faster gnus-thread-latest-date

[Dan Christensen <jdc@uwo.ca>]

The function gnus-thread-latest-date is used by
gnus-thread-sort-by-most-recent-date, and is very slow.  The current
code is:

(defun gnus-thread-latest-date (thread)
  "Return the highest article date in THREAD."
  (let ((previous-time 0))
    (apply 'max
	   (mapcar
	    (lambda (header)
	      (setq previous-time
		    (condition-case ()
			(gnus-float-time (mail-header-parse-date
					  (mail-header-date header)))
		      (error previous-time))))
	    (sort
	     (message-flatten-list thread)
	     (lambda (h1 h2)
	       (< (mail-header-number h1)
		  (mail-header-number h2))))))))

Since all that is desired is the latest date, I don't see why dates that
don't parse are replaced by the date of the article with the previous
number.  Why not just replace them with zero?  And if you do that, then
there is no need for a preliminary sort by number.  So I think the
following version should produce identical output:

(defun gnus-thread-latest-date (thread)
  "Return the highest article date in THREAD."
  (apply 'max
	 (mapcar
	  (lambda (header)
	    (condition-case ()
		(gnus-float-time (mail-header-parse-date
				  (mail-header-date header)))
	      (error 0)))
	  (message-flatten-list thread))))

Can someone check my logic?

In my tests, it is noticeable faster.  Can it be improved further?

I have papers on file for Gnus and Emacs, so feel free to apply
this change.

Dan