solution to 2.68

diff --git a/src/sicp/ex2_68.rkt b/src/sicp/ex2_68.rkt
new file mode 100644 (file)
index 0000000..0606774
--- /dev/null
+++ b/src/sicp/ex2_68.rkt
@@ -0,0 +1,43 @@
+#lang racket
+
+(require "ch2_3.rkt"
+         rackunit)
+
+(define sample-tree
+  (make-code-tree (make-leaf 'A 4)
+                  (make-code-tree
+                   (make-leaf 'B 2)
+                   (make-code-tree (make-leaf 'D 1)
+                                   (make-leaf 'C 1)))))
+
+(define sample-bits '(0 1 1 0 0 1 0 1 0 1 1 1 0))
+
+(define sample-message (decode sample-bits sample-tree))
+
+(define (encode message tree)
+  (if (null? message)
+      '()
+      (append (encode-symbol (car message) tree)
+              (encode (cdr message) tree))))
+
+(define (encode-symbol symbol tree)
+  (cond 
+    [(and (leaf? tree) 
+          (eqv? (symbol-leaf tree) symbol)) '()]
+    [(member? symbol (symbols (left-branch tree))) (cons 0 (encode-symbol symbol (left-branch tree)))]
+    [(member? symbol (symbols (right-branch tree))) (cons 1 (encode-symbol symbol (right-branch tree)))]
+    [(not (member? symbol (symbols tree))) (error "Symbol not in the tree -- ENCODE-SYMBOL " symbol)]))
+
+(define (member? x set)
+  (cond
+    [(null? set) #f]
+    [(eqv? x (car set)) #t]
+    [else (member? x (cdr set))]))
+
+;;; a simple test. encode-decode should give me back the same symbols.
+(check equal? 
+       (decode (encode sample-message 
+                       sample-tree) 
+               sample-tree) 
+       sample-message)
+  
\ No newline at end of file