static int gxf_packet(AVFormatContext *s, AVPacket *pkt) {
    ByteIOContext *pb = s->pb;
    pkt_type_t pkt_type;
    int pkt_len;
    while (!url_feof(pb)) {
        AVStream *st;
        int track_type, track_id, ret;
        int field_nr, field_info, skip = 0;
        int stream_index;
        if (!parse_packet_header(pb, &pkt_type, &pkt_len)) {
            if (!url_feof(pb))
                av_log(s, AV_LOG_ERROR, "GXF: sync lost\n");
            return -1;
        }
        if (pkt_type == PKT_FLT) {
            gxf_read_index(s, pkt_len);
            continue;
        }
        if (pkt_type != PKT_MEDIA) {
            url_fskip(pb, pkt_len);
            continue;
        }
        if (pkt_len < 16) {
            av_log(s, AV_LOG_ERROR, "GXF: invalid media packet length\n");
            continue;
        }
        pkt_len -= 16;
        track_type = get_byte(pb);
        track_id = get_byte(pb);
        stream_index = get_sindex(s, track_id, track_type);
        if (stream_index < 0)
            return stream_index;
        st = s->streams[stream_index];
        field_nr = get_be32(pb);
        field_info = get_be32(pb);
        get_be32(pb); // "timeline" field number
        get_byte(pb); // flags
        get_byte(pb); // reserved
        if (st->codec->codec_id == CODEC_ID_PCM_S24LE ||
            st->codec->codec_id == CODEC_ID_PCM_S16LE) {
            int first = field_info >> 16;
            int last  = field_info & 0xffff; // last is exclusive
            int bps = av_get_bits_per_sample(st->codec->codec_id)>>3;
            if (first <= last && last*bps <= pkt_len) {
                url_fskip(pb, first*bps);
                skip = pkt_len - last*bps;
                pkt_len = (last-first)*bps;
            } else
                av_log(s, AV_LOG_ERROR, "invalid first and last sample values\n");
        }
        ret = av_get_packet(pb, pkt, pkt_len);
        if (skip)
            url_fskip(pb, skip);
        pkt->stream_index = stream_index;
        pkt->dts = field_nr;
        return ret;
    }
    return AVERROR(EIO);
}