3dsurface/TripleBuffer.cpp at master · ubique/3dsurface · GitHub

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#include "TripleBuffer.h"

#include <atomic>
#include <cassert>

namespace xyz {

namespace {

constexpr unsigned consumer_buffer_index_mask{0x00000003};
constexpr unsigned spare_buffer_index_mask{0x0000000c};
constexpr unsigned spare_buffer_index_shift{2};
constexpr unsigned producer_buffer_index_mask{0x00000030};
constexpr unsigned producer_buffer_index_shift{4};
constexpr unsigned spare_buffer_dirty_flag{0x00000040};

} // namespace

TripleBuffer::TripleBuffer(std::array<void *, 3> buffers) : buffers_(buffers) {
  flags_ =
      0 | (1 << spare_buffer_index_shift) | (2 << producer_buffer_index_shift);
}

void *TripleBuffer::producer_buffer() {
  unsigned flags = flags_.load(std::memory_order_acquire);

  flags &= producer_buffer_index_mask;
  flags >>= producer_buffer_index_shift;

  return buffers_[flags];
}

void TripleBuffer::switch_producer_buffer() {
  unsigned flags = flags_.load(std::memory_order_acquire);

  // Spare buffer hasn't been consumed by producer yet.
  if (flags & spare_buffer_dirty_flag) {
    return;
  }

  const unsigned spare_index =
      (flags & spare_buffer_index_mask) >> spare_buffer_index_shift;
  const unsigned producer_index =
      (flags & producer_buffer_index_mask) >> producer_buffer_index_shift;

  flags |= spare_buffer_dirty_flag;
  flags &= ~spare_buffer_index_mask;
  flags &= ~producer_buffer_index_mask;
  flags |= spare_index << producer_buffer_index_shift;
  flags |= producer_index << spare_buffer_index_shift;

  flags_.store(flags, std::memory_order_release);
}

void *TripleBuffer::consumer_buffer() {
  unsigned flags = flags_.load(std::memory_order_acquire);

  flags &= consumer_buffer_index_mask;

  return buffers_[flags];
}

void TripleBuffer::switch_consumer_buffer() {
  unsigned flags = flags_.load(std::memory_order_acquire);

  if (!(flags & spare_buffer_dirty_flag)) {
    return;
  }

  // spare buffer with dirty flag set becomes new consumer buffer
  // current consumer buffer becomes new spare buffer
  // unset dirty flag
  const unsigned spare_index =
      (flags & spare_buffer_index_mask) >> spare_buffer_index_shift;
  const unsigned consumer_index = flags & consumer_buffer_index_mask;

  flags &= ~spare_buffer_dirty_flag;
  flags &= ~spare_buffer_index_mask;
  flags &= ~consumer_buffer_index_mask;
  flags |= spare_index;
  flags |= (consumer_index << spare_buffer_index_shift);

  flags_.store(flags, std::memory_order_release);
}

} // namespace xyz