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Common Lisp implementation running on the JVM
There is a newer version: 1.9.2
;;; assoc.lisp
;;;
;;; Copyright (C) 2003 Peter Graves
;;; $Id$
;;;
;;; This program is free software; you can redistribute it and/or
;;; modify it under the terms of the GNU General Public License
;;; as published by the Free Software Foundation; either version 2
;;; of the License, or (at your option) any later version.
;;;
;;; This program is distributed in the hope that it will be useful,
;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;;; GNU General Public License for more details.
;;;
;;; You should have received a copy of the GNU General Public License
;;; along with this program; if not, write to the Free Software
;;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
;;;
;;; As a special exception, the copyright holders of this library give you
;;; permission to link this library with independent modules to produce an
;;; executable, regardless of the license terms of these independent
;;; modules, and to copy and distribute the resulting executable under
;;; terms of your choice, provided that you also meet, for each linked
;;; independent module, the terms and conditions of the license of that
;;; module.  An independent module is a module which is not derived from
;;; or based on this library.  If you modify this library, you may extend
;;; this exception to your version of the library, but you are not
;;; obligated to do so.  If you do not wish to do so, delete this
;;; exception statement from your version.

;;; From CMUCL.

(in-package "SYSTEM")

(defmacro assoc-guts (test-guy)
  `(do ((alist alist (cdr alist)))
       ((endp alist))
     (if (car alist)
	 (if ,test-guy (return (car alist))))))

(defun assoc (item alist &key key test test-not)
  (cond (test
	 (if key
	     (assoc-guts (funcall test item (funcall key (caar alist))))
	     (assoc-guts (funcall test item (caar alist)))))
	(test-not
	 (if key
	     (assoc-guts (not (funcall test-not item
				       (funcall key (caar alist)))))
	     (assoc-guts (not (funcall test-not item (caar alist))))))
	(t
	 (if key
	     (assoc-guts (eql item (funcall key (caar alist))))
	     (assoc-guts (eql item (caar alist)))))))

(defun assoc-if (predicate alist &key key)
  (if key
      (assoc-guts (funcall predicate (funcall key (caar alist))))
      (assoc-guts (funcall predicate (caar alist)))))

(defun assoc-if-not (predicate alist &key key)
  (if key
      (assoc-guts (not (funcall predicate (funcall key (caar alist)))))
      (assoc-guts (not (funcall predicate (caar alist))))))

(defun rassoc (item alist &key key test test-not)
  (cond (test
	 (if key
	     (assoc-guts (funcall test item (funcall key (cdar alist))))
	     (assoc-guts (funcall test item (cdar alist)))))
	(test-not
	 (if key
	     (assoc-guts (not (funcall test-not item
				       (funcall key (cdar alist)))))
	     (assoc-guts (not (funcall test-not item (cdar alist))))))
	(t
	 (if key
	     (assoc-guts (eql item (funcall key (cdar alist))))
	     (assoc-guts (eql item (cdar alist)))))))

(defun rassoc-if (predicate alist &key key)
  (if key
      (assoc-guts (funcall predicate (funcall key (cdar alist))))
      (assoc-guts (funcall predicate (cdar alist)))))

(defun rassoc-if-not (predicate alist &key key)
  (if key
      (assoc-guts (not (funcall predicate (funcall key (cdar alist)))))
      (assoc-guts (not (funcall predicate (cdar alist))))))

(defun acons (key datum alist)
  (cons (cons key datum) alist))

(defun pairlis (keys data &optional (alist '()))
  (do ((x keys (cdr x))
       (y data (cdr y)))
      ((and (endp x) (endp y)) alist)
    (if (or (endp x) (endp y))
	(error "the lists of keys and data are of unequal length"))
    (setq alist (acons (car x) (car y) alist))))

;;; From SBCL.
(defun copy-alist (alist)
  "Return a new association list which is EQUAL to ALIST."
  (if (endp alist)
      alist
      (let ((result
	     (cons (if (atom (car alist))
		       (car alist)
		       (cons (caar alist) (cdar alist)))
		   nil)))
	(do ((x (cdr alist) (cdr x))
	     (splice result
		     (cdr (rplacd splice
				  (cons
				   (if (atom (car x))
				       (car x)
				       (cons (caar x) (cdar x)))
				   nil)))))
	    ((endp x)))
	result)))