/*
* Copyright © 2016 Mozilla Foundation
*
* This program is made available under an ISC-style license. See the
* accompanying file LICENSE for details.
*/
#ifndef CUBEB_RING_BUFFER_H
#define CUBEB_RING_BUFFER_H
#include "cubeb_utils.h"
#include <algorithm>
#include <atomic>
#include <cstdint>
#include <memory>
#include <thread>
/**
* Single producer single consumer lock-free and wait-free ring buffer.
*
* This data structure allows producing data from one thread, and consuming it
* on another thread, safely and without explicit synchronization. If used on
* two threads, this data structure uses atomics for thread safety. It is
* possible to disable the use of atomics at compile time and only use this data
* structure on one thread.
*
* The role for the producer and the consumer must be constant, i.e., the
* producer should always be on one thread and the consumer should always be on
* another thread.
*
* Some words about the inner workings of this class:
* - Capacity is fixed. Only one allocation is performed, in the constructor.
* When reading and writing, the return value of the method allows checking if
* the ring buffer is empty or full.
* - We always keep the read index at least one element ahead of the write
* index, so we can distinguish between an empty and a full ring buffer: an
* empty ring buffer is when the write index is at the same position as the
* read index. A full buffer is when the write index is exactly one position
* before the read index.
* - We synchronize updates to the read index after having read the data, and
* the write index after having written the data. This means that the each
* thread can only touch a portion of the buffer that is not touched by the
* other thread.
* - Callers are expected to provide buffers. When writing to the queue,
* elements are copied into the internal storage from the buffer passed in.
* When reading from the queue, the user is expected to provide a buffer.
* Because this is a ring buffer, data might not be contiguous in memory,
* providing an external buffer to copy into is an easy way to have linear
* data for further processing.
*/
template <
typename T>
class ring_buffer_base {
public:
/**
* Constructor for a ring buffer.
*
* This performs an allocation, but is the only allocation that will happen
* for the life time of a `ring_buffer_base`.
*
* @param capacity The maximum number of element this ring buffer will hold.
*/
ring_buffer_base(
int capacity)
/* One more element to distinguish from empty and full buffer. */
: capacity_(capacity + 1)
{
assert(storage_capacity() < std::numeric_limits<
int>::max() / 2 &&
"buffer too large for the type of index used.");
assert(capacity_ > 0);
data_.reset(
new T[storage_capacity()]);
/* If this queue is using atomics, initializing those members as the last
* action in the constructor acts as a full barrier, and allow capacity() to
* be thread-safe. */
write_index_ = 0;
read_index_ = 0;
}
/**
* Push `count` zero or default constructed elements in the array.
*
* Only safely called on the producer thread.
*
* @param count The number of elements to enqueue.
* @return The number of element enqueued.
*/
int enqueue_default(
int count) {
return enqueue(nullptr, count); }
/**
* @brief Put an element in the queue
*
* Only safely called on the producer thread.
*
* @param element The element to put in the queue.
*
* @return 1 if the element was inserted, 0 otherwise.
*/
int enqueue(T & element) {
return enqueue(&element, 1); }
/**
* Push `count` elements in the ring buffer.
*
* Only safely called on the producer thread.
*
* @param elements a pointer to a buffer containing at least `count` elements.
* If `elements` is nullptr, zero or default constructed elements are
* enqueued.
* @param count The number of elements to read from `elements`
* @return The number of elements successfully coped from `elements` and
* inserted into the ring buffer.
*/
int enqueue(T * elements,
int count)
{
#ifndef NDEBUG
assert_correct_thread(producer_id);
#endif
int wr_idx = write_index_.load(std::memory_order_relaxed);
int rd_idx = read_index_.load(std::memory_order_acquire);
if (full_internal(rd_idx, wr_idx)) {
return 0;
}
int to_write = std::min(available_write_internal(rd_idx, wr_idx), count);
/* First part, from the write index to the end of the array. */
int first_part = std::min(storage_capacity() - wr_idx, to_write);
/* Second part, from the beginning of the array */
int second_part = to_write - first_part;
if (elements) {
Copy(data_.get() + wr_idx, elements, first_part);
Copy(data_.get(), elements + first_part, second_part);
}
else {
ConstructDefault(data_.get() + wr_idx, first_part);
ConstructDefault(data_.get(), second_part);
}
write_index_.store(increment_index(wr_idx, to_write),
std::memory_order_release);
return to_write;
}
/**
* Retrieve at most `count` elements from the ring buffer, and copy them to
* `elements`, if non-null.
