alac.c

alac decoder,内容详细

            int32_t value;

            value = readbits(alac, readsamplesize);

            /* mask value to readsamplesize size */
            if (readsamplesize != 32)
                value &= (0xffffffff >> (32 - readsamplesize));

            x = value;
        }
        else
        { /* standard rice encoding */
            int extrabits;
            int k; /* size of extra bits */

            /* read k, that is bits as is */
            k = 31 - rice_kmodifier - count_leading_zeros((history >> 9) + 3);

            if (k < 0) k += rice_kmodifier;
            else k = rice_kmodifier;

            if (k != 1)
            {
                extrabits = readbits(alac, k);

                /* multiply x by 2^k - 1, as part of their strange algorithm */
                x = (x << k) - x;

                if (extrabits > 1)
                {
                    x += extrabits - 1;
                }
                else unreadbits(alac, 1);
            }
        }

        x_modified = sign_modifier + x;
        final_val = (x_modified + 1) / 2;
        if (x_modified & 1) final_val *= -1;

        output_buffer[output_count] = final_val;

        sign_modifier = 0;

        /* now update the history */
        history += (x_modified * rice_historymult)
                 - ((history * rice_historymult) >> 9);

        if (x_modified > 0xffff)
            history = 0xffff;

        /* special case: there may be compressed blocks of 0 */
        if ((history < 128) && (output_count+1 < output_size))
        {
            int block_size;

            sign_modifier = 1;

            x = 0;
            while (x <= 8 && readbit(alac))
            {
                x++;
            }

            if (x > 8)
            {
                block_size = readbits(alac, 16);
                block_size &= 0xffff;
            }
            else
            {
                int k;
                int extrabits;

                k = count_leading_zeros(history) + ((history + 16) >> 6 /* / 64 */) - 24;

                extrabits = readbits(alac, k);

                block_size = (((1 << k) - 1) & rice_kmodifier_mask) * x
                           + extrabits - 1;

                if (extrabits < 2)
                {
                    x = 1 - extrabits;
                    block_size += x;
                    unreadbits(alac, 1);
                }
            }

            if (block_size > 0)
            {
                memset(&output_buffer[output_count+1], 0, block_size * 4);
                output_count += block_size;

            }

            if (block_size > 0xffff)
                sign_modifier = 0;

            history = 0;
        }
    }
}

#define SIGN_EXTENDED32(val, bits) ((val << (32 - bits)) >> (32 - bits))

#define SIGN_ONLY(v) \
                     ((v < 0) ? (-1) : \
                                ((v > 0) ? (1) : \
                                           (0)))

static void predictor_decompress_fir_adapt(int32_t *error_buffer,
                                           int32_t *buffer_out,
                                           int output_size,
                                           int readsamplesize,
                                           int16_t *predictor_coef_table,
                                           int predictor_coef_num,
                                           int predictor_quantitization)
{
    int i;

    /* first sample always copies */
    *buffer_out = *error_buffer;

    if (!predictor_coef_num)
    {
        if (output_size <= 1) return;
        memcpy(buffer_out+1, error_buffer+1, (output_size-1) * 4);
        return;
    }

    if (predictor_coef_num == 0x1f) /* 11111 - max value of predictor_coef_num */
    { /* second-best case scenario for fir decompression,
       * error describes a small difference from the previous sample only
       */
        if (output_size <= 1) return;
        for (i = 0; i < output_size - 1; i++)
        {
            int32_t prev_value;
            int32_t error_value;

            prev_value = buffer_out[i];
            error_value = error_buffer[i+1];
            buffer_out[i+1] = SIGN_EXTENDED32((prev_value + error_value), readsamplesize);
        }
        return;
    }

    /* read warm-up samples */
    if (predictor_coef_num > 0)
    {
        int i;
        for (i = 0; i < predictor_coef_num; i++)
        {
            int32_t val;

            val = buffer_out[i] + error_buffer[i+1];

            val = SIGN_EXTENDED32(val, readsamplesize);

            buffer_out[i+1] = val;
        }
    }

#if 0
    /* 4 and 8 are very common cases (the only ones i've seen). these
     * should be unrolled and optimised
     */
    if (predictor_coef_num == 4)
    {
        /* FIXME: optimised general case */
        return;
    }

    if (predictor_coef_table == 8)
    {
        /* FIXME: optimised general case */
        return;
    }
#endif


