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

    Copyright 1992 by Stichting Mathematisch Centrum, Amsterdam, The

    Netherlands.

    All Rights Reserved

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    documentation for any purpose and without fee is hereby granted,

    provided that the above copyright notice appear in all copies and that

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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.

    */

    #include "adpcm.h"

    #include <stdio.h> /*DBG*/

    #ifndef __STDC__

    #define signed

    #endif

    /* Intel ADPCM step variation table */

    static int indexTable[16] = {

    -1, -1, -1, -1, 2, 4, 6, 8,

    -1, -1, -1, -1, 2, 4, 6, 8,

    };

    static 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

    };

    void

    adpcm_coder(indata, outdata, len, state)

    short indata[];

    char outdata[];

    int len;

    struct adpcm_state *state;

    {

    short *inp;/* Input buffer pointer */

    signed char *outp;/* output buffer pointer */

    int val;/* Current input sample value */

    int sign;/* Current adpcm sign bit */

    int delta;/* Current adpcm output value */

    int diff;/* Difference between val and valprev */

    int step;/* Stepsize */

    int valpred;/* Predicted output value */

    int vpdiff;/* Current change to valpred */

    int index;/* Current step change index */

    int outputbuffer;/* place to keep previous 4-bit value */

    int bufferstep;/* toggle between outputbuffer/output */

    outp = (signed char *)outdata;

    inp = indata;

    valpred = state->valprev;

    index = state->index;

    step = stepsizeTable[index];

    bufferstep = 1;

    for ( ; len > 0 ; len-- ) {

    val = *inp++;

    /* 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 << 4) & 0xf0;

    } else {

    *outp++ = (delta & 0x0f) | outputbuffer;

    }

    bufferstep = !bufferstep;

    }

    /* Output last step, if needed */

    if ( !bufferstep )

    *outp++ = outputbuffer;

    state->valprev = valpred;

    state->index = index;

    }

    void

    adpcm_decoder(indata, outdata, len, state)

    char indata[];

    short outdata[];

    int len;

    struct adpcm_state *state;

    {

    signed char *inp;/* Input buffer pointer */

    short *outp;/* output buffer pointer */

    int sign;/* Current adpcm sign bit */

    int delta;/* Current adpcm output value */

    int step;/* Stepsize */

    int valpred;/* Predicted value */

    int vpdiff;/* Current change to valpred */

    int index;/* Current step change index */

    int inputbuffer;/* place to keep next 4-bit value */

    int bufferstep;/* toggle between inputbuffer/input */

    outp = outdata;

    inp = (signed char *)indata;

    valpred = state->valprev;

    index = state->index;

    step = stepsizeTable[index];

    bufferstep = 0;

    for ( ; len > 0 ; len-- ) {

    /* Step 1 - get the delta value */

    if ( bufferstep ) {

    delta = inputbuffer & 0xf;

    } else {

    inputbuffer = *inp++;

    delta = (inputbuffer >> 4) & 0xf;

    }

    bufferstep = !bufferstep;

    /* Step 2 - Find new index value (for later) */

    index += indexTable[delta];

    if ( index < 0 ) index = 0;

    if ( index > 88 ) index = 88;

    /* Step 3 - Separate sign and magnitude */

    sign = delta & 8;

    delta = delta & 7;

    /* Step 4 - Compute difference and new predicted value */

    /*

    ** Computes 'vpdiff = (delta+0.5)*step/4', but see comment

    ** in adpcm_coder.

    */

    vpdiff = step >> 3;

    if ( delta & 4 ) vpdiff += step;

    if ( delta & 2 ) vpdiff += step>>1;

    if ( delta & 1 ) vpdiff += step>>2;

    if ( sign )

    valpred -= vpdiff;

    else

    valpred += vpdiff;

    /* Step 5 - clamp output value */

    if ( valpred > 32767 )

    valpred = 32767;

    else if ( valpred < -32768 )

    valpred = -32768;

    /* Step 6 - Update step value */

    step = stepsizeTable[index];

    /* Step 7 - Output value */

    *outp++ = valpred;

    }

    state->valprev = valpred;

    state->index = index;

    }