Posted on 2007-07-23 08:56
dennis 阅读(966)
评论(0) 编辑 收藏 所属分类:
数据结构与算法 、
计算机科学与基础
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。
(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")))
;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)))))