static int g2m_decode_frame(AVCodecContext *avctx, void *data,
                            int *got_picture_ptr, AVPacket *avpkt)
{
    const uint8_t *buf = avpkt->data;
    int buf_size = avpkt->size;
    G2MContext *c = avctx->priv_data;
    AVFrame *pic = data;
    GetByteContext bc, tbc;
    int magic;
    int got_header = 0;
    uint32_t chunk_size;
    int chunk_type;
    int i;
    int ret;

    if (buf_size < 12) {
        av_log(avctx, AV_LOG_ERROR,
               "Frame should have at least 12 bytes, got %d instead\n",
               buf_size);
        return AVERROR_INVALIDDATA;
    }

    bytestream2_init(&bc, buf, buf_size);

    magic = bytestream2_get_be32(&bc);
    if ((magic & ~0xF) != MKBETAG('G', '2', 'M', '0') ||
        (magic & 0xF) < 2 || (magic & 0xF) > 4) {
        av_log(avctx, AV_LOG_ERROR, "Wrong magic %08X\n", magic);
        return AVERROR_INVALIDDATA;
    }

    if ((magic & 0xF) != 4) {
        av_log(avctx, AV_LOG_ERROR, "G2M2 and G2M3 are not yet supported\n");
        return AVERROR(ENOSYS);
    }

    while (bytestream2_get_bytes_left(&bc) > 5) {
        chunk_size = bytestream2_get_le32(&bc) - 1;
        chunk_type = bytestream2_get_byte(&bc);
        if (chunk_size > bytestream2_get_bytes_left(&bc)) {
            av_log(avctx, AV_LOG_ERROR, "Invalid chunk size %d type %02X\n",
                   chunk_size, chunk_type);
            break;
        }
        switch (chunk_type) {
        case FRAME_INFO:
            c->got_header = 0;
            if (chunk_size < 21) {
                av_log(avctx, AV_LOG_ERROR, "Invalid frame info size %d\n",
                       chunk_size);
                break;
            }
            c->width  = bytestream2_get_be32(&bc);
            c->height = bytestream2_get_be32(&bc);
            if (c->width  < 16 || c->width  > avctx->width ||
                c->height < 16 || c->height > avctx->height) {
                av_log(avctx, AV_LOG_ERROR,
                       "Invalid frame dimensions %dx%d\n",
                       c->width, c->height);
                ret = AVERROR_INVALIDDATA;
                goto header_fail;
            }
            if (c->width != avctx->width || c->height != avctx->height)
                avcodec_set_dimensions(avctx, c->width, c->height);
            c->compression = bytestream2_get_be32(&bc);
            if (c->compression != 2 && c->compression != 3) {
                av_log(avctx, AV_LOG_ERROR,
                       "Unknown compression method %d\n",
                       c->compression);
                return AVERROR_PATCHWELCOME;
            }
            c->tile_width  = bytestream2_get_be32(&bc);
            c->tile_height = bytestream2_get_be32(&bc);
            if (!c->tile_width || !c->tile_height) {
                av_log(avctx, AV_LOG_ERROR,
                       "Invalid tile dimensions %dx%d\n",
                       c->tile_width, c->tile_height);
                ret = AVERROR_INVALIDDATA;
                goto header_fail;
            }
            c->tiles_x = (c->width  + c->tile_width  - 1) / c->tile_width;
            c->tiles_y = (c->height + c->tile_height - 1) / c->tile_height;
            c->bpp = bytestream2_get_byte(&bc);
            chunk_size -= 21;
            bytestream2_skip(&bc, chunk_size);
            if (g2m_init_buffers(c)) {
                ret = AVERROR(ENOMEM);
                goto header_fail;
            }
            got_header = 1;
            break;
        case TILE_DATA:
            if (!c->tiles_x || !c->tiles_y) {
                av_log(avctx, AV_LOG_WARNING,
                       "No frame header - skipping tile\n");
                bytestream2_skip(&bc, bytestream2_get_bytes_left(&bc));
                break;
            }
            if (chunk_size < 2) {
                av_log(avctx, AV_LOG_ERROR, "Invalid tile data size %d\n",
                       chunk_size);
                break;
            }
            c->tile_x = bytestream2_get_byte(&bc);
            c->tile_y = bytestream2_get_byte(&bc);
            if (c->tile_x >= c->tiles_x || c->tile_y >= c->tiles_y) {
                av_log(avctx, AV_LOG_ERROR,
                       "Invalid tile pos %d,%d (in %dx%d grid)\n",
                       c->tile_x, c->tile_y, c->tiles_x, c->tiles_y);
                break;
            }
            chunk_size -= 2;
            ret = 0;
            switch (c->compression) {
            case COMPR_EPIC_J_B:
                av_log(avctx, AV_LOG_ERROR,
                       "ePIC j-b compression is not implemented yet\n");
                return AVERROR(ENOSYS);
            case COMPR_KEMPF_J_B:
                ret = kempf_decode_tile(c, c->tile_x, c->tile_y,
                                        buf + bytestream2_tell(&bc),
                                        chunk_size);
                break;
            }
            if (ret && c->framebuf)
                av_log(avctx, AV_LOG_ERROR, "Error decoding tile %d,%d\n",
                       c->tile_x, c->tile_y);
            bytestream2_skip(&bc, chunk_size);
            break;
        case CURSOR_POS:
            if (chunk_size < 5) {
                av_log(avctx, AV_LOG_ERROR, "Invalid cursor pos size %d\n",
                       chunk_size);
                break;
            }
            c->cursor_x = bytestream2_get_be16(&bc);
            c->cursor_y = bytestream2_get_be16(&bc);
            bytestream2_skip(&bc, chunk_size - 4);
            break;
        case CURSOR_SHAPE:
            if (chunk_size < 8) {
                av_log(avctx, AV_LOG_ERROR, "Invalid cursor data size %d\n",
                       chunk_size);
                break;
            }
            bytestream2_init(&tbc, buf + bytestream2_tell(&bc),
                             chunk_size - 4);
            g2m_load_cursor(avctx, c, &tbc);
            bytestream2_skip(&bc, chunk_size);
            break;
        case CHUNK_CC:
        case CHUNK_CD:
            bytestream2_skip(&bc, chunk_size);
            break;
        default:
            av_log(avctx, AV_LOG_WARNING, "Skipping chunk type %02X\n",
                   chunk_type);
            bytestream2_skip(&bc, chunk_size);
        }
    }
    if (got_header)
        c->got_header = 1;

    if (c->width && c->height && c->framebuf) {
        if ((ret = ff_get_buffer(avctx, pic, 0)) < 0)
            return ret;

        pic->key_frame = got_header;
        pic->pict_type = got_header ? AV_PICTURE_TYPE_I : AV_PICTURE_TYPE_P;

        for (i = 0; i < avctx->height; i++)
            memcpy(pic->data[0] + i * pic->linesize[0],
                   c->framebuf  + i * c->framebuf_stride,
                   c->width * 3);
        g2m_paint_cursor(c, pic->data[0], pic->linesize[0]);

        *got_picture_ptr = 1;
    }

    return buf_size;
header_fail:
    c->width   = c->height  = 0;
    c->tiles_x = c->tiles_y = 0;
    return ret;
}