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+;;; assoc.el --- insert/delete/sort functions on association lists
+
+;; Author: Barry A. Warsaw <[email protected]>
+;; Keywords: extensions
+
+;; This software is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY.  No author or distributor accepts
+;; responsibility to anyone for the consequences of using it or for
+;; whether it serves any particular purpose or works at all, unless he
+;; says so in writing.
+
+;; This software was written as part of the supercite author's
+;; official duty as an employee of the United States Government and is
+;; thus in the public domain.  You are free to use that particular
+;; software as you wish, but WITHOUT ANY WARRANTY WHATSOEVER.  It
+;; would be nice, though if when you use any of this code, you give
+;; due credit to the author.
+
+;;; Commentary:
+
+;; Association list utilities providing insertion, deletion, sorting
+;; fetching off key-value pairs in association lists.
+
+;;; Code:
+
+(defun asort (assoc-symbol key)
+  "Move a specified key-value pair to the head of an alist.
+The alist is referenced by ALIST-SYMBOL. Key-value pair to move to
+head is one matching KEY.  Returns the sorted list and doesn't affect
+the order of any other key-value pair.  Side effect sets alist to new
+sorted list."
+  (set alist-symbol
+       (sort (copy-alist (eval alist-symbol))
+	     (function (lambda (a b) (equal (car a) key))))))
+
+
+(defun aelement (key value)
+  "Makes a list of a cons cell containing car of KEY and cdr of VALUE.
+The returned list is suitable as an element of an alist."
+  (list (cons key value)))
+
+
+(defun aheadsym (alist)
+  "Return the key symbol at the head of ALIST."
+  (car (car alist)))
+
+
+(defun anot-head-p (alist key)
+  "Find out if a specified key-value pair is not at the head of an alist.
+The alist to check is specified by ALIST and the key-value pair is the
+one matching the supplied KEY.  Returns nil if ALIST is nil, or if
+key-value pair is at the head of the alist.  Returns t if key-value
+pair is not at the head of alist.  ALIST is not altered."
+  (not (equal (aheadsym alist) key)))
+
+
+(defun aput (alist-symbol key &optional value)
+  "Inserts a key-value pair into an alist.
+The alist is referenced by ALIST-SYMBOL. The key-value pair is made
+>from KEY and optionally, VALUE. Returns the altered alist or nil if
+ALIST is nil.
+
+If the key-value pair referenced by KEY can be found in the alist, and
+VALUE is supplied non-nil, then the value of KEY will be set to VALUE.
+If VALUE is not supplied, or is nil, the key-value pair will not be
+modified, but will be moved to the head of the alist. If the key-value
+pair cannot be found in the alist, it will be inserted into the head
+of the alist (with value nil if VALUE is nil or not supplied)."
+  (let ((elem (aelement key value))
+	alist)
+    (asort alist-symbol key)
+    (setq alist (eval alist-symbol))
+    (cond ((null alist) (set alist-symbol elem))
+	  ((anot-head-p alist key) (set alist-symbol (nconc elem alist)))
+	  (value (setcar alist (car elem)))
+	  (t alist))))
+
+
+(defun adelete (alist-symbol key)
+  "Delete a key-value pair from the alist.
+Alist is referenced by ALIST-SYMBOL and the key-value pair to remove
+is pair matching KEY.  Returns the altered alist."
+  (asort alist-symbol key)
+  (let ((alist (eval alist-symbol)))
+    (cond ((null alist) nil)
+	  ((anot-head-p alist key) alist)
+	  (t (set alist-symbol (cdr alist))))))
+
+
+(defun aget (alist key &optional keynil-p)
+  "Returns the value in ALIST that is associated with KEY.
+Optional KEYNIL-P describes what to do if the value associated with
+KEY is nil.  If KEYNIL-P is not supplied or is nil, and the value is
+nil, then KEY is returned.  If KEYNIL-P is non-nil, then nil would be
+returned.
+
+If no key-value pair matching KEY could be found in ALIST, or ALIST is
+nil then nil is returned. ALIST is not altered."
+  (let ((copy (copy-alist alist)))
+    (cond ((null alist) nil)
+	  ((progn (asort 'copy key)
+		  (anot-head-p copy key)) nil)
+	  ((cdr (car copy)))
+	  (keynil-p nil)
+	  ((car (car copy)))
+	  (t nil))))
+
+
+(defun amake (alist-symbol keylist &optional valuelist)
+  "Make an association list.
+The association list is attached to the alist referenced by
+ALIST-SYMBOL. Each element in the KEYLIST becomes a key and is
+associated with the value in VALUELIST with the same index. If
+VALUELIST is not supplied or is nil, then each key in KEYLIST is
+associated with nil.
+
+KEYLIST and VALUELIST should have the same number of elements, but
+this isn't enforced.  If VALUELIST is smaller than KEYLIST, remaining
+keys are associated with nil.  If VALUELIST is larger than KEYLIST,
+extra values are ignored.  Returns the created alist."
+  (let ((keycar (car keylist))
+	(keycdr (cdr keylist))
+	(valcar (car valuelist))
+	(valcdr (cdr valuelist)))
+    (cond ((null keycdr)
+	   (aput alist-symbol keycar valcar))
+	  (t
+	   (amake alist-symbol keycdr valcdr)
+	   (aput alist-symbol keycar valcar))))
+  (eval alist-symbol))
+
+(provide 'assoc)
+
+;;; assoc.el ends here