    /* general case */
    if (predictor_coef_num > 0)
    {
        for (i = predictor_coef_num + 1;
             i < output_size;
             i++)
        {
            int j;
            int sum = 0;
            int outval;
            int error_val = error_buffer[i];

            for (j = 0; j < predictor_coef_num; j++)
            {
                sum += (buffer_out[predictor_coef_num-j] - buffer_out[0]) *
                       predictor_coef_table[j];
            }

            outval = (1 << (predictor_quantitization-1)) + sum;
            outval = outval >> predictor_quantitization;
            outval = outval + buffer_out[0] + error_val;
            outval = SIGN_EXTENDED32(outval, readsamplesize);

            buffer_out[predictor_coef_num+1] = outval;

            if (error_val > 0)
            {
                int predictor_num = predictor_coef_num - 1;

                while (predictor_num >= 0 && error_val > 0)
                {
                    int val = buffer_out[0] - buffer_out[predictor_coef_num - predictor_num];
                    int sign = SIGN_ONLY(val);

                    predictor_coef_table[predictor_num] -= sign;

                    val *= sign; /* absolute value */

                    error_val -= ((val >> predictor_quantitization) *
                                  (predictor_coef_num - predictor_num));

                    predictor_num--;
                }
            }
            else if (error_val < 0)
            {
                int predictor_num = predictor_coef_num - 1;

                while (predictor_num >= 0 && error_val < 0)
                {
                    int val = buffer_out[0] - buffer_out[predictor_coef_num - predictor_num];
                    int sign = - SIGN_ONLY(val);

                    predictor_coef_table[predictor_num] -= sign;

                    val *= sign; /* neg value */

                    error_val -= ((val >> predictor_quantitization) *
                                  (predictor_coef_num - predictor_num));

                    predictor_num--;
                }
            }

            buffer_out++;
        }
    }
}

void deinterlace_16(int32_t *buffer_a, int32_t *buffer_b,
                    int16_t *buffer_out,
                    int numchannels, int numsamples,
                    uint8_t interlacing_shift,
                    uint8_t interlacing_leftweight)
{
    int i;
    if (numsamples <= 0) return;

    /* weighted interlacing */
    if (interlacing_leftweight)
    {
        for (i = 0; i < numsamples; i++)
        {
            int32_t difference, midright;
            int16_t left;
            int16_t right;

            midright = buffer_a[i];
            difference = buffer_b[i];


            right = midright - ((difference * interlacing_leftweight) >> interlacing_shift);
            left = right + difference;

            /* output is always little endian */
            if (host_bigendian)
            {
                _Swap16(left);
                _Swap16(right);
            }

            buffer_out[i*numchannels] = left;
            buffer_out[i*numchannels + 1] = right;
        }

        return;
    }

    /* otherwise basic interlacing took place */
    for (i = 0; i < numsamples; i++)
    {
        int16_t left, right;

        left = buffer_a[i];
        right = buffer_b[i];

        /* output is always little endian */
        if (host_bigendian)
        {
            _Swap16(left);
            _Swap16(right);
        }

        buffer_out[i*numchannels] = left;
        buffer_out[i*numchannels + 1] = right;
    }
}

void decode_frame(alac_file *alac,
                  unsigned char *inbuffer,
                  void *outbuffer, int *outputsize)
{
    int channels;
    int32_t outputsamples = alac->setinfo_max_samples_per_frame;

    /* setup the stream */
    alac->input_buffer = inbuffer;
    alac->input_buffer_bitaccumulator = 0;

    channels = readbits(alac, 3);

    *outputsize = outputsamples * alac->bytespersample;

    switch(channels)
    {
    case 0: /* 1 channel */
    {
        int hassize;
        int isnotcompressed;
        int readsamplesize;

        int wasted_bytes;
        int ricemodifier;


        /* 2^result = something to do with output waiting.
         * perhaps matters if we read > 1 frame in a pass?
         */
        readbits(alac, 4);

        readbits(alac, 12); /* unknown, skip 12 bits */

        hassize = readbits(alac, 1); /* the output sample size is stored soon */

        wasted_bytes = readbits(alac, 2); /* unknown ? */

        isnotcompressed = readbits(alac, 1); /* whether the frame is compressed */

        if (hassize)
        {