From: Joel VanderWerf <vjoel@...>
Date: 2005-11-16T08:10:19+09:00
Subject: Re: Reading a signed byte in network byte order

Robert Evans wrote:
> Hi,
> 
> In trying to read 4 bytes as a signed integer from an IO in big- endian
> order, is there already a utility to do this in Ruby? I notice  unpack
> has lots of combinations already, but seemingly not one for  this. Maybe
> I am just missing it?
> 
> Thanks,
> Bob Evans

You can unpack with "N" and then use the following to interpret that
positive Integer as a signed number in 32 bit two's complement
representation and convert it to a positive or negative Integer:

      length = 32 # bits
      max = 2**length-1
      mid = 2**(length-1)
      to_signed = proc {|n| (n>=mid) ? -((n ^ max) + 1) : n}

For example:

irb(main):012:0> to_signed[4294967295]
=> -1

This is a snippet from the implementation of my bit-struct lib (see
RAA). A BitStruct is basically a string with some extra accessors and
convenience methods. Currently it handles fields that are either
multiple bytes or 1-7 bits within a byte. (Eventually, longer odd-size
bit fields would be nice.) Also supports fields for: fixed length char
strings, null-terminated strings, hex octets, decimal octets, floats,
nested BitStructs, and "rest"--the rest of the string after defined
fields. It's *really* useful for playing with net protocols in pure
ruby. (Someday, I'll probably write a C extension for efficiency.)

Example:

require 'bit-struct'

class C < BitStruct
  signed      :foo,     32,     "Something signed"
  unsigned    :bar,     32,     "Something UNsigned"
end

c = C.new

c.foo = -12345678
c.bar = 12345678

puts "-"*40
p c

puts "-"*40
p c.to_s

puts "-"*40
puts c.inspect_detailed

puts "-"*40
p c.to_h

puts "-"*40
puts C.describe

__END__

----------------------------------------
#<C foo=-12345678, bar=12345678>
----------------------------------------
"\377C\236\262\000\274aN"
----------------------------------------
C:
              Something signed = -12345678
            Something UNsigned = 12345678
----------------------------------------
{:bar=>12345678, :foo=>-12345678}
----------------------------------------
    byte: type         name          [size] description
----------------------------------------------------------------------
      @0: signed       foo           [ 32b] Something signed
      @4: unsigned     bar           [ 32b] Something UNsigned


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