libs/vr/libdvrcommon/include/private/dvr/ring_buffer.h

#ifndef ANDROID_DVR_RING_BUFFER_H_
#define ANDROID_DVR_RING_BUFFER_H_

#include <utility>
#include <vector>

namespace android {
namespace dvr {

// A simple ring buffer implementation.
//
// A vector works but you either have to keep track of start_ and size_ yourself
// or erase() from the front which is inefficient.
//
// A deque works but the common usage pattern of Append() PopFront() Append()
// PopFront() looks like it allocates each time size goes from 0 --> 1, which we
// don't want. This class allocates only once.
template <typename T>
class RingBuffer {
 public:
  RingBuffer() { Reset(0); }

  explicit RingBuffer(size_t capacity) { Reset(capacity); }

  RingBuffer(const RingBuffer& other) = default;
  RingBuffer(RingBuffer&& other) noexcept = default;
  RingBuffer& operator=(const RingBuffer& other) = default;
  RingBuffer& operator=(RingBuffer&& other) noexcept = default;

  void Append(const T& val) {
    if (IsFull())
      PopFront();
    Get(size_) = val;
    size_++;
  }

  void Append(T&& val) {
    if (IsFull())
      PopFront();
    Get(size_) = std::move(val);
    size_++;
  }

  bool IsEmpty() const { return size_ == 0; }

  bool IsFull() const { return size_ == buffer_.size(); }

  size_t GetSize() const { return size_; }

  size_t GetCapacity() const { return buffer_.size(); }

  T& Get(size_t i) { return buffer_[(start_ + i) % buffer_.size()]; }

  const T& Get(size_t i) const {
    return buffer_[(start_ + i) % buffer_.size()];
  }

  const T& Back() const { return Get(size_ - 1); }

  T& Back() { return Get(size_ - 1); }

  const T& Front() const { return Get(0); }

  T& Front() { return Get(0); }

  void PopBack() {
    if (size_ != 0) {
      Get(size_ - 1) = T();
      size_--;
    }
  }

  void PopFront() {
    if (size_ != 0) {
      Get(0) = T();
      start_ = (start_ + 1) % buffer_.size();
      size_--;
    }
  }

  void Clear() { Reset(GetCapacity()); }

  void Reset(size_t capacity) {
    start_ = size_ = 0;
    buffer_.clear();
    buffer_.resize(capacity);
  }

 private:
  // Ideally we'd allocate our own memory and use placement new to instantiate
  // instances of T instead of using a vector, but the vector is simpler.
  std::vector<T> buffer_;
  size_t start_, size_;
};

}  // namespace dvr
}  // namespace android

#endif  // ANDROID_DVR_RING_BUFFER_H_