recognizer.scm

; vim: set expandtab:

(declare (unit recognizer))
(module recognizer
        (advance-prime-count
          circle-incr 
          make-prime-counter
          recognizer )

(import
  (chicken base)
  scheme
  srfi-1)

(define-syntax advance-prime-count
  (syntax-rules ()
                ((_ lst)
                 (begin
                   (set-car! lst 0)
                   (set! lst (cdr lst))))))

;;; Increment each element of a circular list
(define (circle-incr lst)
  (do ((i 0 (+ 1 i)))
    ((= 4 i))
    (set! (list-ref lst i) (add1 (list-ref lst i)))))

(define (make-prime-counter)
  (circular-list -100 -100 -100 -100))

(define (recognizer lst)
  (let ((lst (take lst 4)))
    (cond
      ((list= eq? lst '(8 6 2 0))
       'quadruple)

      ((or (list= eq? (drop lst 1) '(6 4 0))
           (list= eq? (drop lst 1) '(6 2 0)))
       'triplet)

      ((list= eq? (drop lst 2) '(8 0))
       'octomus)

      ((list= eq? (drop lst 2) '(6 0))
       'sexy)

      ((list= eq? (drop lst 2) '(4 0))
       'cousin)

      ((list= eq? (drop lst 2) '(2 0))
       'twin)

      ; 59 is the 17th prime - this also marks a full
      ; minute without a prime number occurring
      ((> (list-ref lst 2) 59)
       'combo-breaker)

      (else
        #f))))

); module