From: Austin Ziegler <halostatue@...>
Date: 2007-02-15T07:13:40+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...

Joel VanDerWerf had something that I'll be adapting if I need it elsewhere.

  x = -123
  s = [x].pack("N")

  bits = 32
  max_unsigned = 2 ** bits
  max_signed = 2 ** (bits - 1)
  to_signed = proc { |n| (n >= max_signed) ? n - max_unsigned : n }

  puts to_signed[s.unpack("N").first]

I need it for unsigned->signed words, so:

  x = -123
  s = [x].pack("n")

  bits = 16
  max_unsigned = 2 ** bits
  max_signed = 2 ** (bits - 1)
  to_signed = proc { |n| (n >= max_signed) ? n - max_unsigned : n }

  puts to_signed[s.unpack("n").first]

-austin
-- 
Austin Ziegler * halostatue@gmail.com * http://www.halostatue.ca/
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               * austin@zieglers.ca