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// Ceres Solver - A fast non-linear least squares minimizer
// Copyright 2018 Google Inc. All rights reserved.
// http://ceres-solver.org/
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are met:
//
// * Redistributions of source code must retain the above copyright notice,
// this list of conditions and the following disclaimer.
// * Redistributions in binary form must reproduce the above copyright notice,
// this list of conditions and the following disclaimer in the documentation
// and/or other materials provided with the distribution.
// * Neither the name of Google Inc. nor the names of its contributors may be
// used to endorse or promote products derived from this software without
// specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
// ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
// LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
// CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
// SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
// CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
// ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
// POSSIBILITY OF SUCH DAMAGE.
//
// Author: vitus@google.com (Michael Vitus)
#ifndef CERES_INTERNAL_CONCURRENT_QUEUE_H_
#define CERES_INTERNAL_CONCURRENT_QUEUE_H_
#include <condition_variable>
#include <mutex>
#include <queue>
#include <thread>
#include "glog/logging.h"
namespace ceres {
namespace internal {
// A thread-safe multi-producer, multi-consumer queue for queueing items that
// are typically handled asynchronously by multiple threads. The ConcurrentQueue
// has two states which only affect the Wait call:
//
// (1) Waiters have been enabled (enabled by default or calling
// EnableWaiters). The call to Wait will block until an item is available.
// Push and pop will operate as expected.
//
// (2) StopWaiters has been called. All threads blocked in a Wait() call will
// be woken up and pop any available items from the queue. All future Wait
// requests will either return an element from the queue or return
// immediately if no element is present. Push and pop will operate as
// expected.
//
// A common use case is using the concurrent queue as an interface for
// scheduling tasks for a set of thread workers:
//
// ConcurrentQueue<Task> task_queue;
//
// [Worker threads]:
// Task task;
// while(task_queue.Wait(&task)) {
// ...
// }
//
// [Producers]:
// task_queue.Push(...);
// ..
// task_queue.Push(...);
// ...
// // Signal worker threads to stop blocking on Wait and terminate.
// task_queue.StopWaiters();
//
template <typename T>
class ConcurrentQueue {
public:
// Defaults the queue to blocking on Wait calls.
ConcurrentQueue() = default;
// Atomically push an element onto the queue. If a thread was waiting for an
// element, wake it up.
void Push(const T& value) {
std::lock_guard<std::mutex> lock(mutex_);
queue_.push(value);
work_pending_condition_.notify_one();
}
// Atomically pop an element from the queue. If an element is present, return
// true. If the queue was empty, return false.
bool Pop(T* value) {
CHECK(value != nullptr);
std::lock_guard<std::mutex> lock(mutex_);
return PopUnlocked(value);
}
// Atomically pop an element from the queue. Blocks until one is available or
// StopWaiters is called. Returns true if an element was successfully popped
// from the queue, otherwise returns false.
bool Wait(T* value) {
CHECK(value != nullptr);
std::unique_lock<std::mutex> lock(mutex_);
work_pending_condition_.wait(lock,
[&]() { return !(wait_ && queue_.empty()); });
return PopUnlocked(value);
}
// Unblock all threads waiting to pop a value from the queue, and they will
// exit Wait() without getting a value. All future Wait requests will return
// immediately if no element is present until EnableWaiters is called.
void StopWaiters() {
std::lock_guard<std::mutex> lock(mutex_);
wait_ = false;
work_pending_condition_.notify_all();
}
// Enable threads to block on Wait calls.
void EnableWaiters() {
std::lock_guard<std::mutex> lock(mutex_);
wait_ = true;
}
private:
// Pops an element from the queue. If an element is present, return
// true. If the queue was empty, return false. Not thread-safe. Must acquire
// the lock before calling.
bool PopUnlocked(T* value) {
if (queue_.empty()) {
return false;
}
*value = queue_.front();
queue_.pop();
return true;
}
// The mutex controls read and write access to the queue_ and stop_
// variables. It is also used to block the calling thread until an element is
// available to pop from the queue.
std::mutex mutex_;
std::condition_variable work_pending_condition_;
std::queue<T> queue_;
// If true, signals that callers of Wait will block waiting to pop an
// element off the queue.
bool wait_{true};
};
} // namespace internal
} // namespace ceres
#endif // CERES_INTERNAL_CONCURRENT_QUEUE_H_
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