// SPDX-FileCopyrightText: Copyright (c) Ken Martin, Will Schroeder, Bill Lorensen
// SPDX-License-Identifier: BSD-3-Clause

#include "SMP/STDThread/vtkSMPThreadPool.h"
#include "SMP/Common/vtkSMPToolsImpl.h"

#include <vtkObject.h>

#include <algorithm>
#include <cassert>
#include <condition_variable>
#include <future>
#include <iostream>

namespace vtk
{
namespace detail
{
namespace smp
{
VTK_ABI_NAMESPACE_BEGIN

static constexpr std::size_t NoRunningJob = (std::numeric_limits<std::size_t>::max)();

struct vtkSMPThreadPool::ThreadJob
{
  // This constructor is needed because aggregate initialization can not have default value
  // (prior to C++14)
  // also because emplace_back can not use aggregate initialization (prior to C++20)
  ThreadJob(ProxyData* proxy = nullptr, std::function<void()> function = nullptr)
    : Proxy{ proxy }
    , Function{ std::move(function) }
  {
  }

  ProxyData* Proxy{};             // Proxy that allocated this job
  std::function<void()> Function; // Actual user job
  std::promise<void> Promise;     // Set when job is done
};

struct vtkSMPThreadPool::ThreadData
{
  // stack of jobs, any thread can push, and only push, jobs (and Mutex must be locked)
  std::vector<ThreadJob> Jobs;
  // Current job (used to map thread to Proxy), using an index is okay as only this thread can
  // erase the job and other threads can only push back new jobs not insert. This constraint could
  // be relaxed by using unique ids instead.
  std::size_t RunningJob{ NoRunningJob };
  std::thread SystemThread;                  // the system thread, not really used
  std::mutex Mutex;                          // thread mutex, used for Jobs manipulation
  std::condition_variable ConditionVariable; // thread cv, used to wake up the thread
};

struct vtkSMPThreadPool::ProxyThreadData
{
  // This constructor is needed because aggregate initialization can not have default value
  // (prior to C++14)
  // also because emplace_back can not use aggregate initialization (prior to C++20)
  ProxyThreadData(ThreadData* threadData = nullptr, std::size_t id = 0)
    : Thread{ threadData }
    , Id{ id }
  {
  }

  ThreadData* Thread{}; // The thread data from the pool
  std::size_t Id{};     // Virtual thread ID, mainly used for thread local variables
};

struct vtkSMPThreadPool::ProxyData
{
  vtkSMPThreadPool* Pool{};                   // Pool that created this proxy
  ProxyData* Parent{};                        // either null (for top level) or the parent
  std::vector<ProxyThreadData> Threads;       // Threads used by this
  std::size_t NextThread{};                   // Round-robin thread for jobs
  std::vector<std::future<void>> JobsFutures; // Used to know when job is done
  std::mutex Mutex;                           // Used to synchronize
};

void vtkSMPThreadPool::RunJob(
  ThreadData& data, std::size_t jobIndex, std::unique_lock<std::mutex>& lock)
{
  assert(lock.owns_lock() && "Caller must have locked mutex");
  assert(jobIndex < data.Jobs.size() && "jobIndex out of range");

  const auto oldRunningJob = data.RunningJob; // store old running job for nested threads
  data.RunningJob = jobIndex;                 // Set thread running job
  auto function = std::move(data.Jobs[data.RunningJob].Function);
  lock.unlock(); // MSVC: warning C26110 is a false positive

  try
  {
    function(); // run the function
  }
  catch (const std::exception& e)
  {
    vtkErrorWithObjectMacro(nullptr,
      "Function called by " << vtkSMPThreadPool::GetInstance().GetThreadId()
                            << " has thrown an exception. The exception is ignored. what():\n"
                            << e.what());
  }
  catch (...)
  {
    vtkErrorWithObjectMacro(nullptr,
      "Function called by " << vtkSMPThreadPool::GetInstance().GetThreadId()
                            << " has thrown an unknown exception. The exception is ignored.");
  }

  lock.lock();
  data.Jobs[data.RunningJob].Promise.set_value();
  data.Jobs.erase(data.Jobs.begin() + jobIndex);
  data.RunningJob = oldRunningJob;
}

vtkSMPThreadPool::Proxy::Proxy(std::unique_ptr<ProxyData>&& data)
  : Data{ std::move(data) }
{
}

vtkSMPThreadPool::Proxy::~Proxy()
{
  if (!this->Data->JobsFutures.empty())
  {
    vtkErrorWithObjectMacro(nullptr, "Proxy not joined. Terminating.");
    std::terminate();
  }
}

