Re: Using the GNU GMP Library for Bignums in Emacs

[Jefferson Carpenter]

On 4/24/2018 5:45 AM, Helmut Eller wrote:
> On Mon, Apr 23 2018, Paul Eggert wrote:
>
> > Even in the standard C library (which lacks bignums), the %x printf
> > format is supposed to be used only with unsigned integers. Nowadays
> > GCC even optionally warns about using %x on signed integers.
>
> Maybe %x should then print the (shortest) two's complement
> representation for bignums.  In Common Lisp:
>
> (defun 2comp (x)
>    (format nil "~x" (logand x (1- (ash 1 (+ 1 (integer-length x)))))))
>
> (2comp -1) => "1"
> (2comp -2) => "2"
> (2comp -3) => "5"
> (2comp -15) => "11"
> (2comp -16) => "10"
>
> In practice, only integers smaller than most-negative-fixnum would use
> this rule.
>
> Helmut

I don't think that would make sense, because you can't tell whether a 
given output string was generated from a positive or a negative number.

For instance, (2comp -6) = (2comp 10) = "A".

"Two's complement" isn't a representation of integers; "N-bit two's 
complement" is a representation of integers from -2^N to 2^N-1, for 
positive integer N.

So, I think that function should take another argument, the number of 
bits `n` to output.

Jefferson