Add implementation of exercise 3-28

Author: ramsayleung

chapter3/exercise3-28.rkt

@@ -0,0 +1,41 @@
+#lang racket
+
+(require "digital-circuit.rkt")
+(require "after-delay.rkt")
+;;; The implementation of `or-gate` is in `digital-circuit.rkt`
+(module+ test
+  (require rackunit)
+
+
+  (test-case "Test for or-gate"
+             (define a1 (make-wire))
+             (define a2 (make-wire))
+             (define output (make-wire))
+
+             (or-gate a1 a2 output)
+
+             ;; 0 OR 0 = 0
+             (set-signal! a1 0)
+             (set-signal! a2 0)
+             (propagate)
+             (check-equal? (get-signal output)0)
+
+             ;; 0 OR 1 = 1
+             (set-signal! a1 0)
+             (set-signal! a2 1)
+             (propagate)
+             (check-equal? (get-signal output)1)
+
+             ;; 1 OR 0 = 1
+             (set-signal! a1 1)
+             (set-signal! a2 0)
+             (propagate)
+             (check-equal? (get-signal output) 1)
+
+             ;; 1 OR 1 = 1
+             (set-signal! a1 1)
+             (set-signal! a2 1)
+             (propagate)
+             (check-equal? (get-signal output )1))
+
+  )