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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() : wait_(true) {}
+
+  // 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_;
+};
+
+
+}  // namespace internal
+}  // namespace ceres
+
+#endif  // CERES_INTERNAL_CONCURRENT_QUEUE_H_