dvi_adpcm_encode.c

/***********************************************************
Copyright 1992 by Stichting Mathematisch Centrum, Amsterdam, The
Netherlands.

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******************************************************************/

/*
** Intel/DVI ADPCM coder/decoder.
**
** The algorithm for this coder was taken from the IMA Compatability Project
** proceedings, Vol 2, Number 2; May 1992.
**
** Version 1.2, 18-Dec-92.
**
** Change log:
** - Fixed a stupid bug, where the delta was computed as
**   stepsize*code/4 in stead of stepsize*(code+0.5)/4.
** - There was an off-by-one error causing it to pick
**   an incorrect delta once in a blue moon.
** - The NODIVMUL define has been removed. Computations are now always done
**   using shifts, adds and subtracts. It turned out that, because the standard
**   is defined using shift/add/subtract, you needed bits of fixup code
**   (because the div/mul simulation using shift/add/sub made some rounding
**   errors that real div/mul don't make) and all together the resultant code
**   ran slower than just using the shifts all the time.
** - Changed some of the variable names to be more meaningful.
*/

// modified May 30 2018 by jph for use in Moddable SDK]

#include "stdint.h"

/* Intel ADPCM step variation table */
static const int indexTable[16] = {
    -1, -1, -1, -1, 2, 4, 6, 8,
    -1, -1, -1, -1, 2, 4, 6, 8,
};

static const int stepsizeTable[89] = {
    7, 8, 9, 10, 11, 12, 13, 14, 16, 17,
    19, 21, 23, 25, 28, 31, 34, 37, 41, 45,
    50, 55, 60, 66, 73, 80, 88, 97, 107, 118,
    130, 143, 157, 173, 190, 209, 230, 253, 279, 307,
    337, 371, 408, 449, 494, 544, 598, 658, 724, 796,
    876, 963, 1060, 1166, 1282, 1411, 1552, 1707, 1878, 2066,
    2272, 2499, 2749, 3024, 3327, 3660, 4026, 4428, 4871, 5358,
    5894, 6484, 7132, 7845, 8630, 9493, 10442, 11487, 12635, 13899,
    15289, 16818, 18500, 20350, 22385, 24623, 27086, 29794, 32767
};
    
/* vat uses four bytes for state, so we do, too, to stay compatible */
struct dvi_adpcm_state {
	int16_t valpred;    /* Previous predicted value */
	uint8_t index;     /* Index into stepsize table */
	uint8_t reserved;  /* reserved */
};

static struct dvi_adpcm_state state = {0};


/*
* Initialize encoder state.
*/
void dvi_adpcm_init(void)
{
	state.valpred = 0;
	state.index = 0;
	state.reserved = 0;
} /* dvi_adpcm_init */

/*
* Insert state into output packet only if 'header_flag' is true.  Not
* inserting a header allows piecing together a packet from several
* audio chunks.  If we prefix a header, we still encode the remaining
* samples.  The DVI standard says to skip the first sample, since it
* is already known to the receiver from the header.  'True' DVI blocks
* thus always contain an odd number of samples (see the definition of
* wSamplesPerBlock and the encoding routine in the DVI ADPCM Wave
* Type, e.g., found in the Microsoft Development Library.)
*/
int dvi_adpcm_encode(void *in_buf, int in_size, void *out_buf, int *out_size)
{
  signed char *out_sbuf = out_buf;
  int val;              /* Current input sample value */
  int sign;             /* Current adpcm sign bit */
  int delta;            /* Current adpcm output value */
  int diff;             /* Difference between val and valpred */
  int step;             /* Stepsize */
  int valpred;          /* Predicted output value */
  int vpdiff;           /* Current change to valpred */
  int index;            /* Current step change index */
  int outputbuffer = 0; /* place to keep previous 4-bit value */
  int bufferstep;       /* toggle between outputbuffer/output */
  int16_t *s;           /* output buffer for linear encoding */

  in_size /= 2;
  s = (int16_t *)in_buf;

  valpred = *s++;
  in_size -= 1;		//@@

  /* insert state into output buffer */
  *out_size = in_size / 2;

  ((struct dvi_adpcm_state *)out_buf)->valpred = valpred;		//@@ convert to little endian
  ((struct dvi_adpcm_state *)out_buf)->index = state.index;
  *out_size += sizeof(struct dvi_adpcm_state);
  out_sbuf  += sizeof(struct dvi_adpcm_state);

  index = state.index;
  step  = stepsizeTable[index];
  bufferstep = 1;  /* in/out: encoder state */

  for ( ; in_size > 0; in_size--) {
    val = *s++;

    /* Step 1 - compute difference with previous value */
    diff = val - valpred;
    sign = (diff < 0) ? 8 : 0;
    if ( sign ) diff = (-diff);

    /* Step 2 - Divide and clamp */
    /* Note:
    ** This code *approximately* computes:
    **    delta = diff*4/step;
    **    vpdiff = (delta+0.5)*step/4;
    ** but in shift step bits are dropped. The net result of this is
    ** that even if you have fast mul/div hardware you cannot put it to
    ** good use since the fixup would be too expensive.
    */
    delta = 0;
    vpdiff = (step >> 3);
  
    if ( diff >= step ) {
      delta = 4;
      diff -= step;
      vpdiff += step;
    }
    step >>= 1;
    if ( diff >= step  ) {
      delta |= 2;
      diff -= step;
      vpdiff += step;
    }
    step >>= 1;
    if ( diff >= step ) {
      delta |= 1;
      vpdiff += step;
    }

    /* Step 3 - Update previous value */
    if ( sign )
      valpred -= vpdiff;
    else
      valpred += vpdiff;

    /* Step 4 - Clamp previous value to 16 bits */
    if ( valpred > 32767 )
      valpred = 32767;
    else if ( valpred < -32768 )
      valpred = -32768;

    /* Step 5 - Assemble value, update index and step values */
    delta |= sign;
  
    index += indexTable[delta];
    if ( index < 0 ) index = 0;
    if ( index > 88 ) index = 88;
    step = stepsizeTable[index];

    /* Step 6 - Output value */
    if ( bufferstep ) {
      outputbuffer = delta & 0x0F;
    } else {
      *out_sbuf++ = ((delta << 4) & 0xf0) | outputbuffer;
    }
    bufferstep = !bufferstep;
  }

  /* Output last step, if needed */
  if ( !bufferstep ) *out_sbuf++ = outputbuffer;
    
  state.valpred   = valpred;
  state.index     = index;
  return 0;
} /* dvi_adpcm_encode */