Introduce a variable to control ecomplete sorting

* lisp/ecomplete.el (ecomplete-sort-predicate): New variable.
(ecomplete-get-matches): Use it.

doc/misc/message.texi

@@ -1485,6 +1485,9 @@ choose one of these completions, use the @kbd{M-n} command to move
 down to the list.  Use @kbd{M-n} and @kbd{M-p} to move down and up the
 list, and @kbd{RET} to choose a completion.
 
+The @code{ecomplete-sort-predicate} variable controls how
+@code{ecomplete} matches are sorted.
+
 @node Spelling
 @section Spelling
 @cindex spelling

etc/NEWS

@@ -230,6 +230,10 @@ are implemented in C using the Jansson library.
 It's a simple convenience function for looking up MIME types based on
 file name extensions.
 
++++
+** The ecomplete sorting has changed to a decay-based algorithm.  This
+can be controlled by the new `ecomplete-sort-predicate' variable.
+
 
 * Changes in Emacs 27.1 on Non-Free Operating Systems
 

lisp/ecomplete.el

@@ -70,6 +70,19 @@
   :type '(symbol :tag "Coding system")
   :group 'ecomplete)
 
+(defcustom ecomplete-sort-predicate 'ecomplete-decay
+  "Predicate to use when sorting matched.
+The predicate is called with two parameters that represent the
+completion.  Each parameter is a list where the first element is
+the times the completion has been used, the second is the
+timestamp of the most recent usage, and the third item is the
+string that was matched."
+  :type '(radio (function-item :tag "Sort by usage and newness" ecomplete-decay)
+		(function-item :tag "Sort by times used" ecomplete-usage)
+		(function-item :tag "Sort by newness" ecomplete-newness)
+		(function :tag "Other"))
+  :group 'ecomplete)
+
 ;;; Internal variables.
 
 (defvar ecomplete-database nil)
@@ -122,8 +135,7 @@
 	   (loop for (key count time text) in elems
 		 when (string-match match text)
 		 collect (list count time text))
-	   (lambda (l1 l2)
-	     (> (car l1) (car l2))))))
+           ecomplete-sort-predicate)))
     (when (> (length candidates) 10)
       (setcdr (nthcdr 10 candidates) nil))
     (unless (zerop (length candidates))
@@ -189,6 +201,21 @@ matches."
 	(forward-char 1)))
     (buffer-string)))
 
+(defun ecomplete-usage (l1 l2)
+  (> (car l1) (car l2)))
+
+(defun ecomplete-newness (l1 l2)
+  (> (cadr l1) (cadr l2)))
+
+(defun ecomplete-decay (l1 l2)
+  (> (ecomplete-decay-1 l1) (ecomplete-decay-1 l2)))
+
+(defun ecomplete-decay-1 (elem)
+  ;; We subtract 5% from the item for each week it hasn't been used.
+  (/ (car elem)
+     (expt 1.05 (/ (- (float-time) (cadr elem))
+                   (* 7 24 60 60)))))
+
 (provide 'ecomplete)
 
 ;;; ecomplete.el ends here