apedec.c

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        /* We are pure silence, just memset the output buffer. */
        memset(decoded0, 0, blockstodecode * sizeof(int32_t));
        memset(decoded1, 0, blockstodecode * sizeof(int32_t));
    } else {
        while (blockstodecode--) {
            *decoded0++ = ape_decode_value(ctx, &ctx->riceY);
            if (stereo)
                *decoded1++ = ape_decode_value(ctx, &ctx->riceX);
        }
    }

    if (ctx->blocksdecoded == ctx->currentframeblocks)
        range_dec_normalize(ctx);   /* normalize to use up all bytes */
}

static void init_entropy_decoder(APEContext * ctx)
{
    /* Read the CRC */
    ctx->CRC = bytestream_get_be32(&ctx->ptr);

    /* Read the frame flags if they exist */
    ctx->frameflags = 0;
    if ((ctx->fileversion > 3820) && (ctx->CRC & 0x80000000)) {
        ctx->CRC &= ~0x80000000;

        ctx->frameflags = bytestream_get_be32(&ctx->ptr);
    }

    /* Keep a count of the blocks decoded in this frame */
    ctx->blocksdecoded = 0;

    /* Initialise the rice structs */
    ctx->riceX.k = 10;
    ctx->riceX.ksum = (1 << ctx->riceX.k) * 16;
    ctx->riceY.k = 10;
    ctx->riceY.ksum = (1 << ctx->riceY.k) * 16;

    /* The first 8 bits of input are ignored. */
    ctx->ptr++;

    range_start_decoding(ctx);
}

static const int32_t initial_coeffs[4] = {
    360, 317, -109, 98
};

static void init_predictor_decoder(APEContext * ctx)
{
    APEPredictor *p = &ctx->predictor;

    /* Zero the history buffers */
    memset(p->historybuffer, 0, PREDICTOR_SIZE * sizeof(int32_t));
    p->buf = p->historybuffer;

    /* Initialise and zero the co-efficients */
    memcpy(p->coeffsA[0], initial_coeffs, sizeof(initial_coeffs));
    memcpy(p->coeffsA[1], initial_coeffs, sizeof(initial_coeffs));
    memset(p->coeffsB, 0, sizeof(p->coeffsB));

    p->filterA[0] = p->filterA[1] = 0;
    p->filterB[0] = p->filterB[1] = 0;
    p->lastA[0]   = p->lastA[1]   = 0;
}

/** Get inverse sign of integer (-1 for positive, 1 for negative and 0 for zero) */
static inline int APESIGN(int32_t x) {
    return (x < 0) - (x > 0);
}

static int predictor_update_filter(APEPredictor *p, const int decoded, const int filter, const int delayA, const int delayB, const int adaptA, const int adaptB)
{
    int32_t predictionA, predictionB;

    p->buf[delayA]     = p->lastA[filter];
    p->buf[adaptA]     = APESIGN(p->buf[delayA]);
    p->buf[delayA - 1] = p->buf[delayA] - p->buf[delayA - 1];
    p->buf[adaptA - 1] = APESIGN(p->buf[delayA - 1]);

    predictionA = p->buf[delayA    ] * p->coeffsA[filter][0] +
                  p->buf[delayA - 1] * p->coeffsA[filter][1] +
                  p->buf[delayA - 2] * p->coeffsA[filter][2] +
                  p->buf[delayA - 3] * p->coeffsA[filter][3];

    /*  Apply a scaled first-order filter compression */
    p->buf[delayB]     = p->filterA[filter ^ 1] - ((p->filterB[filter] * 31) >> 5);
    p->buf[adaptB]     = APESIGN(p->buf[delayB]);
    p->buf[delayB - 1] = p->buf[delayB] - p->buf[delayB - 1];
    p->buf[adaptB - 1] = APESIGN(p->buf[delayB - 1]);
    p->filterB[filter] = p->filterA[filter ^ 1];

    predictionB = p->buf[delayB    ] * p->coeffsB[filter][0] +
                  p->buf[delayB - 1] * p->coeffsB[filter][1] +
                  p->buf[delayB - 2] * p->coeffsB[filter][2] +
                  p->buf[delayB - 3] * p->coeffsB[filter][3] +
                  p->buf[delayB - 4] * p->coeffsB[filter][4];

    p->lastA[filter] = decoded + ((predictionA + (predictionB >> 1)) >> 10);
    p->filterA[filter] = p->lastA[filter] + ((p->filterA[filter] * 31) >> 5);

