; -*- scheme -*-

(define (create kind energy) (list energy kind))
(define kind cadr)
(define energy car)
(define set-energy! set-car!)
(define (check! o) (if (< (energy o) 0) (error (kind o) "is invalid")))

(define (is-meat f) (member f (list 'Beef 'Dead-rabbit 'Dead-human)))

(define (invalidate! o) (set-energy! o -1))
(define invalidate-animal! invalidate!)
(define (invalidate-food! o) 
  (if (is-meat (kind o)) 
      (set-energy! o -1)))

(define (can-eat a f)
  (or (and (equal? a 'Cow)    (equal? f 'Grass))
      (and (equal? a 'Rabbit) (equal? f 'Carrot))
      (and (equal? a 'Human)  (or (equal? f 'Carrot) (is-meat f)))))

(define (eat animal food)
  (check! animal)
  (check! food)
  (if (can-eat (kind animal) (kind food))
      (begin
	(set-energy! animal (+ (energy animal) (energy food)))
	(invalidate-food! food))
      (error (kind animal) "can't eat" (kind food))))

(define (slaughter animal)
  (check! animal)
  (let* ((a (kind animal))
	 (e (energy animal))
	 (new-kind (cond ((equal? a 'Cow) 'Beef)
			 ((equal? a 'Rabbit) 'Dead-rabbit)
			 ((equal? a 'Human) 'Dead-human)
			 (else (error "can't slaughter" (kind animal))))))
    (invalidate-animal! animal)
    (create new-kind e)))

(define grass (create 'Grass 5))
(define carrot (create 'Carrot 10))

(define a_rabbit (create 'Rabbit 100))
(define a_cow (create 'Cow 1000))
(define a_human (create 'Human 300))
(define another_human (create 'Human 350))

(for-each (lambda (o)
   (display (kind o))
   (display " -> ")
   (display (energy o))) (list a_rabbit a_cow a_human))

(eat a_rabbit carrot)
(eat a_cow grass)

(define a_dead_rabbit (slaughter a_rabbit))
(define a_beef (slaughter a_cow))

(eat a_human carrot)
(eat a_human carrot)
(eat a_human a_beef)
(eat a_human a_dead_rabbit)

(eat a_human (slaughter another_human))

(if (= (energy a_human) 1785) '() (error "failed"))


;(eat (slaughter (create 'Cow 10)) grass) ; meat (food) can't eat
;(slaughter (slaughter (create 'Cow 10))) ; meat (food) can't be slaughtered
;(eat carrot grass)      ; vegetable (food) can't eat
;(slaughter carrot)     ; vegetable (food) can't be slaughtered

;(eat (create 'Cow 10) carrot) ; cow do not eat carrot
;(eat (create 'Cow 10) (slaughter (create 'Cow 10))) ; cow do not eat beef
;(eat a_human (create 'Cow 10)) ; can't eat live animals
;(eat a_human grass)     ; human do not eat grass
;(eat a_human a_beef)    ; a_beef is already eaten
;(eat a_cow grass)       ; a_cow is dead, it can't eat
;(slaughter a_cow)      ; a_cow is dead, it can't be slaughtered again