From: Tim Pease <tim.pease@...>
Date: 2007-03-09T11:11:29+09:00
Subject: Re: Efficient processing of binary data streams in Ruby?

On 3/8/07, theosib@gmail.com <theosib@gmail.com> wrote:
> I'm writing a Ruby program that has to process binary data from files
> and sockets.  Data items are in bytes, 16-bit words, or 32-bit words,
> and I cannot predict in advance whether the data will be msb-first or
> lsb-first, so I end up writing things like this:
>
>     def unpack_16(x)
>         @msb_first ? ((x[0]<<8)|x[1]) : ((x[1]<<8)|x[0])
>     end
>
>     def pack_16(x)
>         y = "xx"
>         if (@msb_first)
>             y[0] = x>>8
>             y[1] = x&255
>         else
>             y[0] = x&255
>             y[1] = x>>8
>         end
>     end
>
> I expect, however, that this will be painfully slow, and I can't
> imagine that this hasn't been though of before.  Is there a better way
> to do this that will result in much better performance?
>

def unpack_16( str )
  @msb_first ? str.unpack('n') : str.unpack('S')
end

def pack_16( num )
  @msb_first ? [num].pack('n') : [num].pack('S')
end


That will work for little-endian processors (Intel) but not for
big-endian processors (PowerPC, Sparc). For these methods to work on
the latter you'll have to do something like this ...

def unpack_16( str )
  str = str.reverse unless @msb_first
  str.unpack('n')
end

def pack_16( num )
  str = [num].pack('n')
  str.reverse unless @msb_first
end


Just define the desired method based on the processor type -- which
can be figued out by doing this ...

LITTLE_ENDIAN = [42].pack('I')[0] == 42

if LITTLE_ENDIAN
  # define little endian methods here
else
  # define big endian methods here
end

Hope that helps

Blessings,
TwP