(module chinese-zodiac mzscheme
  (require (lib "etc.ss"))
  
  ;; ranking->animal: number -> symbol
  ;; Given a ranking from 0-11, returns the corresponding animal.
  (define (ranking->animal ranking)
    (cond
      [(= ranking 0) 'Rat]
      [(= ranking 1) 'Ox]
      [(= ranking 2) 'Tiger]
      [(= ranking 3) 'Rabbit]
      [(= ranking 4) 'Dragon]
      [(= ranking 5) 'Snake]
      [(= ranking 6) 'Horse]
      [(= ranking 7) 'Goat]
      [(= ranking 8) 'Monkey]
      [(= ranking 9) 'Rooster]
      [(= ranking 10) 'Dog]
      [(= ranking 11) 'Pig]
      [else (error 'ranking->animal "Illegal ranking ~a" ranking)]))

  
  ;; animal->ranking: symbol -> number
  ;; Given an animal, returns the corresponding rank.
  (define (animal->ranking an-animal)
    (define (loop i)
      (cond 
        [(symbol=? an-animal (ranking->animal i)) i]
        [else (loop (add1 i))]))
    (loop 0))
    
  ;; emmity: symbol -> symbol
  ;; Given an-animal, returns the name of its greatest enemy.
  (define (emmity an-animal)
    (ranking->animal (modulo (+ 6 (animal->ranking an-animal)) 12)))
    
  
  ;; An affinity-pair is a structure
  ;; (make-affinity-pair a1 a2)
  ;; where a1 and a2 are animals.
  (define-struct affinity-pair (first-animal second-animal) #f)
  
  
  ;; affinities: symbol -> affinity-pair
  ;; Returns an affinity-pair of the animals with the strongest affinity
  ;; to an-animal.
  (define (affinities an-animal)
    (make-affinity-pair
     (ranking->animal
      (modulo (+ (animal->ranking an-animal) 4) 12))
     (ranking->animal
      (modulo (- (animal->ranking an-animal) 4) 12))))
     
    
  
  )