// Ceres Solver - A fast non-linear least squares minimizer
// Copyright 2023 Google Inc. All rights reserved.
// http://ceres-solver.org/
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// Author: vitus@google.com (Michael Vitus)

#include "ceres/thread_pool.h"

#include <cmath>
#include <limits>

#include "ceres/internal/config.h"

namespace ceres::internal {
namespace {

// Constrain the total number of threads to the amount the hardware can support.
int GetNumAllowedThreads(int requested_num_threads) {
  return std::min(requested_num_threads, ThreadPool::MaxNumThreadsAvailable());
}

}  // namespace

int ThreadPool::MaxNumThreadsAvailable() {
  const int num_hardware_threads = std::thread::hardware_concurrency();
  // hardware_concurrency() can return 0 if the value is not well defined or not
  // computable.
  return num_hardware_threads == 0 ? std::numeric_limits<int>::max()
                                   : num_hardware_threads;
}

ThreadPool::ThreadPool() = default;

ThreadPool::ThreadPool(int num_threads) { Resize(num_threads); }

ThreadPool::~ThreadPool() {
  std::lock_guard<std::mutex> lock(thread_pool_mutex_);
  // Signal the thread workers to stop and wait for them to finish all scheduled
  // tasks.
  Stop();
  for (std::thread& thread : thread_pool_) {
    thread.join();
  }
}

void ThreadPool::Resize(int num_threads) {
  std::lock_guard<std::mutex> lock(thread_pool_mutex_);

  const int num_current_threads = thread_pool_.size();
  if (num_current_threads >= num_threads) {
    return;
  }

  const int create_num_threads =
      GetNumAllowedThreads(num_threads) - num_current_threads;

  for (int i = 0; i < create_num_threads; ++i) {
    thread_pool_.emplace_back(&ThreadPool::ThreadMainLoop, this);
  }
}

void ThreadPool::AddTask(const std::function<void()>& func) {
  task_queue_.Push(func);
}

int ThreadPool::Size() {
  std::lock_guard<std::mutex> lock(thread_pool_mutex_);
  return thread_pool_.size();
}

void ThreadPool::ThreadMainLoop() {
  std::function<void()> task;
  while (task_queue_.Wait(&task)) {
    task();
  }
}

void ThreadPool::Stop() { task_queue_.StopWaiters(); }

}  // namespace ceres::internal