*
* Only safely called on the consumer side.
*
* @param elements A pointer to a buffer with space for at least `count`
* elements. If `elements` is `nullptr`, `count` element will be discarded.
* @param count The maximum number of elements to dequeue.
* @return The number of elements written to `elements`.
*/
int dequeue(T * elements,
int count)
{
#ifndef NDEBUG
assert_correct_thread(consumer_id);
#endif
int rd_idx = read_index_.load(std::memory_order_relaxed);
int wr_idx = write_index_.load(std::memory_order_acquire);
if (empty_internal(rd_idx, wr_idx)) {
return 0;
}
int to_read = std::min(available_read_internal(rd_idx, wr_idx), count);
int first_part = std::min(storage_capacity() - rd_idx, to_read);
int second_part = to_read - first_part;
if (elements) {
Copy(elements, data_.get() + rd_idx, first_part);
Copy(elements + first_part, data_.get(), second_part);
}
read_index_.store(increment_index(rd_idx, to_read),
std::memory_order_release);
return to_read;
}
/**
* Get the number of available element for consuming.
*
* Only safely called on the consumer thread.
*
* @return The number of available elements for reading.
*/
int available_read()
const
{
#ifndef NDEBUG
assert_correct_thread(consumer_id);
#endif
return available_read_internal(
read_index_.load(std::memory_order_relaxed),
write_index_.load(std::memory_order_acquire));
}
/**
* Get the number of available elements for consuming.
*
* Only safely called on the producer thread.
*
* @return The number of empty slots in the buffer, available for writing.
*/
int available_write()
const
{
#ifndef NDEBUG
assert_correct_thread(producer_id);
#endif
return available_write_internal(
read_index_.load(std::memory_order_acquire),
write_index_.load(std::memory_order_relaxed));
}
/**
* Get the total capacity, for this ring buffer.
*
* Can be called safely on any thread.
*
* @return The maximum capacity of this ring buffer.
*/
int capacity()
const {
return storage_capacity() - 1; }
/**
* Reset the consumer and producer thread identifier, in case the thread are
* being changed. This has to be externally synchronized. This is no-op when
* asserts are disabled.
*/
void reset_thread_ids()
{
#ifndef NDEBUG
consumer_id = producer_id = std::thread::id();
#endif
}
private:
/** Return true if the ring buffer is empty.
*
* @param read_index the read index to consider
* @param write_index the write index to consider
* @return true if the ring buffer is empty, false otherwise.
**/
bool empty_internal(
int read_index,
int write_index)
const
{
return write_index == read_index;
}
/** Return true if the ring buffer is full.
*
* This happens if the write index is exactly one element behind the read
* index.
*
* @param read_index the read index to consider
* @param write_index the write index to consider
* @return true if the ring buffer is full, false otherwise.
**/
bool full_internal(
int read_index,
int write_index)
const
{
return (write_index + 1) % storage_capacity() == read_index;
}
/**
* Return the size of the storage. It is one more than the number of elements
* that can be stored in the buffer.
*
* @return the number of elements that can be stored in the buffer.
*/
int storage_capacity()
const {
return capacity_; }
/**
* Returns the number of elements available for reading.
*
* @return the number of available elements for reading.
*/
int available_read_internal(
int read_index,
int write_index)
const
{
if (write_index >= read_index) {
return write_index - read_index;
}
else {
return write_index + storage_capacity() - read_index;
}
}
/**
* Returns the number of empty elements, available for writing.
*
* @return the number of elements that can be written into the array.
*/
int available_write_internal(
int read_index,
int write_index)
const
{
/* We substract one element here to always keep at least one sample
* free in the buffer, to distinguish between full and empty array. */
int rv = read_index - write_index - 1;
if (write_index >= read_index) {
rv += storage_capacity();
}
return rv;
}
/**
* Increments an index, wrapping it around the storage.
*
* @param index a reference to the index to increment.
* @param increment the number by which `index` is incremented.
* @return the new index.
*/
int increment_index(
int index,
int increment)
const
{
assert(increment >= 0);
return (index + increment) % storage_capacity();
}
/**
* @brief This allows checking that enqueue (resp. dequeue) are always called
* by the right thread.
*
* @param id the id of the thread that has called the calling method first.
