How to: 96 bit Date-time arithmetic?

How to: 96 bit Date-time arithmetic?

[Siegfried Heintze]

How do I add delta times and subtract absolute times stored in the 96 bit (3
integer) format? Apparently the functions encode and decode will help me
convert to and from the 96 bit time format.

I want to increment times by 1 day and have emacs properly figure out when
there is a Feb 28.

Re: How to: 96 bit Date-time arithmetic?

[Siegfried Heintze]

While I am still curious about 96 bit arithmetic, here an alternative
attempt. Why does not it work?

(setq x (decode-time (current-time)))
(setq y (list (car x) (cadr x)  (caddr x) (1- (cadddr x))
       (nth 5 x) (nth 6 x) (nth 7 x)(nth 8 x) ))
(apply 'encode-time x) ; no error

(apply 'encode-time y) ; error -- why?

Re: How to: 96 bit Date-time arithmetic?

[lawrence mitchell]

Siegfried Heintze wrote:

> While I am still curious about 96 bit arithmetic, here an alternative
> attempt. Why does not it work?

> (setq x (decode-time (current-time)))
> (setq y (list (car x) (cadr x)  (caddr x) (1- (cadddr x))
>        (nth 5 x) (nth 6 x) (nth 7 x)(nth 8 x) ))
       ^ (nth 4 x)
> (apply 'encode-time x) ; no error

> (apply 'encode-time y) ; error -- why?

in `nth', N counts from zero.

Hence you want (nth 4 x) before (nth 5 x)
Notice that:

(length x)
    => 9
(length y)
    => 8

I'd probably do
(setcar (nthcdr 3 x) (1- (nth 3 x)))
instead, as it's somewhat easier to read.

On another note of style, you probably don't want to use setq
unless you've already bound x in a `let' form, or x is indeed a
global variable. e.g.
(let ((x (decode-time (current-time))))
  (setcar (nthcdr 3 x) (1- (nth 3 x)))
  x)
To return the modified value of x.
-- 
lawrence mitchell <wence@gmx.li>

Re: How to: 96 bit Date-time arithmetic?

[Christian Lemburg]

"Siegfried Heintze" <siegfried@heintze.com> writes:

> How do I add delta times and subtract absolute times stored in the 96 bit (3
> integer) format? Apparently the functions encode and decode will help me
> convert to and from the 96 bit time format.
> 
> I want to increment times by 1 day and have emacs properly figure out when
> there is a Feb 28.


(defun my-emacs-time->seconds (time)
  "Convert emacs time format to seconds as one 32 bit integer."
  (let ((high (lsh (car time) 16))
	(low (cadr time)))
    (logior high low)))

(defun my-seconds->emacs-time (time)
  "Convert one 32 bit integer to emacs time format."
  (list (lsh time -16) (logand time (1- (lsh 1 16))) 0))


-- 
Christian Lemburg, <lemburg@aixonix.de>, http://www.clemburg.com/
 Hand, n.:
 	A singular instrument worn at the end of a human arm and
 commonly thrust into somebody's pocket.
 		-- Ambrose Bierce, "The Devil's Dictionary"

Re: How to: 96 bit Date-time arithmetic?

[Alan Shutko]

Christian Lemburg <lemburg@aixonix.de> writes:

>   "Convert emacs time format to seconds as one 32 bit integer."

It should be noted that integers are only 32-bit on 64-bit
platforms.  On most platforms, integers are 28-bit.

It's probably better to use time-add from time-date.el (included in
Gnus and thus in Emacs).

-- 
Alan Shutko <ats@acm.org> - In a variety of flavors!
They can't get rid of Morn before he speaks!!!!