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/*
* Copyright © 2022 Mozilla Foundation
*
* This program is made available under an ISC-style license. See the
* accompanying file LICENSE for details.
*/
/**
*/
#ifndef CUBEB_TRIPLE_BUFFER
#define CUBEB_TRIPLE_BUFFER
#include <atomic>
// Single producer / single consumer wait-free triple buffering
// implementation, for when a producer wants to publish data to a consumer
// without blocking, but when a queue is wastefull, because it's OK for the
// consumer to miss data updates.
template <typename T> class triple_buffer {
public:
// Write a new value into the triple buffer. Returns true if a value was
// overwritten.
// Producer-side only.
bool write(T & input)
{
storage[input_idx] = input;
return publish();
}
// Get the latest value from the triple buffer.
// Consumer-side only.
T & read()
{
update();
return storage[output_idx];
}
// Returns true if a new value has been published by the consumer without
// having been consumed yet.
// Consumer-side only.
bool updated()
{
return (shared_state.load(std::memory_order_relaxed) & BACK_DIRTY_BIT) != 0;
}
// Reset state and indices to initial values.
void invalidate()
{
shared_state.store(0, std::memory_order_release);
input_idx = 1;
output_idx = 2;
}
private:
// Publish a value to the consumer. Returns true if the data was overwritten
// without having been read.
bool publish()
{
auto former_back_idx = shared_state.exchange(input_idx | BACK_DIRTY_BIT,
std::memory_order_acq_rel);
input_idx = former_back_idx & BACK_INDEX_MASK;
return (former_back_idx & BACK_DIRTY_BIT) != 0;
}
// Get a new value from the producer, if a new value has been produced.
bool update()
{
bool was_updated = updated();
if (was_updated) {
auto former_back_idx =
shared_state.exchange(output_idx, std::memory_order_acq_rel);
output_idx = former_back_idx & BACK_INDEX_MASK;
}
return was_updated;
}
T storage[3];
// Mask used to extract back-buffer index
const uint8_t BACK_INDEX_MASK = 0b11;
// Bit set by producer to signal updates
const uint8_t BACK_DIRTY_BIT = 0b100;
// Shared state: a dirty bit, and an index.
std::atomic<uint8_t> shared_state = {0};
// Output index, private to the consumer.
uint8_t output_idx = 1;
// Input index, private to the producer.
uint8_t input_idx = 2;
};
#endif // CUBEB_TRIPLE_BUFFER