static int build_def_list(Picture *def, Picture **in, int len, int is_long, int sel){
    int i[2]={0};
    int index=0;

    while(i[0]<len || i[1]<len){
        while(i[0]<len && !(in[ i[0] ] && (in[ i[0] ]->reference & sel)))
            i[0]++;
        while(i[1]<len && !(in[ i[1] ] && (in[ i[1] ]->reference & (sel^3))))
            i[1]++;
        if(i[0] < len){
            in[ i[0] ]->pic_id= is_long ? i[0] : in[ i[0] ]->frame_num;
            split_field_copy(&def[index++], in[ i[0]++ ], sel  , 1);
        }
        if(i[1] < len){
            in[ i[1] ]->pic_id= is_long ? i[1] : in[ i[1] ]->frame_num;
            split_field_copy(&def[index++], in[ i[1]++ ], sel^3, 0);
        }
    }

    return index;
}