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Diffstat (limited to 'media/libcubeb/src/cubeb_ringbuffer.h')
| -rw-r--r-- | media/libcubeb/src/cubeb_ringbuffer.h | 468 |
1 files changed, 468 insertions, 0 deletions
diff --git a/media/libcubeb/src/cubeb_ringbuffer.h b/media/libcubeb/src/cubeb_ringbuffer.h new file mode 100644 index 0000000000..f020351566 --- /dev/null +++ b/media/libcubeb/src/cubeb_ringbuffer.h @@ -0,0 +1,468 @@ +/* + * 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 |