From: Daniel Martin <martin@...>
Date: 2006-07-30T08:03:48+09:00
Subject: Re: How can I parse binary files?

Joel VanderWerf <vjoel@path.berkeley.edu> writes:

> 2. Reading BitStructs from a file.
>
> I'm not sure there's an easier way to do it than in your code, Daniel,
> but I'm open to suggestions. All of ruby's IO (including sockets) uses
> Strings, so we always have to read a String and then construct a
> BitStruct from that string using BitStruct.new.

Well, the main issue is that I thought it was a bit clumsy to have to
both ask for the byte length and do the reading.  Now, I'm not sure
this can be done for a structure with a "rest" parameter, but for
anything else it should be possible for the user of bit-struct to do
the read and structure init in one step, say:
  mrk_head = MRKHeader.read(f)
or
  mrk_head = MRKHeader.new 
  mrk_head << f
Actually, those two steps should be combinable into something like:
  mrk_head = MRKHeader.new << f

The point is that the user of bit-struct should be able to avoid
knowing the actual byte length if at all possible.

This might even be possible to do with something that has a rest
parameter, so long as you have a maximum size limitation on "rest",
and then just use however many bytes you get back in one read call
(reasonable behavior for sockets).

> 3. Defining data wrappers.
>
> Hm.... I've needed that too. I'll think about it.

I can think of at least two ways:

one is for most built-in field types to have overrideable data_in and
data_out procedures that take a single argument and by default just
return their argument but someone who needs, say, a date stored as
seconds-since-1970 could then just subclass BitStruct::UnsignedField
and override data_in and data_out.

The other is to add an extra data_in and data_out option to all fields
whose values should be Procs.

Actually, it's probably feasible to implement both.