    if (!decoded) // no need updating filter coefficients
        return p->filterA[filter];

    if (decoded > 0) {
        p->coeffsA[filter][0] -= p->buf[adaptA    ];
        p->coeffsA[filter][1] -= p->buf[adaptA - 1];
        p->coeffsA[filter][2] -= p->buf[adaptA - 2];
        p->coeffsA[filter][3] -= p->buf[adaptA - 3];

        p->coeffsB[filter][0] -= p->buf[adaptB    ];
        p->coeffsB[filter][1] -= p->buf[adaptB - 1];
        p->coeffsB[filter][2] -= p->buf[adaptB - 2];
        p->coeffsB[filter][3] -= p->buf[adaptB - 3];
        p->coeffsB[filter][4] -= p->buf[adaptB - 4];
    } else {
        p->coeffsA[filter][0] += p->buf[adaptA    ];
        p->coeffsA[filter][1] += p->buf[adaptA - 1];
        p->coeffsA[filter][2] += p->buf[adaptA - 2];
        p->coeffsA[filter][3] += p->buf[adaptA - 3];

        p->coeffsB[filter][0] += p->buf[adaptB    ];
        p->coeffsB[filter][1] += p->buf[adaptB - 1];
        p->coeffsB[filter][2] += p->buf[adaptB - 2];
        p->coeffsB[filter][3] += p->buf[adaptB - 3];
        p->coeffsB[filter][4] += p->buf[adaptB - 4];
    }
    return p->filterA[filter];
}

static void predictor_decode_stereo(APEContext * ctx, int count)
{
    int32_t predictionA, predictionB;
    APEPredictor *p = &ctx->predictor;
    int32_t *decoded0 = ctx->decoded0;
    int32_t *decoded1 = ctx->decoded1;

    while (count--) {
        /* Predictor Y */
        predictionA = predictor_update_filter(p, *decoded0, 0, YDELAYA, YDELAYB, YADAPTCOEFFSA, YADAPTCOEFFSB);
        predictionB = predictor_update_filter(p, *decoded1, 1, XDELAYA, XDELAYB, XADAPTCOEFFSA, XADAPTCOEFFSB);
        *(decoded0++) = predictionA;
        *(decoded1++) = predictionB;

        /* Combined */
        p->buf++;

        /* Have we filled the history buffer? */
        if (p->buf == p->historybuffer + HISTORY_SIZE) {
            memmove(p->historybuffer, p->buf, PREDICTOR_SIZE * sizeof(int32_t));
            p->buf = p->historybuffer;
        }
    }
}

static void predictor_decode_mono(APEContext * ctx, int count)
{
    APEPredictor *p = &ctx->predictor;
    int32_t *decoded0 = ctx->decoded0;
    int32_t predictionA, currentA, A;

    currentA = p->lastA[0];

    while (count--) {
        A = *decoded0;

        p->buf[YDELAYA] = currentA;
        p->buf[YDELAYA - 1] = p->buf[YDELAYA] - p->buf[YDELAYA - 1];

        predictionA = p->buf[YDELAYA    ] * p->coeffsA[0][0] +
                      p->buf[YDELAYA - 1] * p->coeffsA[0][1] +
                      p->buf[YDELAYA - 2] * p->coeffsA[0][2] +
                      p->buf[YDELAYA - 3] * p->coeffsA[0][3];

        currentA = A + (predictionA >> 10);

        p->buf[YADAPTCOEFFSA]     = APESIGN(p->buf[YDELAYA    ]);
        p->buf[YADAPTCOEFFSA - 1] = APESIGN(p->buf[YDELAYA - 1]);

        if (A > 0) {
            p->coeffsA[0][0] -= p->buf[YADAPTCOEFFSA    ];
            p->coeffsA[0][1] -= p->buf[YADAPTCOEFFSA - 1];
            p->coeffsA[0][2] -= p->buf[YADAPTCOEFFSA - 2];
            p->coeffsA[0][3] -= p->buf[YADAPTCOEFFSA - 3];
        } else if (A < 0) {
            p->coeffsA[0][0] += p->buf[YADAPTCOEFFSA    ];
            p->coeffsA[0][1] += p->buf[YADAPTCOEFFSA - 1];
            p->coeffsA[0][2] += p->buf[YADAPTCOEFFSA - 2];
            p->coeffsA[0][3] += p->buf[YADAPTCOEFFSA - 3];
        }

        p->buf++;

