static int ass_encode_frame(AVCodecContext *avctx,
                            unsigned char *buf, int bufsize,
                            const AVSubtitle *sub)
{
    ASSEncodeContext *s = avctx->priv_data;
    int i, len, total_len = 0;

    for (i=0; i<sub->num_rects; i++) {
        char ass_line[2048];
        const char *ass = sub->rects[i]->ass;

        if (sub->rects[i]->type != SUBTITLE_ASS) {
            av_log(avctx, AV_LOG_ERROR, "Only SUBTITLE_ASS type supported.\n");
            return -1;
        }

        if (strncmp(ass, "Dialogue: ", 10)) {
            av_log(avctx, AV_LOG_ERROR, "AVSubtitle rectangle ass \"%s\""
                   " does not look like a SSA markup\n", ass);
            return AVERROR_INVALIDDATA;
        }

        if (avctx->codec->id == AV_CODEC_ID_ASS) {
            long int layer;
            char *p;

            if (i > 0) {
                av_log(avctx, AV_LOG_ERROR, "ASS encoder supports only one "
                       "ASS rectangle field.\n");
                return AVERROR_INVALIDDATA;
            }

            ass += 10; // skip "Dialogue: "
            /* parse Layer field. If it's a Marked field, the content
             * will be "Marked=N" instead of the layer num, so we will
             * have layer=0, which is fine. */
            layer = strtol(ass, &p, 10);

#define SKIP_ENTRY(ptr) do {        \
    char *sep = strchr(ptr, ',');   \
    if (sep)                        \
        ptr = sep + 1;              \
} while (0)

            SKIP_ENTRY(p); // skip layer or marked
            SKIP_ENTRY(p); // skip start timestamp
            SKIP_ENTRY(p); // skip end timestamp
            snprintf(ass_line, sizeof(ass_line), "%d,%ld,%s", ++s->id, layer, p);
            ass_line[strcspn(ass_line, "\r\n")] = 0;
            ass = ass_line;
        }
        len = av_strlcpy(buf+total_len, ass, bufsize-total_len);

        if (len > bufsize-total_len-1) {
            av_log(avctx, AV_LOG_ERROR, "Buffer too small for ASS event.\n");
            return -1;
        }

        total_len += len;
    }

    return total_len;
}