;;; ring.el --- handle rings of marks

;; Copyright (C) 1992 Free Software Foundation, Inc.

;; This file is part of GNU Emacs.

;; GNU Emacs 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 1, or (at your option)
;; any later version.

;; GNU Emacs 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 GNU Emacs; see the file COPYING.  If not, write to
;; the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.

;;; This code defines a ring data structure. A ring is a 
;;;     (hd-index tl-index . vector) 
;;; list. You can insert to, remove from, and rotate a ring. When the ring
;;; fills up, insertions cause the oldest elts to be quietly dropped.
;;;
;;; HEAD = index of the newest item on the ring.
;;; TAIL = index of the oldest item on the ring.
;;;
;;; These functions are used by the input history mechanism, but they can
;;; be used for other purposes as well.

(provide 'history)

(defun ring-p (x) 
  "T if X is a ring; NIL otherwise."
  (and (consp x) (integerp (car x))
       (consp (cdr x)) (integerp (car (cdr x)))
       (vectorp (cdr (cdr x)))))

(defun make-ring (size)
  "Make a ring that can contain SIZE elts"
  (cons 1 (cons 0 (make-vector (+ size 1) nil))))

(defun ring-plus1 (index veclen)
  "INDEX+1, with wraparound"
  (let ((new-index (+ index 1)))
    (if (= new-index veclen) 0 new-index)))

(defun ring-minus1 (index veclen)
  "INDEX-1, with wraparound"
  (- (if (= 0 index) veclen index) 1))

(defun ring-length (ring)
  "Number of elts in the ring."
  (let ((hd (car ring)) (tl (car (cdr ring)))  (siz (length (cdr (cdr ring)))))
    (let ((len (if (<= hd tl) (+ 1 (- tl hd)) (+ 1 tl (- siz hd)))))
      (if (= len siz) 0 len))))

(defun ring-empty-p (ring)
  (= 0 (ring-length ring)))

(defun ring-insert (ring item)
  "Insert a new item onto the ring. If the ring is full, dump the oldest
item to make room."       
  (let* ((vec (cdr (cdr ring)))  (len (length vec))
	 (new-hd (ring-minus1 (car ring) len)))
      (setcar ring new-hd)
      (aset vec new-hd item)
      (if (ring-empty-p ring) ;overflow -- dump one off the tail.
	  (setcar (cdr ring) (ring-minus1 (car (cdr ring)) len)))))

(defun ring-remove (ring)
  "Remove the oldest item retained on the ring."
  (if (ring-empty-p ring) (error "Ring empty")
      (let ((tl (car (cdr ring)))  (vec (cdr (cdr ring))))
	(set-car (cdr ring) (ring-minus1 tl (length vec)))
	(aref vec tl))))

;;; This isn't actually used in this package. I just threw it in in case
;;; someone else wanted it. If you want rotating-ring behavior on your history
;;; retrieval (analagous to kill ring behavior), this function is what you
;;; need. I should write the yank-input and yank-pop-input-or-kill to go with
;;; this, and not bind it to a key by default, so it would be available to
;;; people who want to bind it to a key. But who would want it? Blech.
(defun ring-rotate (ring n)
  (if (not (= n 0))
      (if (ring-empty-p ring) ;Is this the right error check?
	  (error "ring empty")
	  (let ((hd (car ring))  (tl (car (cdr ring)))  (vec (cdr (cdr ring))))
	    (let ((len (length vec)))
	      (while (> n 0)
		(setq tl (ring-plus1 tl len))
		(aset ring tl (aref ring hd))
		(setq hd (ring-plus1 hd len))
		(setq n (- n 1)))
	      (while (< n 0)
		(setq hd (ring-minus1 hd len))
		(aset vec hd (aref vec tl))
		(setq tl (ring-minus1 tl len))
		(setq n (- n 1))))
	    (set-car ring hd)
	    (set-car (cdr ring) tl)))))

(defun comint-mod (n m)
  "Returns N mod M.  M is positive.
Answer is guaranteed to be non-negative, and less than m."
  (let ((n (% n m)))
    (if (>= n 0) n
	(+ n
	   (if (>= m 0) m (- m)))))) ; (abs m)

(defun ring-ref (ring index)
  (let ((numelts (ring-length ring)))
    (if (= numelts 0) (error "indexed empty ring")
	(let* ((hd (car ring))  (tl (car (cdr ring)))  (vec (cdr (cdr ring)))
	       (index (comint-mod index numelts))
	       (vec-index (comint-mod (+ index hd) 
				      (length vec))))
	  (aref vec vec-index)))))

;;; ring.el ends here