        /* Have we filled the history buffer? */
        if (p->buf == p->historybuffer + HISTORY_SIZE) {
            memmove(p->historybuffer, p->buf, PREDICTOR_SIZE * sizeof(int32_t));
            p->buf = p->historybuffer;
        }

        p->filterA[0] = currentA + ((p->filterA[0] * 31) >> 5);
        *(decoded0++) = p->filterA[0];
    }

    p->lastA[0] = currentA;
}

static void do_init_filter(APEFilter *f, int16_t * buf, int order)
{
    f->coeffs = buf;
    f->historybuffer = buf + order;
    f->delay       = f->historybuffer + order * 2;
    f->adaptcoeffs = f->historybuffer + order;

    memset(f->historybuffer, 0, (order * 2) * sizeof(int16_t));
    memset(f->coeffs, 0, order * sizeof(int16_t));
    f->avg = 0;
}

static void init_filter(APEContext * ctx, APEFilter *f, int16_t * buf, int order)
{
    do_init_filter(&f[0], buf, order);
    do_init_filter(&f[1], buf + order * 3 + HISTORY_SIZE, order);
}

static inline void do_apply_filter(int version, APEFilter *f, int32_t *data, int count, int order, int fracbits)
{
    int res;
    int absres;

    while (count--) {
        /* round fixedpoint scalar product */
        res = (scalarproduct(f->delay - order, f->coeffs, order) + (1 << (fracbits - 1))) >> fracbits;

        if (*data < 0)
            vector_add(f->coeffs, f->adaptcoeffs - order, order);
        else if (*data > 0)
            vector_sub(f->coeffs, f->adaptcoeffs - order, order);

        res += *data;

        *data++ = res;

        /* Update the output history */
        *f->delay++ = av_clip_int16(res);

        if (version < 3980) {
            /* Version ??? to < 3.98 files (untested) */
            f->adaptcoeffs[0]  = (res == 0) ? 0 : ((res >> 28) & 8) - 4;
            f->adaptcoeffs[-4] >>= 1;
            f->adaptcoeffs[-8] >>= 1;
        } else {
            /* Version 3.98 and later files */

            /* Update the adaption coefficients */
            absres = (res < 0 ? -res : res);

            if (absres > (f->avg * 3))
                *f->adaptcoeffs = ((res >> 25) & 64) - 32;
            else if (absres > (f->avg * 4) / 3)
                *f->adaptcoeffs = ((res >> 26) & 32) - 16;
            else if (absres > 0)
                *f->adaptcoeffs = ((res >> 27) & 16) - 8;
            else
                *f->adaptcoeffs = 0;

            f->avg += (absres - f->avg) / 16;

            f->adaptcoeffs[-1] >>= 1;
            f->adaptcoeffs[-2] >>= 1;
            f->adaptcoeffs[-8] >>= 1;
        }

        f->adaptcoeffs++;

        /* Have we filled the history buffer? */
        if (f->delay == f->historybuffer + HISTORY_SIZE + (order * 2)) {
            memmove(f->historybuffer, f->delay - (order * 2),
                    (order * 2) * sizeof(int16_t));
            f->delay = f->historybuffer + order * 2;
            f->adaptcoeffs = f->historybuffer + order;
        }
    }
}

static void apply_filter(APEContext * ctx, APEFilter *f,
                         int32_t * data0, int32_t * data1,
                         int count, int order, int fracbits)
{
    do_apply_filter(ctx->fileversion, &f[0], data0, count, order, fracbits);
    if (data1)
        do_apply_filter(ctx->fileversion, &f[1], data1, count, order, fracbits);
}

static void ape_apply_filters(APEContext * ctx, int32_t * decoded0,
                              int32_t * decoded1, int count)
{
    int i;

    for (i = 0; i < APE_FILTER_LEVELS; i++) {
        if (!ape_filter_orders[ctx->fset][i])
            break;
        apply_filter(ctx, ctx->filters[i], decoded0, decoded1, count, ape_filter_orders[ctx->fset][i], ape_filter_fracbits[ctx->fset][i]);
    }
}

static void init_frame_decoder(APEContext * ctx)
{
    int i;
    init_entropy_decoder(ctx);
    init_predictor_decoder(ctx);

    for (i = 0; i < APE_FILTER_LEVELS; i++) {
        if (!ape_filter_orders[ctx->fset][i])
            break;
        init_filter(ctx, ctx->filters[i], ctx->filterbuf[i], ape_filter_orders[ctx->fset][i]);
    }
}

static void ape_unpack_mono(APEContext * ctx, int count)
{
    int32_t left;
    int32_t *decoded0 = ctx->decoded0;
    int32_t *decoded1 = ctx->decoded1;

    if (ctx->frameflags & APE_FRAMECODE_STEREO_SILENCE) {
        entropy_decode(ctx, count, 0);
        /* We are pure silence, so we're done. */
        av_log(ctx->avctx, AV_LOG_DEBUG, "pure silence mono\n");
        return;
    }

    entropy_decode(ctx, count, 0);
    ape_apply_filters(ctx, decoded0, NULL, count);

