How to efficiently unpack float, int16, int32, etc. data from void* array in C++?

Question

I’ve got an array of PCM data; it can be 16-bit, 24-bit packed, 32-bit, etc. It can be signed, or unsigned, and it can be 32 or 64-bit floating point. It is currently stored as a void** matrix, indexed first by channel, then by frame. The goal is to allow my library to take in any PCM format and buffer it, without requiring manipulation of the data to fit a designated structure. If the A/D converter spits out 24-bit packed arrays of interleaved PCM, I need to accept it gracefully. I also need to support 16-bit non-interleaved, as well as any permutation of the above formats.

I know the bit depth and other information at runtime, and I’m trying to code efficiently while not duplicating code. What I need is an effective way to cast the matrix, put PCM data into the matrix, and then pull it out later.

I can cast the matrix to int32_t, or int16_t for the 32 and 16-bit signed PCM respectively; I’ll probably have to store the 24-bit PCM in an int32_t for 32-bit, 8-bit byte systems as well.

Can anyone recommend a good way to put data into this array, and pull it out later? I’d like to avoid large sections of code which look like:

switch (mFormat) {
case 1:  // unsigned 8 bit
  for (int i = 0; i < mChannels; i++)
    framesArray = (uint8_t*)pcm[i];
  break;
case 2:  // signed 8 bit
  for (int i = 0; i < mChannels; i++)
    framesArray = (int8_t*)pcm[i];
  break;
case 3:  // unsigned 16 bit
...

Limitations: I’m working in C/C++, no templates, no RTTI, no STL. Think embedded. Things get trickier when I have to port this to a DSP with 16-bit bytes.

Does anybody have any useful macros they might be willing to share?

Answer 1

This one will match typecodes to casting functions. The basic idea is that it creates a set of tiny conversion functions for each type, and an array of function pointers, and then indexes into that array based on the data format in order to find the correct conversion function to call.

Usage example:

int main ()
{   
    void** pcm;
    int currentChannel;
    int currentFrame;
    int mFormat;

    // gets data casted to our type
    STORETYPE translatedFrameData = GET_FRAMEDATA(pcm, currentChannel, currentFrame, mFormat);  

    return 0;
}

The header file:

// this is a big type, we cast to this one
#define STORETYPE int32_t

// these functions get a single frame
typedef STORETYPE (*getterFunction)(void**, int, int);

// this macros make an array that maps format codes to cast functions
#define BEGIN_RESERVE_FORMAT_CODES getterFunction __getter_array[] = {
#define RESERVE_FORMAT_CODE(code) __get__##code##__,
#define END_RESERVE_FORMAT_CODES };

//
#define FORMAT_DEFINITION(code, format) STORETYPE __get__##code##__(void**pcm, int channel, int frame) \
{ return (STORETYPE) ((format**)pcm)[channel][frame]; }

// get corresponding function 
#define GET_FRAMEDATA( pcm, channel, frame, format ) __getter_array[format](pcm,channel,frame)

//serious part, define needed types
FORMAT_DEFINITION(0, uint8_t)
FORMAT_DEFINITION(1, int8_t)
FORMAT_DEFINITION(2, uint16_t)
FORMAT_DEFINITION(3, int16_t)

//actually this makes the array which binds types
BEGIN_RESERVE_FORMAT_CODES
    RESERVE_FORMAT_CODE(0)
    RESERVE_FORMAT_CODE(1)
    RESERVE_FORMAT_CODE(2)
    RESERVE_FORMAT_CODE(3)
END_RESERVE_FORMAT_CODES

//WATCH OUT FOR SEQUENCE

hope helps