static void mpegvideo_extract_headers(AVCodecParserContext *s,
                                      AVCodecContext *avctx,
                                      const uint8_t *buf, int buf_size)
{
    ParseContext1 *pc = s->priv_data;
    const uint8_t *buf_end;
    const uint8_t *buf_start= buf;
    uint32_t start_code;
    int frame_rate_index, ext_type, bytes_left;
    int frame_rate_ext_n, frame_rate_ext_d;
    int picture_structure, top_field_first, repeat_first_field, progressive_frame;
    int horiz_size_ext, vert_size_ext, bit_rate_ext;

    s->repeat_pict = 0;
    buf_end = buf + buf_size;
    while (buf < buf_end) {
        start_code= -1;
        buf= ff_find_start_code(buf, buf_end, &start_code);
        bytes_left = buf_end - buf;
        switch(start_code) {
        case PICTURE_START_CODE:
            if (bytes_left >= 2) {
                s->pict_type = (buf[1] >> 3) & 7;
            }
            break;
        case SEQ_START_CODE:
            if (bytes_left >= 7) {
                pc->width  = (buf[0] << 4) | (buf[1] >> 4);
                pc->height = ((buf[1] & 0x0f) << 8) | buf[2];
                avcodec_set_dimensions(avctx, pc->width, pc->height);
                frame_rate_index = buf[3] & 0xf;
                pc->frame_rate.den = avctx->time_base.den = ff_frame_rate_tab[frame_rate_index].num;
                pc->frame_rate.num = avctx->time_base.num = ff_frame_rate_tab[frame_rate_index].den;
                avctx->bit_rate = ((buf[4]<<10) | (buf[5]<<2) | (buf[6]>>6))*400;
                avctx->codec_id = CODEC_ID_MPEG1VIDEO;
                avctx->sub_id = 1;
            }
            break;
        case EXT_START_CODE:
            if (bytes_left >= 1) {
                ext_type = (buf[0] >> 4);
                switch(ext_type) {
                case 0x1: 
                    if (bytes_left >= 6) {
                        horiz_size_ext = ((buf[1] & 1) << 1) | (buf[2] >> 7);
                        vert_size_ext = (buf[2] >> 5) & 3;
                        bit_rate_ext = ((buf[2] & 0x1F)<<7) | (buf[3]>>1);
                        frame_rate_ext_n = (buf[5] >> 5) & 3;
                        frame_rate_ext_d = (buf[5] & 0x1f);
                        pc->progressive_sequence = buf[1] & (1 << 3);
                        avctx->has_b_frames= !(buf[5] >> 7);
                        pc->width  |=(horiz_size_ext << 12);
                        pc->height |=( vert_size_ext << 12);
                        avctx->bit_rate += (bit_rate_ext << 18) * 400;
                        avcodec_set_dimensions(avctx, pc->width, pc->height);
                        avctx->time_base.den = pc->frame_rate.den * (frame_rate_ext_n + 1);
                        avctx->time_base.num = pc->frame_rate.num * (frame_rate_ext_d + 1);
                        avctx->codec_id = CODEC_ID_MPEG2VIDEO;
                        avctx->sub_id = 2; 
                    }
                    break;
                case 0x8: 
                    if (bytes_left >= 5) {
                        picture_structure = buf[2]&3;
                        top_field_first = buf[3] & (1 << 7);
                        repeat_first_field = buf[3] & (1 << 1);
                        progressive_frame = buf[4] & (1 << 7);
                        
                        if (repeat_first_field) {
                            if (pc->progressive_sequence) {
                                if (top_field_first)
                                    s->repeat_pict = 4;
                                else
                                    s->repeat_pict = 2;
                            } else if (progressive_frame) {
                                s->repeat_pict = 1;
                            }
                        }
                    }
                    break;
                }
            }
            break;
        case -1:
            goto the_end;
        default:
            

            if (start_code >= SLICE_MIN_START_CODE &&
                start_code <= SLICE_MAX_START_CODE)
                goto the_end;
            break;
        }
    }
 the_end: ;
}