static void mp3_write_xing(AVFormatContext *s)
{
    MP3Context       *mp3 = s->priv_data;
    AVCodecContext *codec = s->streams[mp3->audio_stream_idx]->codec;
    AVDictionaryEntry *enc = av_dict_get(s->streams[mp3->audio_stream_idx]->metadata, "encoder", NULL, 0);
    AVIOContext *dyn_ctx;
    int32_t        header;
    MPADecodeHeader  mpah;
    int srate_idx, i, channels;
    int bitrate_idx;
    int best_bitrate_idx;
    int best_bitrate_error = INT_MAX;
    int ret;
    int ver = 0;
    int lsf, bytes_needed;

    if (!s->pb->seekable || !mp3->write_xing)
        return;

    for (i = 0; i < FF_ARRAY_ELEMS(avpriv_mpa_freq_tab); i++) {
        const uint16_t base_freq = avpriv_mpa_freq_tab[i];

        if      (codec->sample_rate == base_freq)     ver = 0x3; // MPEG 1
        else if (codec->sample_rate == base_freq / 2) ver = 0x2; // MPEG 2
        else if (codec->sample_rate == base_freq / 4) ver = 0x0; // MPEG 2.5
        else continue;

        srate_idx = i;
        break;
    }
    if (i == FF_ARRAY_ELEMS(avpriv_mpa_freq_tab)) {
        av_log(s, AV_LOG_WARNING, "Unsupported sample rate, not writing Xing "
               "header.\n");
        return;
    }

    switch (codec->channels) {
    case 1:  channels = MPA_MONO;                                          break;
    case 2:  channels = MPA_STEREO;                                        break;
    default: av_log(s, AV_LOG_WARNING, "Unsupported number of channels, "
                    "not writing Xing header.\n");
             return;
    }

    /* dummy MPEG audio header */
    header  =  0xff                                  << 24; // sync
    header |= (0x7 << 5 | ver << 3 | 0x1 << 1 | 0x1) << 16; // sync/audio-version/layer 3/no crc*/
    header |= (srate_idx << 2) << 8;
    header |= channels << 6;

    lsf = !((header & (1 << 20) && header & (1 << 19)));

    mp3->xing_offset = xing_offtbl[ver != 3][channels == 1] + 4;
    bytes_needed     = mp3->xing_offset + XING_SIZE;

    for (bitrate_idx = 1; bitrate_idx < 15; bitrate_idx++) {
        int bit_rate = 1000 * avpriv_mpa_bitrate_tab[lsf][3 - 1][bitrate_idx];
        int error    = FFABS(bit_rate - codec->bit_rate);

        if (error < best_bitrate_error){
            best_bitrate_error = error;
            best_bitrate_idx   = bitrate_idx;
        }
    }

    for (bitrate_idx = best_bitrate_idx; bitrate_idx < 15; bitrate_idx++) {
        int32_t mask = bitrate_idx << (4 + 8);
        header |= mask;

        avpriv_mpegaudio_decode_header(&mpah, header);

        if (bytes_needed <= mpah.frame_size)
            break;

        header &= ~mask;
    }

    ret = avio_open_dyn_buf(&dyn_ctx);
    if (ret < 0)
        return;

    avio_wb32(dyn_ctx, header);

    avpriv_mpegaudio_decode_header(&mpah, header);

    av_assert0(mpah.frame_size >= bytes_needed);

    ffio_fill(dyn_ctx, 0, mp3->xing_offset - 4);
    ffio_wfourcc(dyn_ctx, "Xing");
    avio_wb32(dyn_ctx, 0x01 | 0x02 | 0x04 | 0x08);  // frames / size / TOC / vbr scale

    mp3->size = mpah.frame_size;
    mp3->want = 1;

    avio_wb32(dyn_ctx, 0);  // frames
    avio_wb32(dyn_ctx, 0);  // size

    // TOC
    for (i = 0; i < XING_TOC_SIZE; i++)
        avio_w8(dyn_ctx, 255 * i / XING_TOC_SIZE);

    // vbr quality
    // we write it, because some (broken) tools always expect it to be present
    avio_wb32(dyn_ctx, 0);

    // encoder short version string
    if (enc) {
        uint8_t encoder_str[9] = { 0 };
        memcpy(encoder_str, enc->value, FFMIN(strlen(enc->value), sizeof(encoder_str)));
        avio_write(dyn_ctx, encoder_str, sizeof(encoder_str));
    } else
        ffio_fill(dyn_ctx, 0, 9);

    avio_w8(dyn_ctx, 0);      // tag revision 0 / unknown vbr method
    avio_w8(dyn_ctx, 0);      // unknown lowpass filter value
    ffio_fill(dyn_ctx, 0, 8); // empty replaygain fields
    avio_w8(dyn_ctx, 0);      // unknown encoding flags
    avio_w8(dyn_ctx, 0);      // unknown abr/minimal bitrate

    // encoder delay
    if (codec->initial_padding >= 1 << 12) {
        av_log(s, AV_LOG_WARNING, "Too many samples of initial padding.\n");
        avio_wb24(dyn_ctx, 0);
    } else {
        avio_wb24(dyn_ctx, codec->initial_padding << 12);
    }

    avio_w8(dyn_ctx,   0); // misc
    avio_w8(dyn_ctx,   0); // mp3gain
    avio_wb16(dyn_ctx, 0); // preset

    // audio length and CRCs (will be updated later)
    avio_wb32(dyn_ctx, 0); // music length
    avio_wb16(dyn_ctx, 0); // music crc
    avio_wb16(dyn_ctx, 0); // tag crc

    ffio_fill(dyn_ctx, 0, mpah.frame_size - bytes_needed);

    mp3->xing_frame_size   = avio_close_dyn_buf(dyn_ctx, &mp3->xing_frame);
    mp3->xing_frame_offset = avio_tell(s->pb);
    avio_write(s->pb, mp3->xing_frame, mp3->xing_frame_size);

    mp3->audio_size = mp3->xing_frame_size;
}