From: Harris Reynolds <hreynolds2@...>
Date: 2007-02-16T00:32:18+09:00
Subject: Re: Converting Bytes to  a Negative Integer

I really like the to_signed proc idea at the bottom of this message... the procedure doesn't work however in all cases... note the Ruby code below:

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

x = -123
s = [x].pack("N")
puts to_signed[s.unpack("N").first]

y = 7777777
s = [y].pack("N")
puts to_signed[s.unpack("N").first]

# try converting 7777777 when you already have the bytes
z = ""
[129, 237, 173, 241].each { |e| z << e }
puts to_signed[z.unpack("N").first]

# OUTPUT
>ruby signed_test.rb
-123
7777777
-2115129871

Coverting the bytes of 7777777 yields a pretty nasty negative number in this case.

~harris



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/ 
* austin@halostatue.ca * http://www.halostatue.ca/feed/ 
* austin@zieglers.ca



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