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Re: Using the GNU GMP Library for Bignums in Emacs

From: Jefferson Carpenter
Subject: Re: Using the GNU GMP Library for Bignums in Emacs
Date: Sun, 3 Jun 2018 23:44:55 +0000
User-agent: Mozilla/5.0 (Windows NT 6.2; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0

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.


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.


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