static int mjpegb_decode_frame(AVCodecContext *avctx,

                              void *data, int *got_frame,

                              AVPacket *avpkt)

{

    const uint8_t *buf = avpkt->data;

    int buf_size = avpkt->size;

    MJpegDecodeContext *s = avctx->priv_data;

    const uint8_t *buf_end, *buf_ptr;

    GetBitContext hgb; 

    uint32_t dqt_offs, dht_offs, sof_offs, sos_offs, second_field_offs;

    uint32_t field_size, sod_offs;

    int ret;



    buf_ptr = buf;

    buf_end = buf + buf_size;

    s->got_picture = 0;



read_header:

    

    s->restart_interval = 0;

    s->restart_count = 0;

    s->mjpb_skiptosod = 0;



    if (buf_end - buf_ptr >= 1 << 28)

        return AVERROR_INVALIDDATA;



    init_get_bits(&hgb, buf_ptr, (buf_end - buf_ptr)*8);



    skip_bits(&hgb, 32); 



    if (get_bits_long(&hgb, 32) != MKBETAG('m','j','p','g'))

    {

        av_log(avctx, AV_LOG_WARNING, "not mjpeg-b (bad fourcc)\n");

        return AVERROR_INVALIDDATA;

    }



    field_size = get_bits_long(&hgb, 32); 

    av_log(avctx, AV_LOG_DEBUG, "field size: 0x%x\n", field_size);

    skip_bits(&hgb, 32); 

    second_field_offs = read_offs(avctx, &hgb, buf_end - buf_ptr, "second_field_offs is %d and size is %d\n");

    av_log(avctx, AV_LOG_DEBUG, "second field offs: 0x%x\n", second_field_offs);



    dqt_offs = read_offs(avctx, &hgb, buf_end - buf_ptr, "dqt is %d and size is %d\n");

    av_log(avctx, AV_LOG_DEBUG, "dqt offs: 0x%x\n", dqt_offs);

    if (dqt_offs)

    {

        init_get_bits(&s->gb, buf_ptr+dqt_offs, (buf_end - (buf_ptr+dqt_offs))*8);

        s->start_code = DQT;

        if (ff_mjpeg_decode_dqt(s) < 0 &&

            (avctx->err_recognition & AV_EF_EXPLODE))

          return AVERROR_INVALIDDATA;

    }



    dht_offs = read_offs(avctx, &hgb, buf_end - buf_ptr, "dht is %d and size is %d\n");

    av_log(avctx, AV_LOG_DEBUG, "dht offs: 0x%x\n", dht_offs);

    if (dht_offs)

    {

        init_get_bits(&s->gb, buf_ptr+dht_offs, (buf_end - (buf_ptr+dht_offs))*8);

        s->start_code = DHT;

        ff_mjpeg_decode_dht(s);

    }



    sof_offs = read_offs(avctx, &hgb, buf_end - buf_ptr, "sof is %d and size is %d\n");

    av_log(avctx, AV_LOG_DEBUG, "sof offs: 0x%x\n", sof_offs);

    if (sof_offs)

    {

        init_get_bits(&s->gb, buf_ptr+sof_offs, (buf_end - (buf_ptr+sof_offs))*8);

        s->start_code = SOF0;

        if (ff_mjpeg_decode_sof(s) < 0)

            return -1;

    }



    sos_offs = read_offs(avctx, &hgb, buf_end - buf_ptr, "sos is %d and size is %d\n");

    av_log(avctx, AV_LOG_DEBUG, "sos offs: 0x%x\n", sos_offs);

    sod_offs = read_offs(avctx, &hgb, buf_end - buf_ptr, "sof is %d and size is %d\n");

    av_log(avctx, AV_LOG_DEBUG, "sod offs: 0x%x\n", sod_offs);

    if (sos_offs)

    {

        init_get_bits(&s->gb, buf_ptr + sos_offs,

                      8 * FFMIN(field_size, buf_end - buf_ptr - sos_offs));

        s->mjpb_skiptosod = (sod_offs - sos_offs - show_bits(&s->gb, 16));

        s->start_code = SOS;

        if (ff_mjpeg_decode_sos(s, NULL, NULL) < 0 &&

            (avctx->err_recognition & AV_EF_EXPLODE))

          return AVERROR_INVALIDDATA;

    }



    if (s->interlaced) {

        s->bottom_field ^= 1;

        

        if (s->bottom_field != s->interlace_polarity && second_field_offs)

        {

            buf_ptr = buf + second_field_offs;

            goto read_header;

            }

    }



    



    if(!s->got_picture) {

        av_log(avctx, AV_LOG_WARNING, "no picture\n");

        return buf_size;

    }



    if ((ret = av_frame_ref(data, s->picture_ptr)) < 0)

        return ret;

    *got_frame = 1;



    if (!s->lossless && avctx->debug & FF_DEBUG_QP) {

        av_log(avctx, AV_LOG_DEBUG, "QP: %d\n",

               FFMAX3(s->qscale[0], s->qscale[1], s->qscale[2]));

    }



    return buf_size;

}
