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- ;#lang planet neil/sicp
- #lang racket
- (require (planet soegaard/sicp:2:1/sicp))
- (define wave einstein)
- (define (make-leaf symbol weight)
- (list 'leaf symbol weight))
- (define (leaf? obj)
- (eq? (car obj) '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))
- (caddr tree)))
- (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))
- (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 (error "bad bit -- CHOOSE-BRANCH" bit))))
- (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))))))
- (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 sample-symbol (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)
- (define (encode-symbol-1 symbol tree)
- (cond ((leaf? tree) '())
- ((container? symbol (symbols (left-branch tree)))
- (cons '0 (encode-symbol symbol (left-branch tree))))
- (else (cons '1 (encode-symbol symbol (right-branch tree))))))
- (if (container? symbol (symbols tree))
- (encode-symbol-1 symbol tree)
- (error "bad tree -- CONTAINER? symbol" symbol)))
- (define (container? symbol list-of-symbol)
- (cond ((null? list-of-symbol) false)
- ((eq? symbol (car list-of-symbol)) true)
- (else (container? symbol (cdr list-of-symbol)))))
- (encode sample-symbol sample-tree)
- (define (generate-huffman-tree pairs)
- (successive-merge (make-leaf-set pairs)))
- (define (successive-merge order-set)
- (define (successive-merge-1 x order-set)
- (if (null? order-set)
- x
- (successive-merge (adjoin-set (make-code-tree x (car order-set))
- (cdr order-set)))))
- (successive-merge-1 (car order-set) (cdr order-set)))
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