vtkSMPThreadPool::Proxy::Proxy(Proxy&&) noexcept = default;
vtkSMPThreadPool::Proxy& vtkSMPThreadPool::Proxy::operator=(Proxy&&) noexcept = default;

void vtkSMPThreadPool::Proxy::Join()
{
  if (this->IsTopLevel()) // wait for all futures, all jobs are done by other threads
  {
    for (auto& future : this->Data->JobsFutures)
    {
      future.wait();
    }
  }
  else // nested run code in calling thread too
  {
    // Run jobs associated with this thread and proxy
    ThreadData& threadData = *this->Data->Threads[0].Thread;
    assert(threadData.SystemThread.get_id() == std::this_thread::get_id());

    while (true)
    {
      // protect access in case other thread push work for current thread
      std::unique_lock<std::mutex> lock{ threadData.Mutex };

      auto it = std::find_if(threadData.Jobs.begin(), threadData.Jobs.end(),
        [this](ThreadJob& job) { return job.Proxy == this->Data.get(); });

      if (it == threadData.Jobs.end()) // no remaining job associated to this proxy
      {
        break;
      }

      const auto jobIndex = static_cast<std::size_t>(std::distance(threadData.Jobs.begin(), it));
      RunJob(threadData, jobIndex, lock);
    }

    for (auto& future : this->Data->JobsFutures)
    {
      future.wait();
    }
  }

  this->Data->JobsFutures.clear();
}

void vtkSMPThreadPool::Proxy::DoJob(std::function<void()> job)
{
  this->Data->NextThread = (this->Data->NextThread + 1) % this->Data->Threads.size();
  auto& proxyThread = this->Data->Threads[this->Data->NextThread];

  if (!this->IsTopLevel() && this->Data->NextThread == 0) // when nested, thread 0 is "this_thread"
  {
    assert(std::this_thread::get_id() == proxyThread.Thread->SystemThread.get_id());

    std::unique_lock<std::mutex> lock{ proxyThread.Thread->Mutex };
    proxyThread.Thread->Jobs.emplace_back(this->Data.get(), std::move(job));
  }
  else
  {
    std::unique_lock<std::mutex> lock{ proxyThread.Thread->Mutex };

    auto& jobs = proxyThread.Thread->Jobs;
    jobs.emplace_back(this->Data.get(), std::move(job));
    this->Data->JobsFutures.emplace_back(jobs.back().Promise.get_future());

    lock.unlock();

    proxyThread.Thread->ConditionVariable.notify_one();
  }
}

std::vector<std::reference_wrapper<std::thread>> vtkSMPThreadPool::Proxy::GetThreads() const
{
  std::vector<std::reference_wrapper<std::thread>> output;

  for (auto& proxyThread : this->Data->Threads)
  {
    output.emplace_back(proxyThread.Thread->SystemThread);
  }

  return output;
}

bool vtkSMPThreadPool::Proxy::IsTopLevel() const noexcept
{
  return this->Data->Parent == nullptr;
}

vtkSMPThreadPool::vtkSMPThreadPool()
{
  const std::size_t threadCount = static_cast<std::size_t>(
    vtkSMPToolsImpl<BackendType::STDThread>::GetEstimatedDefaultNumberOfThreads());
  this->Threads.reserve(threadCount);
  for (std::size_t i{}; i < threadCount; ++i)
  {
    std::unique_ptr<ThreadData> data{ new ThreadData{} };
    data->SystemThread = this->MakeThread();
    this->Threads.emplace_back(std::move(data));
  }

  this->Initialized.store(true, std::memory_order_release);
}

vtkSMPThreadPool::~vtkSMPThreadPool()
{
  this->Joining.store(true, std::memory_order_release);

  for (auto& threadData : this->Threads)
  {
    threadData->ConditionVariable.notify_one();
  }

  for (auto& threadData : this->Threads)
  {
    threadData->SystemThread.join();
  }
}

vtkSMPThreadPool::Proxy vtkSMPThreadPool::AllocateThreads(std::size_t threadCount)
{
  if (threadCount == 0 || threadCount > this->ThreadCount())
  {
    threadCount = this->ThreadCount();
  }

  std::unique_ptr<ProxyData> proxy{ new ProxyData{} };
  proxy->Pool = this;
  proxy->Threads.reserve(threadCount);

