text: // Copyright Fedor Indutny and other Node contributors.
  //
  // 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.
  
  #include "node_v8_platform.h"
  
  #include "node.h"
  #include "util.h"
  #include "util-inl.h"
  #include "uv.h"
  #include "v8-platform.h"
  
  namespace node {
  
  using v8::Task;
  using v8::Isolate;
  
  // The last task to encounter before killing the worker
  class StopTask : public Task {
   public:
    void Run() {}
  };
  
  static StopTask stop_task_;
  
  
  Platform::Platform(unsigned int worker_count) : worker_count_(worker_count) {
    workers_ = new uv_thread_t[worker_count_];
  
    for (unsigned int i = 0; i < worker_count_; i++) {
      int err;
  
      err = uv_thread_create(worker_at(i), WorkerBody, this);
      CHECK_EQ(err, 0);
    }
  }
  
  
  Platform::~Platform() {
    // Push stop task
    for (unsigned int i = 0; i < worker_count(); i++)
      global_queue()->Push(&stop_task_);
  
    // And wait for workers to exit
    for (unsigned int i = 0; i < worker_count(); i++) {
      int err;
  
      err = uv_thread_join(worker_at(i));
      CHECK_EQ(err, 0);
    }
    delete[] workers_;
  }
  
  
  void Platform::CallOnBackgroundThread(Task* task,
                                        ExpectedRuntime expected_runtime) {
    global_queue()->Push(task);
  }
  
  
  void Platform::CallOnForegroundThread(Isolate* isolate, Task* task) {
    // TODO(indutny): create per-isolate thread pool
    global_queue()->Push(task);
  }
  
  
  double Platform::MonotonicallyIncreasingTime() {
    // uv_hrtime() returns a uint64_t but doubles can only represent integrals up
    // to 2^53 accurately.  Take steps to prevent loss of precision on overflow.
    const uint64_t timestamp = uv_hrtime();
    const uint64_t billion = 1000 * 1000 * 1000;
    const uint64_t seconds = timestamp / billion;
    const uint64_t nanoseconds = timestamp % billion;
    return seconds + 1.0 / nanoseconds;
  }
  
  
  void Platform::WorkerBody(void* arg) {
    Platform* p = static_cast<Platform*>(arg);
  
    for (;;) {
      Task* task = p->global_queue()->Shift();
      if (task == &stop_task_)
        break;
  
      task->Run();
      delete task;
    }
  }
  
  
  TaskQueue::TaskQueue() : read_off_(0), write_off_(0) {
    CHECK_EQ(0, uv_cond_init(&read_cond_));
    CHECK_EQ(0, uv_cond_init(&write_cond_));
    CHECK_EQ(0, uv_mutex_init(&mutex_));
  }
  
  
  TaskQueue::~TaskQueue() {
    uv_mutex_lock(&mutex_);
    CHECK_EQ(read_off_, write_off_);
    uv_mutex_unlock(&mutex_);
    uv_cond_destroy(&read_cond_);
    uv_cond_destroy(&write_cond_);
    uv_mutex_destroy(&mutex_);
  }
  
  
  void TaskQueue::Push(Task* task) {
    uv_mutex_lock(&mutex_);
  
    while (can_write() == false)
      uv_cond_wait(&write_cond_, &mutex_);  // Wait until there is a free slot.
  
    ring_[write_off_] = task;
    write_off_ = next(write_off_);
    uv_cond_signal(&read_cond_);
    uv_mutex_unlock(&mutex_);
  }
  
  
  Task* TaskQueue::Shift() {
    uv_mutex_lock(&mutex_);
  
    while (can_read() == false)
      uv_cond_wait(&read_cond_, &mutex_);
  
    Task* task = ring_[read_off_];
    if (can_write() == false)
      uv_cond_signal(&write_cond_);  // Signal waiters that we freed up a slot.
    read_off_ = next(read_off_);
    uv_mutex_unlock(&mutex_);
  
    return task;
  }
  
  
  unsigned int TaskQueue::next(unsigned int n) {
    return (n + 1) % ARRAY_SIZE(TaskQueue {}.ring_);
  }
  
  
  bool TaskQueue::can_read() const {
    return read_off_ != write_off_;
  }
  
  
  // The read pointer chases the write pointer in the circular queue.
  // This method checks that the write pointer hasn't advanced so much
  // that it has gone full circle and caught up with the read pointer.
  //
  // can_write() returns false when there is an empty slot but the read pointer
  // points to the first element and the write pointer to the last element.
  // That should be rare enough that it is not worth the extra bookkeeping
  // to work around that.  It's not harmful either, just mildly inefficient.
  bool TaskQueue::can_write() const {
    return next(write_off_) != read_off_;
  }
  
  
  }  // namespace node
  