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// Copyright 2016 The Cobalt Authors. All Rights Reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// Module Overview: Starboard Queue module
//
// Defines a C++-only synchronized queue implementation, built entirely on top
// of Starboard synchronization primitives. It can be safely used by both
// clients and implementations.
#ifndef STARBOARD_QUEUE_H_
#define STARBOARD_QUEUE_H_
#include <algorithm>
#include <deque>
#include "starboard/condition_variable.h"
#include "starboard/export.h"
#include "starboard/mutex.h"
#include "starboard/time.h"
#include "starboard/types.h"
#ifndef __cplusplus
#error "Only C++ files can include this header."
#endif
namespace starboard {
// Synchronized, blocking queue, based on starboard::ConditionVariable. This
// class is designed for T to be a pointer type, or something otherwise
// inherently Nullable, as there is no way to distinguish T() from "no result."
template <typename T>
class Queue {
public:
Queue() : condition_(mutex_), wake_(false) {}
~Queue() {}
// Polls for an item, returning the default value of T if nothing is present.
T Poll() {
ScopedLock lock(mutex_);
if (!queue_.empty()) {
T entry = queue_.front();
queue_.pop_front();
return entry;
}
return T();
}
// Gets the item at the front of the queue, blocking until there is such an
// item, or the queue is woken up. If there are multiple waiters, this Queue
// guarantees that only one waiter will receive any given queue item.
T Get() {
ScopedLock lock(mutex_);
while (queue_.empty()) {
if (wake_) {
wake_ = false;
return T();
}
condition_.Wait();
}
T entry = queue_.front();
queue_.pop_front();
return entry;
}
// Gets the item at the front of the queue, blocking until there is such an
// item, or the given timeout duration expires, or the queue is woken up. If
// there are multiple waiters, this Queue guarantees that only one waiter will
// receive any given queue item.
T GetTimed(SbTime duration) {
ScopedLock lock(mutex_);
SbTimeMonotonic start = SbTimeGetMonotonicNow();
while (queue_.empty()) {
if (wake_) {
wake_ = false;
return T();
}
SbTimeMonotonic elapsed = SbTimeGetMonotonicNow() - start;
if (elapsed >= duration) {
return T();
}
condition_.WaitTimed(duration - elapsed);
}
T entry = queue_.front();
queue_.pop_front();
return entry;
}
// Pushes |value| onto the back of the queue, waking up a single waiter, if
// any exist.
void Put(T value) {
ScopedLock lock(mutex_);
queue_.push_back(value);
condition_.Signal();
}
// Forces one waiter to wake up empty-handed.
void Wake() {
ScopedLock lock(mutex_);
wake_ = true;
condition_.Signal();
}
// It is guaranteed that after this function returns there is no element in
// the queue equals to |value|. This is useful to remove un-processed values
// before destroying the owning object.
// Note that it is the responsibility of the caller to free any resources
// associated with |value| after this function returns. It is possible that
// another thread is still using |value|, the caller should make sure that
// this is properly coordinated with the free of resources. Usually this can
// be solved by calling Remove() on the same thread that calls Get().
void Remove(T value) {
ScopedLock lock(mutex_);
queue_.erase(std::remove(queue_.begin(), queue_.end(), value),
queue_.end());
}
void Clear() {
ScopedLock lock(mutex_);
queue_.clear();
}
size_t Size() {
ScopedLock lock(mutex_);
return queue_.size();
}
private:
Mutex mutex_;
ConditionVariable condition_;
std::deque<T> queue_;
bool wake_;
};
} // namespace starboard
#endif // STARBOARD_QUEUE_H_