1 (define (test-case actual expected)
2   (newline)
3   (display "Actual:   ")
4   (display actual)
5   (newline)
6   (display "Expected: ")
7   (display expected)
8   (newline))
9 
10 (define (make-leaf symbol weight)
11   (list 'leaf symbol weight))
12 (define (leaf? object)
13   (eq? (car object) 'leaf))
14 (define (symbol-leaf x) (cadr x))
15 (define (weight-leaf x) (caddr x))
16 
17 (define (make-code-tree left right)
18   (list left
19 	right
20 	(append (symbols left) (symbols right))
21 	(+ (weight left) (weight right))))
22 
23 (define (left-branch tree) (car tree))
24 (define (right-branch tree) (cadr tree))
25 (define (symbols tree)
26   (if (leaf? tree)
27       (list (symbol-leaf tree))
28       (caddr tree)))
29 (define (weight tree)
30   (if (leaf? tree)
31       (weight-leaf tree)
32       (cadddr tree)))
33 
34 (define (decode bits tree)
35   (define (decode-1 bits current-branch)
36     (if (null? bits)
37 	'()
38 	(let ((next-branch
39 	       (choose-branch (car bits) current-branch)))
40 	  (if (leaf? next-branch)
41 	      (cons (symbol-leaf next-branch)
42 		    (decode-1 (cdr bits) tree))
43 	      (decode-1 (cdr bits) next-branch)))))
44   (decode-1 bits tree))
45 (define (choose-branch bit branch)
46   (cond ((= bit 0) (left-branch branch))
47 	((= bit 1) (right-branch branch))
48 	(else (error "bad bit -- CHOOSE-BRANCH" bit))))
49 
50 (define (adjoin-set x set)
51   (cond ((null? set) (list x))
52 	((< (weight x) (weight (car set))) (cons x set))
53 	(else (cons (car set)
54 		    (adjoin-set x (cdr set))))))
55 (define (make-leaf-set pairs)
56   (if (null? pairs)
57       '()
58       (let ((pair (car pairs)))
59 	(adjoin-set (make-leaf (car pair)
60 			       (cadr pair))
61 		    (make-leaf-set (cdr pairs))))))
62 
63 ;; Exercise 2.67.  Define an encoding tree and a sample message:
64 
65 (define sample-tree
66   (make-code-tree (make-leaf 'A 4)
67                   (make-code-tree
68                    (make-leaf 'B 2)
69                    (make-code-tree (make-leaf 'D 1)
70                                    (make-leaf 'C 1)))))
71 
72 (define sample-message '(0 1 1 0 0 1 0 1 0 1 1 1 0))
73 (test-case (decode sample-message sample-tree) '(A D A B B C A))