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/*
* Copyright (c) 2015 The WebRTC project authors. All Rights Reserved.
*
* Use of this source code is governed by a BSD-style license
* that can be found in the LICENSE file in the root of the source
* tree. An additional intellectual property rights grant can be found
* in the file PATENTS. All contributing project authors may
* be found in the AUTHORS file in the root of the source tree.
*/
#ifndef RTC_BASE_SWAP_QUEUE_H_
#define RTC_BASE_SWAP_QUEUE_H_
#include <stddef.h>
#include <utility>
#include <vector>
#include "rtc_base/checks.h"
#include "rtc_base/constructormagic.h"
#include "rtc_base/criticalsection.h"
#include "rtc_base/system/unused.h"
#include "rtc_base/thread_annotations.h"
namespace webrtc {
namespace internal {
// (Internal; please don't use outside this file.)
template <typename T>
bool NoopSwapQueueItemVerifierFunction(const T&) {
return true;
}
} // namespace internal
// Functor to use when supplying a verifier function for the queue.
template <typename T,
bool (*QueueItemVerifierFunction)(const T&) =
internal::NoopSwapQueueItemVerifierFunction>
class SwapQueueItemVerifier {
public:
bool operator()(const T& t) const { return QueueItemVerifierFunction(t); }
};
// This class is a fixed-size queue. A producer calls Insert() to insert
// an element of type T at the back of the queue, and a consumer calls
// Remove() to remove an element from the front of the queue. It's safe
// for the producer(s) and the consumer(s) to access the queue
// concurrently, from different threads.
//
// To avoid the construction, copying, and destruction of Ts that a naive
// queue implementation would require, for each "full" T passed from
// producer to consumer, SwapQueue<T> passes an "empty" T in the other
// direction (an "empty" T is one that contains nothing of value for the
// consumer). This bidirectional movement is implemented with swap().
//
// // Create queue:
// Bottle proto(568); // Prepare an empty Bottle. Heap allocates space for
// // 568 ml.
// SwapQueue<Bottle> q(N, proto); // Init queue with N copies of proto.
// // Each copy allocates on the heap.
// // Producer pseudo-code:
// Bottle b(568); // Prepare an empty Bottle. Heap allocates space for 568 ml.
// loop {
// b.Fill(amount); // Where amount <= 568 ml.
// q.Insert(&b); // Swap our full Bottle for an empty one from q.
// }
//
// // Consumer pseudo-code:
// Bottle b(568); // Prepare an empty Bottle. Heap allocates space for 568 ml.
// loop {
// q.Remove(&b); // Swap our empty Bottle for the next-in-line full Bottle.
// Drink(&b);
// }
//
// For a well-behaved Bottle class, there are no allocations in the
// producer, since it just fills an empty Bottle that's already large
// enough; no deallocations in the consumer, since it returns each empty
// Bottle to the queue after having drunk it; and no copies along the
// way, since the queue uses swap() everywhere to move full Bottles in
// one direction and empty ones in the other.
template <typename T, typename QueueItemVerifier = SwapQueueItemVerifier<T>>
class SwapQueue {
public:
// Creates a queue of size size and fills it with default constructed Ts.
explicit SwapQueue(size_t size) : queue_(size) {
RTC_DCHECK(VerifyQueueSlots());
}
// Same as above and accepts an item verification functor.
SwapQueue(size_t size, const QueueItemVerifier& queue_item_verifier)
: queue_item_verifier_(queue_item_verifier), queue_(size) {
RTC_DCHECK(VerifyQueueSlots());
}
// Creates a queue of size size and fills it with copies of prototype.
SwapQueue(size_t size, const T& prototype) : queue_(size, prototype) {
RTC_DCHECK(VerifyQueueSlots());
}
// Same as above and accepts an item verification functor.
SwapQueue(size_t size,
const T& prototype,
const QueueItemVerifier& queue_item_verifier)
: queue_item_verifier_(queue_item_verifier), queue_(size, prototype) {
RTC_DCHECK(VerifyQueueSlots());
}
// Resets the queue to have zero content wile maintaining the queue size.
void Clear() {
rtc::CritScope cs(&crit_queue_);
next_write_index_ = 0;
next_read_index_ = 0;
num_elements_ = 0;
}
// Inserts a "full" T at the back of the queue by swapping *input with an
// "empty" T from the queue.
// Returns true if the item was inserted or false if not (the queue was full).
// When specified, the T given in *input must pass the ItemVerifier() test.
// The contents of *input after the call are then also guaranteed to pass the
// ItemVerifier() test.
bool Insert(T* input) RTC_WARN_UNUSED_RESULT {
RTC_DCHECK(input);
rtc::CritScope cs(&crit_queue_);
RTC_DCHECK(queue_item_verifier_(*input));
if (num_elements_ == queue_.size()) {
return false;
}
using std::swap;
swap(*input, queue_[next_write_index_]);
++next_write_index_;
if (next_write_index_ == queue_.size()) {
next_write_index_ = 0;
}
++num_elements_;
RTC_DCHECK_LT(next_write_index_, queue_.size());
RTC_DCHECK_LE(num_elements_, queue_.size());
return true;
}
// Removes the frontmost "full" T from the queue by swapping it with
// the "empty" T in *output.
// Returns true if an item could be removed or false if not (the queue was
// empty). When specified, The T given in *output must pass the ItemVerifier()
// test and the contents of *output after the call are then also guaranteed to
// pass the ItemVerifier() test.
bool Remove(T* output) RTC_WARN_UNUSED_RESULT {
RTC_DCHECK(output);
rtc::CritScope cs(&crit_queue_);
RTC_DCHECK(queue_item_verifier_(*output));
if (num_elements_ == 0) {
return false;
}
using std::swap;
swap(*output, queue_[next_read_index_]);
++next_read_index_;
if (next_read_index_ == queue_.size()) {
next_read_index_ = 0;
}
--num_elements_;
RTC_DCHECK_LT(next_read_index_, queue_.size());
RTC_DCHECK_LE(num_elements_, queue_.size());
return true;
}
private:
// Verify that the queue slots complies with the ItemVerifier test.
bool VerifyQueueSlots() {
rtc::CritScope cs(&crit_queue_);
for (const auto& v : queue_) {
RTC_DCHECK(queue_item_verifier_(v));
}
return true;
}
rtc::CriticalSection crit_queue_;
// TODO(peah): Change this to use std::function() once we can use C++11 std
// lib.
QueueItemVerifier queue_item_verifier_ RTC_GUARDED_BY(crit_queue_);
// (next_read_index_ + num_elements_) % queue_.size() =
// next_write_index_
size_t next_write_index_ RTC_GUARDED_BY(crit_queue_) = 0;
size_t next_read_index_ RTC_GUARDED_BY(crit_queue_) = 0;
size_t num_elements_ RTC_GUARDED_BY(crit_queue_) = 0;
// queue_.size() is constant.
std::vector<T> queue_ RTC_GUARDED_BY(crit_queue_);
RTC_DISALLOW_COPY_AND_ASSIGN(SwapQueue);
};
} // namespace webrtc
#endif // RTC_BASE_SWAP_QUEUE_H_