;;; Copyright (C) Peter McGoron 2024
;;; 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, version 3 of the License.
;;; 
;;; 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, see <https://www.gnu.org/licenses/>.

;;; Singly linked list with tail pointer, message passing style.
;;; 
;;; LINKED-LIST:
;;; 
;;; (PUSH! VAL): Pushes a value to the head of the list.
;;; (PUSH-TAIL! VAL): Pushes a value to the end of the list.
;;; (SET-CDR! VAL): Set the cdr of TAIL to VAL. This will be overrwitten
;;;    if PUSH-TAIL! is called again.
;;; (TO-LIST): Returns a list structure.

(define linked-list:new
  (lambda ()
    (letrec
        ((head '())
         (tail '())
         (this
          (object/abstract
           'push!
           (lambda (val)
             (set! head (cons val head))
             (if (null? tail)
                 (set! tail head))
             this)
           'push-tail!
           (lambda (val)
             (if (null? tail)
                 (this 'push! val)
                 (begin
                   (set-cdr! tail (cons val '()))
                   (set! tail (cdr tail))))
             this)
           'set-cdr!
           (lambda (val)
             (if (null? tail)
                 (error "cannot set cdr of empty list")
                 (set-cdr! tail val))
             this)
           'to-list
           (lambda () head))))
      this)))