    /* Now apply the predictor decoding */
    predictor_decode_mono(ctx, count);

    /* Pseudo-stereo - just copy left channel to right channel */
    if (ctx->channels == 2) {
        while (count--) {
            left = *decoded0;
            *(decoded1++) = *(decoded0++) = left;
        }
    }
}

static void ape_unpack_stereo(APEContext * ctx, int count)
{
    int32_t left, right;
    int32_t *decoded0 = ctx->decoded0;
    int32_t *decoded1 = ctx->decoded1;

    if (ctx->frameflags & APE_FRAMECODE_STEREO_SILENCE) {
        /* We are pure silence, so we're done. */
        av_log(ctx->avctx, AV_LOG_DEBUG, "pure silence stereo\n");
        return;
    }

    entropy_decode(ctx, count, 1);
    ape_apply_filters(ctx, decoded0, decoded1, count);

    /* Now apply the predictor decoding */
    predictor_decode_stereo(ctx, count);

    /* Decorrelate and scale to output depth */
    while (count--) {
        left = *decoded1 - (*decoded0 / 2);
        right = left + *decoded0;

        *(decoded0++) = left;
        *(decoded1++) = right;
    }
}

static int ape_decode_frame(AVCodecContext * avctx,
                            void *data, int *data_size,
                            uint8_t * buf, int buf_size)
{
    APEContext *s = avctx->priv_data;
    int16_t *samples = data;
    int nblocks;
    int i, n;
    int blockstodecode;
    int bytes_used;

    if (buf_size == 0 && !s->samples) {
        *data_size = 0;
        return 0;
    }

    /* should not happen but who knows */
    if (BLOCKS_PER_LOOP * 2 * avctx->channels > *data_size) {
        av_log (avctx, AV_LOG_ERROR, "Packet size is too big to be handled in lavc! (max is %d where you have %d)\n", *data_size, s->samples * 2 * avctx->channels);
        return -1;
    }

    if(!s->samples){
        s->data = av_realloc(s->data, (buf_size + 3) & ~3);
        s->dsp.bswap_buf(s->data, buf, buf_size >> 2);
        s->ptr = s->last_ptr = s->data;
        s->data_end = s->data + buf_size;

        nblocks = s->samples = bytestream_get_be32(&s->ptr);
        n =  bytestream_get_be32(&s->ptr);
        if(n < 0 || n > 3){
            av_log(avctx, AV_LOG_ERROR, "Incorrect offset passed\n");
            s->data = NULL;
            return -1;
        }
        s->ptr += n;

        s->currentframeblocks = nblocks;
        buf += 4;
        if (s->samples <= 0) {
            *data_size = 0;
            return buf_size;
        }

        memset(s->decoded0,  0, sizeof(s->decoded0));
        memset(s->decoded1,  0, sizeof(s->decoded1));

        /* Initialize the frame decoder */
        init_frame_decoder(s);
    }

    if (!s->data) {
        *data_size = 0;
        return buf_size;
    }

    nblocks = s->samples;
    blockstodecode = FFMIN(BLOCKS_PER_LOOP, nblocks);

    if ((s->channels == 1) || (s->frameflags & APE_FRAMECODE_PSEUDO_STEREO))
        ape_unpack_mono(s, blockstodecode);
    else
        ape_unpack_stereo(s, blockstodecode);

    for (i = 0; i < blockstodecode; i++) {
        *samples++ = s->decoded0[i];
        if(s->channels == 2)
            *samples++ = s->decoded1[i];
    }

    s->samples -= blockstodecode;

    *data_size = blockstodecode * 2 * s->channels;
    bytes_used = s->samples ? s->ptr - s->last_ptr : buf_size;
    s->last_ptr = s->ptr;
    return bytes_used;
}

AVCodec ape_decoder = {
    "ape",
    CODEC_TYPE_AUDIO,
    CODEC_ID_APE,
    sizeof(APEContext),
    ape_decode_init,
    NULL,
    ape_decode_close,
    ape_decode_frame,
};