dep/cubeb: Minimize and update to 54217bc

dep/cubeb/src/cubeb_triple_buffer.h

@@ -0,0 +1,80 @@
+/*
+ * Copyright © 2022 Mozilla Foundation
+ *
+ * This program is made available under an ISC-style license.  See the
+ * accompanying file LICENSE for details.
+ */
+
+/**
+ * Adapted and ported to C++ from https://crates.io/crates/triple_buffer
+ */
+
+#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;
+  }
+
+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