From: Ben Nagy <ben@...>
Date: 2006-10-23T15:37:41+09:00
Subject: Re: Binary file modification

> -----Original Message-----
> From: list-bounce@example.com 
> [mailto:list-bounce@example.com] On Behalf Of Rob Lee
[...]
> I've been reading the content-length data from 
> the file 
> using something similar to :
> 
> f=open(config[:file],"rb")
> f.pos=AUDIO_CONTENT_HEADER_OFFSET
> length=f.read(3).unpack("H2H2H2").hex.to_i

That should die because Array#hex doesn't exist, but I get the idea. If you
do this kind of thing a lot with different kinds of data then you should
note that to_i and to_s both take optional base arguments, so
foo.unpack('H*').first.to_i(16) should work

[...]
> I'd like to be able to reverse this process and take any an integer 
> value (1024 in the case shown below) and write it to a binary 
> file with 
> some header and footer data - something like :
[...]
> mydata += ["1024"].pack("someformat")
[...]
> However I'm a bit stuck on how to pack the data (if this is 
> the correct 
> solution).

Here's the problem - if you need exactly three bytes then you are going to
have to apply your own padding, since the pack routines will only pack
directly as a long or a short which will mostly be 4 and 2 bytes - both of
which could cause you problems. My hacks always involve <<'ing a single byte
integer onto a string. In your case, here is a horrible oneliner which you
should not use because it is gross.

num=1024
num.to_s(16)[0..2].instance_eval {(self.reverse + '0' * (6 -
self.length)).reverse}.scan(/../).inject('') {|s,byte| s << byte.to_i(16)}

You could also try googling BitStruct, which is a ruby library that might
help by defining these headers and footers as structure objects.

Cheers,

ben