scheme实现huffman编码的完整代码

    来自sicp的完整代码，包括书中给出的代码以及习题，实现了huffman树的生成、解码、编码过程，总共67行代码，同样的代码有空用java、ruby改写下，看看会有什么不同。

ruby 代码

(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))
;合并最低权重的两个节点
(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)))
;解码
(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 (choose-branch bit branch)
  (cond ((= bit 0) (left-branch branch))
        ((= bit 1) (right-branch branch))
        (else (display "bad bit --CHOOSE-BRANCH"))))
(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-leaf-set pairs)
  (if (null? pairs)
      '()
      (let ((pair (car pairs)))
        (adjoin-set (make-leaf (car pair) (cadr pair)) (make-leaf-set (cdr pairs))))))

;编码
(define (encode message tree)
  (if (null? message)
      '()
      (append (encode-symbol (car message) tree)
              (encode (cdr message) tree))))
(define (encode-symbol symbol tree)
  (define (iter branch)
    (if (leaf? branch)
        '()
        (if (memq symbol (symbols (left-branch branch)))
            (cons 0 (iter (left-branch branch)))
            (cons 1 (iter (right-branch branch))))
        ))
  (if (memq symbol (symbols tree))
      (iter tree)
      (display "bad symbol -- UNKNOWN SYMBOL")))
;根据符号权重列表，生成hufman树
(define (generate-huffman-tree pairs)
  (successive-merge (make-leaf-set pairs)))

(define (successive-merge leaf-set)
  (if (null? (cdr leaf-set))
      (car leaf-set)
      (successive-merge (adjoin-set (make-code-tree (car leaf-set)
                                                    (cadr leaf-set))
                                    (cddr leaf-set)))))