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cubeb_triple_buffer.h (2562B)

      1 /*
      2 * Copyright © 2022 Mozilla Foundation
      3 *
      4 * This program is made available under an ISC-style license.  See the
      5 * accompanying file LICENSE for details.
      6 */
      7 
      8 /**
      9 * Adapted and ported to C++ from https://crates.io/crates/triple_buffer
     10 */
     11 
     12 #ifndef CUBEB_TRIPLE_BUFFER
     13 #define CUBEB_TRIPLE_BUFFER
     14 
     15 #include <atomic>
     16 
     17 // Single producer / single consumer wait-free triple buffering
     18 // implementation, for when a producer wants to publish data to a consumer
     19 // without blocking, but when a queue is wastefull, because it's OK for the
     20 // consumer to miss data updates.
     21 template <typename T> class triple_buffer {
     22 public:
     23  // Write a new value into the triple buffer. Returns true if a value was
     24  // overwritten.
     25  // Producer-side only.
     26  bool write(T & input)
     27  {
     28    storage[input_idx] = input;
     29    return publish();
     30  }
     31  // Get the latest value from the triple buffer.
     32  // Consumer-side only.
     33  T & read()
     34  {
     35    update();
     36    return storage[output_idx];
     37  }
     38  // Returns true if a new value has been published by the consumer without
     39  // having been consumed yet.
     40  // Consumer-side only.
     41  bool updated()
     42  {
     43    return (shared_state.load(std::memory_order_relaxed) & BACK_DIRTY_BIT) != 0;
     44  }
     45  // Reset state and indices to initial values.
     46  void invalidate()
     47  {
     48    shared_state.store(0, std::memory_order_release);
     49    input_idx = 1;
     50    output_idx = 2;
     51  }
     52 
     53 private:
     54  // Publish a value to the consumer. Returns true if the data was overwritten
     55  // without having been read.
     56  bool publish()
     57  {
     58    auto former_back_idx = shared_state.exchange(input_idx | BACK_DIRTY_BIT,
     59                                                 std::memory_order_acq_rel);
     60    input_idx = former_back_idx & BACK_INDEX_MASK;
     61    return (former_back_idx & BACK_DIRTY_BIT) != 0;
     62  }
     63  // Get a new value from the producer, if a new value has been produced.
     64  bool update()
     65  {
     66    bool was_updated = updated();
     67    if (was_updated) {
     68      auto former_back_idx =
     69          shared_state.exchange(output_idx, std::memory_order_acq_rel);
     70      output_idx = former_back_idx & BACK_INDEX_MASK;
     71    }
     72    return was_updated;
     73  }
     74  T storage[3];
     75  // Mask used to extract back-buffer index
     76  const uint8_t BACK_INDEX_MASK = 0b11;
     77  // Bit set by producer to signal updates
     78  const uint8_t BACK_DIRTY_BIT = 0b100;
     79  // Shared state: a dirty bit, and an index.
     80  std::atomic<uint8_t> shared_state = {0};
     81  // Output index, private to the consumer.
     82  uint8_t output_idx = 1;
     83  // Input index, private to the producer.
     84  uint8_t input_idx = 2;
     85 };
     86 
     87 #endif // CUBEB_TRIPLE_BUFFER