;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; A Simulator for Digital Circuits
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

(define (half-adder a b s c)
  (let ((d (make-wire)) (e (make-wire)))
    (or-gate a b d)
    (and-gate a b c)
    (inverter c e)
    (and-gate d e s)
    'ok))

(define (full-adder a b c-in sum c-out)
  (let ((s (make-wire))
        (c1 (make-wire))
        (c2 (make-wire)))
    (half-adder b c-in s c1)
    (half-adder a s sum c2)
    (or-gate c1 c2 c-out)
    'ok))

(define (inverter input output)
  (define (invert-input)
    (let ((new-value (logical-not (get-signal input))))
      (after-delay inverter-delay
                   (lambda ()
                     (set-signal! output new-value)))))
  (add-action! input invert-input)
  'ok)

(define (logical-not s)
  (cond ((= s 0) 1)
        ((= s 1) 0)
        (else (error "Invalid signal" s))))

(define (and-gate a1 a2 output)
  (define (and-action-procedure)
    (let ((new-value
           (logical-and (get-signal a1) (get-signal a2))))
      (after-delay and-gate-delay
                   (lambda ()
                     (set-signal! output new-value)))))
  (add-action! a1 and-action-procedure)
  (add-action! a2 and-action-procedure)
  'ok)

(define (logical-and a b)
  (cond ((and (= a 0) (= b 0)) 0)
	((and (= a 1) (= b 0)) 0)
	((and (= a 0) (= b 1)) 0)
	((and (= a 1) (= b 1)) 1)
        (else (error "Invalid signals" a b))))

(define (make-wire)
  (let ((signal-value 0) (action-procedures '()))
    (define (set-my-signal! new-value)
      (if (not (= signal-value new-value))
          (begin (set! signal-value new-value)
                 (call-each action-procedures))
          'done))
    (define (accept-action-procedure! proc)
      (set! action-procedures (cons proc action-procedures))
      (proc))
    (define (dispatch m)
      (cond ((eq? m 'get-signal) signal-value)
            ((eq? m 'set-signal!) set-my-signal!)
            ((eq? m 'add-action!) accept-action-procedure!)
            (else (error "Unknown operation -- WIRE" m))))
    dispatch))

(define (call-each procedures)
  (if (null? procedures)
      'done
      (begin
        ((car procedures))
        (call-each (cdr procedures)))))

(define (get-signal wire)
  (wire 'get-signal))
(define (set-signal! wire new-value)
  ((wire 'set-signal!) new-value))
(define (add-action! wire action-procedure)
  ((wire 'add-action!) action-procedure))

(define (after-delay delay action)
  (add-to-agenda! (+ delay (current-time the-agenda))
                  action
                  the-agenda))

(define (propagate)
  (if (empty-agenda? the-agenda)
      'done
      (let ((first-item (first-agenda-item the-agenda)))
        (first-item)
        (remove-first-agenda-item! the-agenda)
        (propagate))))

(define (probe name wire)
  (add-action! wire
               (lambda ()        
                 (newline)
                 (display name)
                 (display " ")
                 (display (current-time the-agenda))
                 (display "  New-value = ")
                 (display (get-signal wire)))))

(define inverter-delay 2)
(define and-gate-delay 3)
(define or-gate-delay 5)

(define (make-time-segment time queue)
  (cons time queue))
(define (segment-time s) (car s))
(define (segment-queue s) (cdr s))

(define (make-agenda) (list 0))
(define (current-time agenda) (car agenda))
(define (set-current-time! agenda time)
  (set-car! agenda time))
(define (segments agenda) (cdr agenda))
(define (set-segments! agenda segments)
  (set-cdr! agenda segments))
(define (first-segment agenda) (car (segments agenda)))
(define (rest-segments agenda) (cdr (segments agenda)))

(define (empty-agenda? agenda)
  (null? (segments agenda)))

(define (add-to-agenda! time action agenda)
  (define (belongs-before? segments)
    (or (null? segments)
        (< time (segment-time (car segments)))))
  (define (make-new-time-segment time action)
    (let ((q (make-queue)))
      (insert-queue! q action)
      (make-time-segment time q)))
  (define (add-to-segments! segments)
    (if (= (segment-time (car segments)) time)
        (insert-queue! (segment-queue (car segments))
                       action)
        (let ((rest (cdr segments)))
          (if (belongs-before? rest)
              (set-cdr!
               segments
               (cons (make-new-time-segment time action)
                     (cdr segments)))
              (add-to-segments! rest)))))
  (let ((segments (segments agenda)))
    (if (belongs-before? segments)
        (set-segments!
         agenda
         (cons (make-new-time-segment time action)
               segments))
        (add-to-segments! segments))))

(define (remove-first-agenda-item! agenda)
  (let ((q (segment-queue (first-segment agenda))))
    (delete-queue! q)
    (if (empty-queue? q)
        (set-segments! agenda (rest-segments agenda)))))

(define (first-agenda-item agenda)
  (if (empty-agenda? agenda)
      (error "Agenda is empty -- FIRST-AGENDA-ITEM")
      (let ((first-seg (first-segment agenda)))
        (set-current-time! agenda (segment-time first-seg))
        (front-queue (segment-queue first-seg)))))

(define the-agenda (make-agenda))

;------------------------------------------------------------------------------

(define (rev-bitify n c)
  (if (= c 0)
      nil
      (cons (remainder n 2) (rev-bitify (floor (/ n 2)) (- c 1)))))

(define word-bits 32)

(define (word-bitify n)
  (reverse (rev-bitify n word-bits)))

(define (unbitify l n)
  (if (null? l)
      0
      (+ (* (car l) (expt 2 (- n 1)))
	 (unbitify (cdr l) (dec n)))))

(define (word-unbitify l)
  (unbitify l word-bits))

(define (make-wire-list n)
  (if (= n 0) nil (cons (make-wire) (make-wire-list (dec n)))))

(define (ripple-carry-adder-circuit)
  (define addend (make-wire-list word-bits))
  (define augend (make-wire-list word-bits))  
  
  (define sum (make-wire-list word-bits))
  
  (define carry (make-wire))

  (ripple-carry-adder addend augend sum carry)

  (lambda (a b)
    (map (lambda (x y) (set-signal! x y)) addend (word-bitify a)) 
    (map (lambda (x y) (set-signal! x y)) augend (word-bitify b))
    (propagate)
    (word-unbitify (map (lambda (x) (get-signal x)) sum))))

(define mega-sum (ripple-carry-adder-circuit))

(debug "\n")
(assert '(mega-sum 12345678 87654321) 99999999)
(assert '(mega-sum 111111111 222222222) 333333333)
(assert '(mega-sum 256 (mega-sum 1024 768)) 2048)