From: Jason Voegele <jason@...>
Date: 2002-09-24T09:24:03+09:00
Subject: Bignum from 2s Complement

For a Ruby program I'm writing, I'm receiving binary data over a 
network.  For most of the data I need to read, String#unpack works like 
a charm.  However, some of the data I receive is 64 bit 2s complement 
integers, and since String#unpack does not have a mode for 64 bit 
numbers I'm having trouble figuring out how to get a Bignum from the 
binary data.

As I've mentioned, the data is in 2s complement representation[1] such 
that 1 is encoded as "\001\000\000\000\000\000\000\000" and -1 is 
encoded as "\377\377\377\377\377\377\377\377" (those are both little 
endian, by the way).

My question is, what's the best way to make a Ruby integer out of this 
binary string?  I've already tried constructing a hexadecimal 
representation of the bytes and passing that to the Kernel::Integer 
method before realizing that that method is doing direct conversion 
rather than presuming 2s complement representation.  I also tried 
massaging the result of this process by doing a bit-wise complement and 
adding one (trying to perform the 2s complement myself), but that 
doesn't work either.

Any ideas?  Please note that I need a solution that works for big endian 
and little endian, since the data can arrive in either order.

Thanks,
Jason Voegele

[1] The data is encoded like this:
        lsb = __cdr_lsb(val)      # least significant 4 bytes
        msb = __cdr_msb(val)  # most significant 4 bytes
        lval = Kernel::big_endian? ? [msb, lsb] : [lsb, msb]
        lval.pack('ll')

I've added the big_endian? method to Kernel to determine whether the 
underlying platform is big endian or not.