15:47
Fri
21
Oct 2011

Circular Buffer, Cyclic Buffer or Ring Buffer is a data structure that effectively manages a queue of some items. Items can be added at the back and removed from the front. It has limited capacity because it is based on preallocated array. Functionality is implemented using two pointers or indices - pointing to the first and past the last valid element. The Begin pointer is incremented whenever an item is popped from the front so that it "chases" the End pointer, which is incremented whenever a new item is pushed to the back. They can both wrap around the size of the array. Both operations are done very effectively - in constant time O(1) and no reallocations are needed. This makes circular buffers perfect solution for queues of some data streams, like video or audio.

It's not very sophisticated data structure, but there is one problem. Sample codes of circular buffers you can find on the Internet, just like for many other data structures, operate usually on a single object of some user-defined type. What if we need a buffer for raw binary data, stored as array of bytes? We can treat single bytes as data items, but enqueueing and dequeueing single bytes with separate function calls would not be efficient. We can, on the other hand, define some block of data (like 4096 bytes) as the type of item, but this limits us to operating on on such block at a time.

Best solution would be to write an implementation that operates on binary data in form of (const char *bytes, size_t byte_count) and allows writing and reading arbitrary amount of data in a single call, just like functions for writing and reading files do. The only problem that arises in such code is that sometimes the block of data you want to write to or read from the buffer is not in a continuous region of memory, but wraps around to the beginning of the array so we have to process it on two parts - first at the end of the array and the second at the beginning.

Here is my C++ implementation of a circular buffer for raw binary data:

#include <algorithm> // for std::min

class CircularBuffer
{
public:
  CircularBuffer(size_t capacity);
  ~CircularBuffer();

  size_t size() const { return size_; }
  size_t capacity() const { return capacity_; }
  // Return number of bytes written.
  size_t write(const char *data, size_t bytes);
  // Return number of bytes read.
  size_t read(char *data, size_t bytes);

private:
  size_t beg_index_, end_index_, size_, capacity_;
  char *data_;
};

CircularBuffer::CircularBuffer(size_t capacity)
  : beg_index_(0)
  , end_index_(0)
  , size_(0)
  , capacity_(capacity)
{
  data_ = new char[capacity];
}

CircularBuffer::~CircularBuffer()
{
  delete [] data_;
}

size_t CircularBuffer::write(const char *data, size_t bytes)
{
  if (bytes == 0) return 0;

  size_t capacity = capacity_;
  size_t bytes_to_write = std::min(bytes, capacity - size_);

  // Write in a single step
  if (bytes_to_write <= capacity - end_index_)
  {
    memcpy(data_ + end_index_, data, bytes_to_write);
    end_index_ += bytes_to_write;
    if (end_index_ == capacity) end_index_ = 0;
  }
  // Write in two steps
  else
  {
    size_t size_1 = capacity - end_index_;
    memcpy(data_ + end_index_, data, size_1);
    size_t size_2 = bytes_to_write - size_1;
    memcpy(data_, data + size_1, size_2);
    end_index_ = size_2;
  }

  size_ += bytes_to_write;
  return bytes_to_write;
}

size_t CircularBuffer::read(char *data, size_t bytes)
{
  if (bytes == 0) return 0;

  size_t capacity = capacity_;
  size_t bytes_to_read = std::min(bytes, size_);

  // Read in a single step
  if (bytes_to_read <= capacity - beg_index_)
  {
    memcpy(data, data_ + beg_index_, bytes_to_read);
    beg_index_ += bytes_to_read;
    if (beg_index_ == capacity) beg_index_ = 0;
  }
  // Read in two steps
  else
  {
    size_t size_1 = capacity - beg_index_;
    memcpy(data, data_ + beg_index_, size_1);
    size_t size_2 = bytes_to_read - size_1;
    memcpy(data + size_1, data_, size_2);
    beg_index_ = size_2;
  }

  size_ -= bytes_to_read;
  return bytes_to_read;
}

Similar phenomenon can be observed in API of the FMOD sound library. Just like graphical textures in DirectX, sound samples in FMOD can also be "locked" to get pointer to a raw memory we can read or fill. But DirectX textures lie in the continuous memory region, so we get a single pointer. The only difficult thing in understanding locking textures is the concept of "stride", which can be greater than the width of a single row. Here in FMOD the Sound::lock() method returns two pointers and two lengths, probably because the locked region can wrap over end of internally used circular buffer like the one shown above.

Comments (4) | Tags: c++ algorithms

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