static int mov_text_decode_frame(AVCodecContext *avctx,
                            void *data, int *got_sub_ptr, AVPacket *avpkt)
{
    AVSubtitle *sub = data;
    MovTextContext *m = avctx->priv_data;
    int ret;
    AVBPrint buf;
    char *ptr = avpkt->data;
    char *end;
    int text_length, tsmb_type, ret_tsmb;
    uint64_t tsmb_size;
    const uint8_t *tsmb;
    if (!ptr || avpkt->size < 2)
        return AVERROR_INVALIDDATA;
    /*
     * A packet of size two with value zero is an empty subtitle
     * used to mark the end of the previous non-empty subtitle.
     * We can just drop them here as we have duration information
     * already. If the value is non-zero, then it's technically a
     * bad packet.
     */
    if (avpkt->size == 2)
        return AV_RB16(ptr) == 0 ? 0 : AVERROR_INVALIDDATA;
    /*
     * The first two bytes of the packet are the length of the text string
     * In complex cases, there are style descriptors appended to the string
     * so we can't just assume the packet size is the string size.
     */
    text_length = AV_RB16(ptr);
    end = ptr + FFMIN(2 + text_length, avpkt->size);
    ptr += 2;
    tsmb_size = 0;
    m->tracksize = 2 + text_length;
    m->style_entries = 0;
    m->box_flags = 0;
    m->count_s = 0;
    // Note that the spec recommends lines be no longer than 2048 characters.
    av_bprint_init(&buf, 0, AV_BPRINT_SIZE_UNLIMITED);
    if (text_length + 2 != avpkt->size) {
        while (m->tracksize + 8 <= avpkt->size) {
            // A box is a minimum of 8 bytes.
            tsmb = ptr + m->tracksize - 2;
            tsmb_size = AV_RB32(tsmb);
            tsmb += 4;
            tsmb_type = AV_RB32(tsmb);
            tsmb += 4;
            if (tsmb_size == 1) {
                if (m->tracksize + 16 > avpkt->size)
                    break;
                tsmb_size = AV_RB64(tsmb);
                tsmb += 8;
                m->size_var = 16;
            } else
                m->size_var = 8;
            //size_var is equal to 8 or 16 depending on the size of box
            if (tsmb_size == 0) {
                av_log(avctx, AV_LOG_ERROR, "tsmb_size is 0\n");
                return AVERROR_INVALIDDATA;
            }
            if (tsmb_size > avpkt->size - m->tracksize)
                break;
            for (size_t i = 0; i < box_count; i++) {
                if (tsmb_type == box_types[i].type) {
                    if (m->tracksize + m->size_var + box_types[i].base_size > avpkt->size)
                        break;
                    ret_tsmb = box_types[i].decode(tsmb, m, avpkt);
                    if (ret_tsmb == -1)
                        break;
                }
            }
            m->tracksize = m->tracksize + tsmb_size;
        }
        text_to_ass(&buf, ptr, end, m);
    } else
        text_to_ass(&buf, ptr, end, m);
    ret = ff_ass_add_rect(sub, buf.str, m->readorder++, 0, NULL, NULL);
    av_bprint_finalize(&buf, NULL);
    if (ret < 0)
        return ret;
    *got_sub_ptr = sub->num_rects > 0;
    return avpkt->size;
}