadTask(IThreadTask&& other) = default;
            IThreadTask& operator=(IThreadTask&& other) = default;

            /**
             * Run the task.
             */
            virtual void execute() = 0;
        };

        template <typename Func>
        class ThreadTask: public IThreadTask
        {
        public:
            ThreadTask(Func&& func)
                :m_func{std::move(func)}
            {
            }

            ~ThreadTask(void) override = default;
            ThreadTask(const ThreadTask& rhs) = delete;
            ThreadTask& operator=(const ThreadTask& rhs) = delete;
            ThreadTask(ThreadTask&& other) = default;
            ThreadTask& operator=(ThreadTask&& other) = default;

            /**
             * Run the task.
             */
            void execute() override
            {
                m_func();
            }

        private:
            Func m_func;
        };

    public:
        /**
         * A wrapper around a std::future that adds the behavior of futures returned from 
std::async.
         * Specifically, this object will block and wait for execution to finish before going out 
of scope.
         */
        template <typename T>
        class TaskFuture
        {
        public:
            TaskFuture(std::future<T>&& future)
                :m_future{std::move(future)}
            {
            }

            TaskFuture(const TaskFuture& rhs) = delete;
            TaskFuture& operator=(const TaskFuture& rhs) = delete;
            TaskFuture(TaskFuture&& other) = default;
            TaskFuture& operator=(TaskFuture&& other) = default;
            ~TaskFuture(void)
            {
                if(m_future.valid())
                {
                    m_future.get();
                }
            }

            auto get(void)
            {
                return m_future.get();
            }


        private:
            std::future<T> m_future;
        };

    public:
        /**
         * Constructor.
         */
        ThreadPool(void)
            :ThreadPool{std::max(std::thread::hardware_concurrency()/2, 2u) - 1u}
        {
            /*
             * Always create at least one thread.  If hardware_concurrency() returns 0,
             * subtracting one would turn it to UINT_MAX, so get the maximum of
             * hardware_concurrency() and 2 before subtracting 1.
             */
        }

        /**
         * Constructor.
         */
        explicit ThreadPool(const std::uint32_t numThreads)
            :m_done{false},
            m_workQueue{},
            m_threads{}
        {
            try
            {
                for(std::uint32_t i = 0u; i < numThreads; ++i)
                {
                    m_threads.emplace_back(&ThreadPool::worker, this);
                }
            }
            catch(...)
            {
                destroy();
                throw;
            }
        }

        /**
         * Non-copyable.
         */
        ThreadPool(const ThreadPool& rhs) = delete;

        /**
         * Non-assignable.
         */
        ThreadPool& operator=(const ThreadPool& rhs) = delete;

        /**
         * Destructor.
         */
        ~ThreadPool(void)
        {
            destroy();
        }

        auto queueSize() const
        {
            return m_workQueue.size();
        }

        /**
         * Submit a job to be run by the thread pool.
         */
        template <typename Func, typename... Args>
        auto submit(Func&& func, Args&&... args)
        {
            auto boundTask = std::bind(std::forward<Func>(func), std::forward<Args>(args)...);
            using ResultType = std::result_of_t<decltype(boundTask)()>;
            using PackagedTask = std::packaged_task<ResultType()>;
            using TaskType = ThreadTask<PackagedTask>;
            
            PackagedTask task{std::move(boundTask)};
            TaskFuture<ResultType> result{task.get_future()};
            m_workQueue.push(std::make_unique<TaskType>(std::move(task)));
            return result;
        }

    private:
        /**
         * Constantly running function each thread uses to acquire work items from the queue.
         */
        void worker(void)
        {
            while(!m_done)
            {
                std::unique_ptr<IThreadTask> pTask{nullptr};
                if(m_workQueue.waitPop(pTask))
                {
                    pTask->execute();
                }
            }
        }

        /**
         * Invalidates the queue and joins all running threads.
         */
        void destroy(void)
        {
            m_done = true;
            m_workQueue.invalidate();
            for(auto& thread : m_threads)
            {
                if(thread.joinable())
                {
                    thread.join();
                }
            }
        }

    private:
        std::atomic_bool m_done;
        ThreadSafeQueue<std::unique_ptr<IThreadTask>> m_workQueue;
        std::vector<std::thread> m_threads;
    };

    namespace DefaultThreadPool
    {
        /**
         * Get the default thread pool for the application.
         * This pool is created with std::thread::hardware_concurrency() - 1 threads.
         */
        inline ThreadPool& getThreadPool(void)
        {
            static ThreadPool defaultPool;
            return defaultPool;
        }

        inline auto queueSize()
        {
           return getThreadPool().queueSize();
        }

        /**
         * Submit a job to the default thread pool.
         */
        template <typename Func, typename... Args>
        inline auto submitJob(Func&& func, Args&&... args)
        {
            return getThreadPool().submit(
                    std::forward<Func>(func), 
                    std::forward<Args>(args)...);
        }
    }
}

#endif