From: Joel VanderWerf <vjoel@...>
Date: 2006-10-23T16:54:35+09:00
Subject: Re: Binary file modification

Rob Lee wrote:
> Hi
> 
> I've been modifying a binary file that contains various data including 
> some audio.  I'm trying to add my own audio (instead of the audio in the 
> file) which I believe I have done but I need to modify the 
> content-length header in the file which indicates the length of the 
> audio sample.  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
> f.close
> => an integer
> 
> This basically opens the file as a binary file, skips to the audio 
> content header data position and then unpacks 3 bytes into a string 
> (formatted as hex) which is then converted to an integer value.
> 
> 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 = SOMEHEADERDATA
> mydata += ["1024"].pack("someformat")
> mydata += AUDIODATA
> f=open(config[:file],"wb")
> f.write(mydata)
> f.close
> 
> However I'm a bit stuck on how to pack the data (if this is the correct 
> solution).  If someone could point me in the correct direction it would 
> be much appreciated.

Ben's right, you can use BitStruct for this:

require 'bit-struct'

class AudioData < BitStruct

   unsigned    :audio_length,    3*8,    :endian => :little
   rest        :data

   # Note: don't use :length as the name of a field, because it will
   # conflict with the #length method inherited from String.

   # the :endian option can also be :big, :network (== :big), or :native
end

audio_data = AudioData.new

data = "foo bar baz"
audio_data.data = data
audio_data.audio_length = data.length

p audio_data
p audio_data.to_s

__END__

Output:

#<AudioData audio_length=11, data="foo bar baz">
"\v\000\000foo bar baz"

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