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+/*
+ * 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 rd_idx = read_index_.load(std::memory_order::memory_order_relaxed);
+ int wr_idx = write_index_.load(std::memory_order::memory_order_relaxed);
+
+ 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::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 wr_idx = write_index_.load(std::memory_order::memory_order_acquire);
+ int rd_idx = read_index_.load(std::memory_order::memory_order_relaxed);
+
+ 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::memory_order_relaxed);
+
+ 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::memory_order_relaxed),
+ write_index_.load(std::memory_order::memory_order_relaxed));
+ }
+ /**
+ * 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::memory_order_relaxed),
+ write_index_.load(std::memory_order::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