[Dev-luatex] A few newbie questions

Hans van der Meer H.vanderMeer at uva.nl
Wed Jan 28 10:21:12 CET 2009


Yue Wang has some good points. I second the motion!

Hans van der Meer

On 28 jan 2009, at 07:57, Yue Wang wrote:

> Hi, Taco:
>
> Last week I stayed in my grandma's home (located in a beautiful
> countryside) which has limited internet access. So I planned to study
> the luatex source code during the stay. I read a small subsect of the
> LuaTeX source and most parts of MetaPost source code and have a few
> newbie questions:
>
> I will refer to the mp.w as an example, since it is finished and I am
> more familiar with the C programming language.
>
>
> - wouldn't it be great if we remove the packed data dependency?
> Knuth used packed data since memory space was very valuable during the
> years when he wrote the TeX and MetaFont program. By doing so, he can
> split a memory word (the space needed for a 32bit integer) into two,
> three or four so that he can save more spaces. In pdfTeX and MetaPost,
> almost all the important data structures (like all kinds of nodes,
> boxes, lists) are build on top of the packed data structure. For
> example, In TeX's char_node presentation, instead of using one word
> for font and one word for character, he can use one word to do so by
> defining two web macros font==type and character==subtype. However,
> Memory consumption is not that important these years and in LuaTeX
> other representations uses much more memory (like parsing a
> complicated opentype font and dumping it into a lua table using the
> fontforge library). So wouldn't it be bettter if we remove the packed
> data? It will make the code more readable at the cost of consuming a
> few more memory (the TeX web side is very memory efficient, so there
> won't be big memory footprint even if we double the memory
> consumption) since we can get rid of the messy web or C macros.
>
> For example, in Part 21, mp.w, we define more than a dozen macros to
> get all the variables of a edge which A points to its start. Such code
> may confuse modern programmers who learn to program in C after the
> mid-1990s. They prefer to do something like this:
> typedef fill_node struct {
>  path *path_p;
>  pen *pen_p;
> .....
>  int miterlim_var;
> };
> if we want to create a new fill node, instead of creating its node,
> and setting its path_p to a given pointer p using
> mp_path_p(t)=p;
> it will be more understandable if we can do this:
> node * mp_new_fill_node (pointer, p){
>  fill_node *fn = malloc (sizeof (fill_node)); /* a fill_node*/
>  fn->path_p = p;
>  fn->pen_p=NULL;
>  fn->red_val=0
> ....
>  return fn
> }
>
>
> -Why can't we use IEEE floating number specification?
> TeX/MetaPost have it's own number presentation builtin. But this is a
> very complicated and strange floating number system which represent
> each number as a integer. I think IEEE floating number specification
> is good enough for implementing TeX? Today almost every operating
> system support float/double, and the precision is great. If we can do
> this, Part 7 of the mp.w and luatex.web can be totally removed, and we
> can also clean the conversion macros in the rest of the macros. If we
> remove the dependency of part 7 and part 9, maybe we can also make our
> code more portable on different machines.
>
> - Why should we do memory management ourselves?
> I can see that in mp.w, we maintain a node pool (a fixed continuous
> space in physical memory). When we want to allocate a node, we should
> call mp_get_node, which does several things like finding available
> place for memory allocation, reporting error if the memory is exceed
> then it tries to put everything with different structures into the
> pool. Wouldn't it be great if we just ask the operating system's C
> library to handle these tasks (like using malloc and free as the
> example code of mp_new_fill_node shows). Modern operating system's
> library is more efficient, and it will also make the luatex code looks
> better.
>
> - From the part of the code I read, if I understand correctly, the
> only reason of incorporating fontforge is to get the font Metrics
> data?
> If so, I think FreeType2 is sufficient to accomplish the task? [I
> think freetype2 is much smaller and efficient, and fontforge use
> freetype2]
>
> Yue Wang
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