*/
#ifndef NDEBUG
static void assert_correct_thread(std::thread::id & id)
{
if (id == std::thread::id()) {
id = std::this_thread::get_id();
return;
}
assert(id == std::this_thread::get_id());
}
#endif
/** Index at which the oldest element is at, in samples. */
std::atomic<
int> read_index_;
/** Index at which to write new elements. `write_index` is always at
* least one element ahead of `read_index_`. */
std::atomic<
int> write_index_;
/** Maximum number of elements that can be stored in the ring buffer. */
const int capacity_;
/** Data storage */
std::unique_ptr<T[]> data_;
#ifndef NDEBUG
/** The id of the only thread that is allowed to read from the queue. */
mutable std::thread::id consumer_id;
/** The id of the only thread that is allowed to write from the queue. */
mutable std::thread::id producer_id;
#endif
};
/**
* Adapter for `ring_buffer_base` that exposes an interface in frames.
*/
template <
typename T>
class audio_ring_buffer_base {
public:
/**
* @brief Constructor.
*
* @param channel_count Number of channels.
* @param capacity_in_frames The capacity in frames.
*/
audio_ring_buffer_base(
int channel_count,
int capacity_in_frames)
: channel_count(channel_count),
ring_buffer(frames_to_samples(capacity_in_frames))
{
assert(channel_count > 0);
}
/**
* @brief Enqueue silence.
*
* Only safely called on the producer thread.
*
* @param frame_count The number of frames of silence to enqueue.
* @return The number of frames of silence actually written to the queue.
*/
int enqueue_default(
int frame_count)
{
return samples_to_frames(
ring_buffer.enqueue(nullptr, frames_to_samples(frame_count)));
}
/**
* @brief Enqueue `frames_count` frames of audio.
*
* Only safely called from the producer thread.
*
* @param [in] frames If non-null, the frames to enqueue.
* Otherwise, silent frames are enqueued.
* @param frame_count The number of frames to enqueue.
*
* @return The number of frames enqueued
*/
int enqueue(T * frames,
int frame_count)
{
return samples_to_frames(
ring_buffer.enqueue(frames, frames_to_samples(frame_count)));
}
/**
* @brief Removes `frame_count` frames from the buffer, and
* write them to `frames` if it is non-null.
*
* Only safely called on the consumer thread.
*
* @param frames If non-null, the frames are copied to `frames`.
* Otherwise, they are dropped.
* @param frame_count The number of frames to remove.
*
* @return The number of frames actually dequeud.
*/
int dequeue(T * frames,
int frame_count)
{
return samples_to_frames(
ring_buffer.dequeue(frames, frames_to_samples(frame_count)));
}
/**
* Get the number of available frames of audio for consuming.
*
* Only safely called on the consumer thread.
*
* @return The number of available frames of audio for reading.
*/
int available_read()
const
{
return samples_to_frames(ring_buffer.available_read());
}
/**
* Get the number of available frames of audio for consuming.
*
* Only safely called on the producer thread.
*
* @return The number of empty slots in the buffer, available for writing.
*/
int available_write()
const
{
return samples_to_frames(ring_buffer.available_write());
}
/**
* Get the total capacity, for this ring buffer.
*
* Can be called safely on any thread.
*
* @return The maximum capacity of this ring buffer.
*/
int capacity()
const {
return samples_to_frames(ring_buffer.capacity()); }
private:
/**
* @brief Frames to samples conversion.
*
* @param frames The number of frames.
*
* @return A number of samples.
*/
int frames_to_samples(
int frames)
const {
return frames * channel_count; }
/**
* @brief Samples to frames conversion.
*
* @param samples The number of samples.
*
* @return A number of frames.
*/
int samples_to_frames(
int samples)
const {
return samples / channel_count; }
/** Number of channels of audio that will stream through this ring buffer. */
int channel_count;
/** The underlying ring buffer that is used to store the data. */
ring_buffer_base<T> ring_buffer;
};
/**
* Lock-free instantiation of the `ring_buffer_base` type. This is safe to use
* from two threads, one producer, one consumer (that never change role),
* without explicit synchronization.
*/
template <
typename T>
using lock_free_queue = ring_buffer_base<T>;
/**
* Lock-free instantiation of the `audio_ring_buffer` type. This is safe to use
* from two threads, one producer, one consumer (that never change role),
* without explicit synchronization.
*/
template <
typename T>
using lock_free_audio_ring_buffer = audio_ring_buffer_base<T>;
#endif // CUBEB_RING_BUFFER_H