  // Check if we are in the pool
  ThreadData* threadData = this->GetCallerThreadData();
  if (threadData)
  {
    // Don't lock since we are in the running job, in this thread
    proxy->Parent = threadData->Jobs[threadData->RunningJob].Proxy;
    // First thread is always current thread
    proxy->Threads.emplace_back(threadData, this->GetNextThreadId());
    this->FillThreadsForNestedProxy(proxy.get(), threadCount);
  }
  else
  {
    proxy->Parent = nullptr;
    for (std::size_t i{}; i < threadCount; ++i)
    {
      proxy->Threads.emplace_back(this->Threads[i].get(), this->GetNextThreadId());
    }
  }

  return Proxy{ std::move(proxy) };
}

std::size_t vtkSMPThreadPool::GetThreadId() const noexcept
{
  auto* threadData = this->GetCallerThreadData();

  if (threadData)
  {
    std::unique_lock<std::mutex> lock{ threadData->Mutex }; // protect threadData->Jobs access
    assert(threadData->RunningJob != NoRunningJob && "Invalid state");
    auto& proxyThreads = threadData->Jobs[threadData->RunningJob].Proxy->Threads;
    lock.unlock();

    for (const auto& proxyThread : proxyThreads)
    {
      if (proxyThread.Thread == threadData)
      {
        return proxyThread.Id;
      }
    }
  }

  // Use 1 for any thread outside the pool and 2+ for ids of proxy thread because thread local
  // implementation uses ID "0" for invalid state
  return ExternalThreadID;
}

bool vtkSMPThreadPool::IsParallelScope() const noexcept
{
  return GetCallerThreadData() != nullptr;
}

bool vtkSMPThreadPool::GetSingleThread() const
{
  // Return true if the caller is the thread[0] of the current running proxy

  auto* threadData = GetCallerThreadData();
  if (threadData)
  {
    std::lock_guard<std::mutex> lock{ threadData->Mutex };
    assert(threadData->RunningJob != NoRunningJob && "Invalid state");
    return threadData->Jobs[threadData->RunningJob].Proxy->Threads[0].Thread == threadData;
  }

  return false;
}

std::size_t vtkSMPThreadPool::ThreadCount() const noexcept
{
  return this->Threads.size();
}

vtkSMPThreadPool::ThreadData* vtkSMPThreadPool::GetCallerThreadData() const noexcept
{
  for (const auto& threadData : this->Threads)
  {
    if (threadData->SystemThread.get_id() == std::this_thread::get_id())
    {
      return threadData.get();
    }
  }

  return nullptr;
}

std::thread vtkSMPThreadPool::MakeThread()
{
  return std::thread{ [this]()
    {
      while (!this->Initialized.load(std::memory_order_acquire))
      {
      }

      ThreadData& threadData = *this->GetCallerThreadData();

      // Main loop for threads of the pool
      // When they are woke up, they check for new job and stop if "this->Joining" is true
      // and no more jobs are running
      while (true)
      {
        std::unique_lock<std::mutex> lock{ threadData.Mutex };

        // Job stealing could be implemented but it would require some changes in the process
        // A thread that no longer has work to do could look at other threads' jobs to "steal" a job
        // from them and thus increase parallelism. This must take care to not generate deadlocks
        // and should not increase Proxy parallelism above requested thread count.
        // This goes out of the scope of the current implementation.
        threadData.ConditionVariable.wait(lock, [this, &threadData]
          { return !threadData.Jobs.empty() || this->Joining.load(std::memory_order_acquire); });

        if (threadData.Jobs.empty())
        {
          break; // joining
        }

        RunJob(threadData, threadData.Jobs.size() - 1, lock);
      }
    } };
}

void vtkSMPThreadPool::FillThreadsForNestedProxy(ProxyData* proxy, std::size_t maxCount)
{
  // This function assigns thread for proxies, this function assumes that the calling thread is
  // already part of the assigned thread for the proxy.
  // Otherwise it will assign thread pool threads that are not already used by any of proxy parents

  if (proxy->Parent->Threads.size() == this->Threads.size())
  {
    return; // No thread will be available
  }

  const auto isFree = [proxy](ThreadData* threadData)
  {
    for (auto* parent = proxy->Parent; parent != nullptr; parent = parent->Parent)
    {
      for (auto& proxyThread : parent->Threads)
      {
        if (proxyThread.Thread == threadData)
        {
          return false;
        }
      }
    }

    return true;
  };

  for (auto& threadData : this->Threads)
  {
    if (isFree(threadData.get()))
    {
      proxy->Threads.emplace_back(threadData.get(), this->GetNextThreadId());
    }

    if (proxy->Threads.size() == maxCount)
    {
      break;
    }
  }
}

std::size_t vtkSMPThreadPool::GetNextThreadId() noexcept
{
  return this->NextProxyThreadId.fetch_add(1, std::memory_order_relaxed) + 1;
}

vtkSMPThreadPool& vtkSMPThreadPool::GetInstance()
{
  static vtkSMPThreadPool instance{};
  return instance;
}

VTK_ABI_NAMESPACE_END
} // namespace smp
} // namespace detail
} // namespace vtk
