From: Tim Pease <tim.pease@...>
Date: 2007-02-15T07:11:04+09:00
Subject: Re: Converting Bytes to a Negative Integer

On 2/14/07, James Edward Gray II <james@grayproductions.net> wrote:
> On Feb 14, 2007, at 2:27 PM, Harris Reynolds wrote:
>
> > Let's say I have 4 bytes:
> >
> > 255 255 255 236
>
>  >> bytes = [255, 255, 255, 236].map { |b| b.chr }.join
> => "\377\377\377\354"
>
> > The binary representation of this is:
> >
> > 11111111
> > 11111111
> > 11111111
> > 11101100
>
>  >> bytes.unpack("B*").first.scan(/[01]{8}/)
> => ["11111111", "11111111", "11111111", "11101100"]
>
> > The decimal number should be -20.  Does anyone know how to convert
> > these 4 bytes into an Integer?
>
> This is where I had trouble.  I found the following:
>
>  >> bytes.reverse.unpack("l*").first
> => -20
>
> This is tied to my processor though.  I had to reverse the bytes
> because they are not in the order my machine expects.
>
> unpack() has directives for when the order is known, but all of those
> seemed to be unsigned.
>
> I'm anxious to see the cross-platform solution for this...
>

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

bytes = [255, 255, 255, 236]
str = bytes.pack('C*')
str.reverse! if LITTLE_ENDIAN
str.unpack('i')[0]



Blessings,
TwP