;; This file contains excerpts from the textbook Concrete
;; Abstractions: An Introduction to Computer Science Using Scheme, by
;; Max Hailperin, Barbara Kaiser, and Karl Knight, Copyright (c) 1998
;; by the authors. Full text is available for free at
;; http://www.gustavus.edu/+max/concrete-abstractions.html

;; Chapter 14: Object-Oriented Programming

;; 14.5  An Application: Adventures in the Imaginary Land of Gack

(define-class
  'auto-person
  person-class
  '(threshold
    restlessness)
  '(maybe-act
    act))

(class/set-method!
 auto-person-class 'init
 (lambda (this name place threshold)
   (person^init this name place)
   (auto-person/set-threshold! this threshold)
   (auto-person/set-restlessness! this 0)
   (registry/add registry this)))

(class/set-method!
 auto-person-class 'maybe-act
 (lambda (this)
   (let ((threshold (auto-person/get-threshold this))
         (restlessness (auto-person/get-restlessness this)))
      (if (< restlessness threshold)
          (auto-person/set-restlessness! this
                                         (+ 1 restlessness))
          (begin (auto-person/act this)
                 (auto-person/set-restlessness! this 0))))))

(class/set-method!
 auto-person-class 'act
 (lambda (this)
   (let ((new-place (random-element
                     (place/neighbors
                      (auto-person/place this)))))
     (if new-place
         (auto-person/move-to this new-place)))))