Daniel-Nikpayuk_SICP-Solutions/Chapter 3/3.3/exercise-3.33 (averager)/exercise.lsp
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;; Copyright 2015 Daniel Nikpayuk
;;
;; This file is part of SICP Solutions.
;;
;; SICP Solutions is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License
;; as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.
;;
;; SICP Solutions is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty
;; of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
;;
;; You should have received a copy of the GNU General Public License along with SICP Solutions. If not, see
;; <http://www.gnu.org/licenses/>.
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(define (adder a1 a2 sum)
  (define (process-new-value)
    (cond ((and (has-value? a1) (has-value? a2))
           (set-value! sum
                       (+ (get-value a1) (get-value a2))
                       me))
          ((and (has-value? a1) (has-value? sum))
           (set-value! a2
                       (- (get-value sum) (get-value a1))
                       me))
          ((and (has-value? a2) (has-value? sum))
           (set-value! a1
                       (- (get-value sum) (get-value a2))
                       me))))
  (define (process-forget-value)
    (forget-value! sum me)
    (forget-value! a1 me)
    (forget-value! a2 me)
    (process-new-value))
  (define (me request)
    (cond ((eq? request 'I-have-a-value)  
           (process-new-value))
          ((eq? request 'I-lost-my-value) 
           (process-forget-value))
          (else 
           (error "Unknown request -- ADDER" request))))
  (connect a1 me)
  (connect a2 me)
  (connect sum me)
  me)

(define (multiplier m1 m2 product)
  (define (process-new-value)
    (cond ((or (and (has-value? m1) (= (get-value m1) 0))
               (and (has-value? m2) (= (get-value m2) 0)))
           (set-value! product 0 me))
          ((and (has-value? m1) (has-value? m2))
           (set-value! product
                       (* (get-value m1) (get-value m2))
                       me))
          ((and (has-value? product) (has-value? m1))
           (set-value! m2
                       (/ (get-value product) (get-value m1))
                       me))
          ((and (has-value? product) (has-value? m2))
           (set-value! m1
                       (/ (get-value product) (get-value m2))
                       me))))
  (define (process-forget-value)
    (forget-value! product me)
    (forget-value! m1 me)
    (forget-value! m2 me)
    (process-new-value))
  (define (me request)
    (cond ((eq? request 'I-have-a-value)
           (process-new-value))
          ((eq? request 'I-lost-my-value)
           (process-forget-value))
          (else
           (error "Unknown request -- MULTIPLIER" request))))
  (connect m1 me)
  (connect m2 me)
  (connect product me)
  me)

(define (constant value connector)
  (define (me request)
    (error "Unknown request -- CONSTANT" request))
  (connect connector me)
  (set-value! connector value me)
  me)

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

(define (inform-about-value constraint)
  (constraint 'I-have-a-value))
(define (inform-about-no-value constraint)
  (constraint 'I-lost-my-value))

(define (probe name connector)
  (define (print-probe value)
    (newline)
    (display "Probe: ")
    (display name)
    (display " = ")
    (display value))
  (define (process-new-value)
    (print-probe (get-value connector)))
  (define (process-forget-value)
    (print-probe "?"))
  (define (me request)
    (cond ((eq? request 'I-have-a-value)
           (process-new-value))
          ((eq? request 'I-lost-my-value)
           (process-forget-value))
          (else
           (error "Unknown request -- PROBE" request))))
  (connect connector me)
  me)

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

(define (for-each-except exception procedure list)
  (define (loop items)
    (cond ((null? items) 'done)
          ((eq? (car items) exception) (loop (cdr items)))
          (else (procedure (car items))
                (loop (cdr items)))))
  (loop list))

(define (make-connector)
  (let ((value false) (informant false) (constraints '()))
    (define (set-my-value newval setter)
      (cond ((not (has-value? me))
             (set! value newval)
             (set! informant setter)
             (for-each-except setter
                              inform-about-value
                              constraints))
            ((not (= value newval))
             (error "Contradiction" (list value newval)))
            (else 'ignored)))
    (define (forget-my-value retractor)
      (if (eq? retractor informant)
          (begin (set! informant false)
                 (for-each-except retractor
                                  inform-about-no-value
                                  constraints))
          'ignored))
    (define (connect new-constraint)
      (if (not (memq new-constraint constraints))
          (set! constraints 
                (cons new-constraint constraints)))
      (if (has-value? me)
          (inform-about-value new-constraint))
      'done)
    (define (me request)
      (cond ((eq? request 'has-value?)
             (if informant true false))
            ((eq? request 'value) value)
            ((eq? request 'set-value!) set-my-value)
            ((eq? request 'forget) forget-my-value)
            ((eq? request 'connect) connect)
            (else (error "Unknown operation -- CONNECTOR"
                         request))))
    me))

(define (has-value? connector)
  (connector 'has-value?))
(define (get-value connector)
  (connector 'value))
(define (set-value! connector new-value informant)
  ((connector 'set-value!) new-value informant))
(define (forget-value! connector retractor)
  ((connector 'forget) retractor))
(define (connect connector new-constraint)
  ((connector 'connect) new-constraint))

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;; Just for practices sake:
(define (averager a1 a2 average)
 (define (process-new-value)
  (cond
   ((and (has-value? a1) (has-value? a2))
    (set-value! average (/ (+ (get-value a1) (get-value a2)) 2.0) me)
   )
   ((and (has-value? a1) (has-value? average))
    (set-value! a2 (- (* 2.0 (get-value average)) (get-value a1)) me)
   )
   ((and (has-value? a2) (has-value? average))
    (set-value! a1 (- (* 2.0 (get-value average)) (get-value a2)) me)
   )
  )
 )
 (define (process-forget-value)
  (forget-value! average me)
  (forget-value! a1 me)
  (forget-value! a2 me)
  (process-new-value)
 )
 (define (me request)
  (cond
   ((eq? request 'I-have-a-value) (process-new-value))
   ((eq? request 'I-lost-my-value) (process-forget-value))
   (else (error "Unknown request -- AVERAGER" request))
  )
 )
 (connect a1 me)
 (connect a2 me)
 (connect average me)
 me
)

(define (averager a1 a2 average)
 (define (process-new-value)
  (cond
   ((and (has-value? a1) (has-value? a2))
    (set-value! average (/ (+ (get-value a1) (get-value a2)) 2.0) me)
   )
   ((and (has-value? a1) (has-value? average))
    (set-value! a2 (- (* 2.0 (get-value average)) (get-value a1)) me)
   )
   ((and (has-value? a2) (has-value? average))
    (set-value! a1 (- (* 2.0 (get-value average)) (get-value a2)) me)
   )
  )
 )
 (define (process-forget-value)
  (forget-value! average me)
  (forget-value! a1 me)
  (forget-value! a2 me)
  (process-new-value)
 )
 (define (me request)
  (cond
   ((eq? request 'I-have-a-value) (process-new-value))
   ((eq? request 'I-lost-my-value) (process-forget-value))
   (else (error "Unknown request -- AVERAGER" request))
  )
 )
 (connect a1 me)
 (connect a2 me)
 (connect average me)
 me
)

(define (averager a b c)
 (let
  (
   (d (make-connector))
   (e (make-connector))
  )
  (constant 0.5 d)
  (adder a b e)
  (multiplier d e c)
 )
)

