sicp 2.68 2.69

sicp_2_68_2_69

(define (make-leaf symbol weight)
(list 'leaf symbol weight))
(define (leaf? object)
(eq? (car object) 'leaf))
(define (symbol-leaf x) (cadr x))
(define (weight-leaf x) (caddr x))

(define (adjoin-set x set)
(cond ((null? set) (list x))
((< (weight x) (weight (car set))) (cons x set))
(else (cons (car set)
(adjoin-set x (cdr set))))))

(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))
(caddr tree)))
(define (weight tree)
(if (leaf? tree)
(weight-leaf tree)
(cadddr tree)))

(define (choose-branch bit branch)
(cond ((= bit 0) (left-branch branch))
((= bit 1) (right-branch branch))
(else (error "ïëîõîé áèò -- CHOOSE-BRANCH" bit))))

(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))
(decode-1 (cdr bits) next-branch)))))
(decode-1 bits tree))

(define (member? x seq)
  (cond ((null? seq) #f)
        ((eq? (car seq) x) #t)
        (else (member? x (cdr seq)))))

(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) 
              (eq? symbol (symbol-leaf tree)))
         '())
        ((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))))
        (else (error "should not get here" symbol))))

(define (successive-merge leaf-set) 
   (if (= (length leaf-set) 1) 
       (car leaf-set) 
       (let ((first (car leaf-set)) 
             (second (cadr leaf-set)) 
             (rest (cddr leaf-set))) 
         (successive-merge (adjoin-set (make-code-tree first second) 
                                       rest))))) 
(define (make-leaf-set pairs)
(if (null? pairs)
'()
(let ((pair (car pairs)))
(adjoin-set (make-leaf (car pair)
(cadr pair))
(make-leaf-set (cdr pairs))))))

(define (generate-huffman-tree pairs)
  (successive-merge (make-leaf-set pairs)))

(encode '(A B C D) (generate-huffman-tree '((A 1)(B 2) (C 3) (D 4) (F 5))))