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;; Example 2.68
(define (make-leaf symbol weight)
(list 'leaf symbol weight))
(define (leaf? leaf)
(eq? 'leaf (car leaf)))
(define (symbol-leaf leaf) (cadr leaf))
(define (weight-leaf leaf) (caddr leaf))
(define (make-code-tree left right)
(list left
right
(append (symbols left) (symbols right))
(+ (weight left) (weight right))))
(define (left-branch tree) (car tree))
(define (right-branch tree) (cadr tree))
(define (symbols tree)
(if (leaf? tree)
(list (symbol-leaf tree)) ;; leaf symbol
(caddr tree))) ;; symbols of the tree node (not leaf)
(define (weight tree)
(if (leaf? tree)
(weight-leaf tree)
(cadddr tree)))
(define (decode bits tree)
(define (decode-1 bits current-branch)
(if (null? bits)
'()
(let ((next-branch (choose-branch (car bits) current-branch)))
(if (leaf? next-branch)
(cons (symbol-leaf next-branch)
(decode-1 (cdr bits) tree)) ;; found symbol. Starting from the root again
(decode-1 (cdr bits) next-branch)))))
(decode-1 bits tree))
(define (choose-branch bit branch)
(cond ((= 0 bit) (left-branch branch))
((= 1 bit) (right-branch branch))
(else (error "Bad bit while choosing branch"))))
(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-message '(0 1 1 0 0 1 0 1 0 1 1 1 0))
(define decoded-msg (decode sample-message sample-tree))
(define (encode message tree)
(if (null? message)
'()
(append (encode-symbol (car message) tree)
(encode (cdr message) tree))))
(define (encode-symbol symbol tree)
(if (leaf? tree)
'()
(let ((lb (left-branch tree))
(rb (right-branch tree)))
(cond ((find-symbol symbol lb) (cons '0 (encode-symbol symbol lb)))
((find-symbol symbol rb) (cons '1 (encode-symbol symbol rb)))
(else (error "No such symbol in the Huffman tree"))))))
(define (find-symbol symbol branch)
(define (helper symbol syms)
(if (null? syms)
#f
(if (eq? symbol (car syms))
#t
(helper symbol (cdr syms)))))
(helper symbol (symbols branch)))
;; Start test
sample-message
(encode decoded-msg sample-tree)
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