From: Joel VanderWerf <vjoel@...>
Date: 2009-06-10T09:19:00+09:00
Subject: Re: binaryparse: length in bytes of block definition?

Markus Fischer wrote:
> Now, I wouldn't mind going back to bit-struct for the BSP reading stuff,
> as I find the documentation outstanding well! However, through
> binaryparse I discovered one thing I'm really enjoying: it supports
> direct reading from an IO-type object. So I can pass either a File IO or
> an StringIO object directory, which is really comfortable.

That's a useful addition. I've added that to bit-struct 0.12 now (along 
with the vector stuff), you can get it at:

http://redshift.sourceforge.net/bit-struct

> About your upcoming addition and your example: how would a simple
> integer array look like?
> 
> C-struct example:
> 
> typedef struct {
> 	float		normal[3];
> 	float		dist;
> } dplane_t;
> 
> Ruby (?):
> 
> class MyFloat < BitStruct
>   float :value, 32
> end
> 
> class Plane < BitStruct
>   vector :normal, MyFloat, :length => 3
>   float :dist, 32
> end

and then

   Plane.new.normal[2].value

works, but yes it seems like one layer too many.

> Would multiple dimension be supported, too? Like C-style float[2][2] ?
> 
> class Plane < BitStruct
>   vector :normal, :length => 3 do
>     vector :length => 3 od
>       float 32
>     end
>   end
>   float :dist, 32
> end

Yes, that's supported now:

class MD < BitStruct
   vector :row, :length => 3 do
     vector :col, :length => 10 do
       float :x, 32
     end
   end
end

md = MD.new
   rows = md.row
     row = rows[2]
       cols = row.col
         col = cols[7]
           col.x = 1.23
         cols[7] = col
       row.col = cols
     rows[2] = row
   md.row = rows

p md
p md.row[2].col[7].x

As you can see, the reader method is fine, but the writer has to be
broken down into a sequence of writes to each nested chunk... yuck.

The reason it doesn't allow simply

   md.row[2].col[7].x = 1.23

is that all those intermediate objects have to be something, either
separate (copied) bit-structs, or proxies that delegate back to the
original. If the latter, there is IMO the possibility of confusion
as these proxies propagate around and the original object changes
unexpectedly.

Any other ideas...?

-- 
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407