ハフマン符号。面倒だなーって思ってたら前やった奴みつけたからそれを流用。
前準備
(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 (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) ; symbol
(cadr pair)) ; frequency
(make-leaf-set (cdr pairs))))))
2.67
(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))
(decode sample-message sample-tree)
; => (A D A B B C A)
2.68
(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)
(if (eq? symbol (symbol-leaf tree))
'() #f)
(let ((left-symbols (encode-symbol symbol (left-branch tree))))
(if left-symbols
(cons 0 left-symbols)
(let ((right-symbols (encode-symbol symbol (right-branch tree))))
(if right-symbols
(cons 1 right-symbols)
#f)
)))))
2.69
(define (generate-huffman-tree pairs)
(successive-merge (make-leaf-set pairs)))
(define (successive-merge leaf-set)
(if (= (length leaf-set) 1)
(car leaf-set)
(let ((merged-tree (make-code-tree (car leaf-set) (cadr leaf-set)))
(rest (cddr leaf-set)))
(successive-merge
(cond
((eq? rest '()) (list merged-tree))
((< (weight merged-tree) (weight (car rest))) (append (list merged-tree) rest))
(else (append (list (car rest)) (list merged-tree) (cdr rest))))
))))
2.70
(define rock-symbols '((NA 16) (YIP 9) (SHA 3) (A 2) (GET 2) (JOB 2) (BOOM 1) (WAH 1))) (define rock-tree (generate-huffman-tree rock-symbols)) (encode '(GET A JOB) rock-tree) ; => (1 1 1 1 0 1 1 1 0 1 1 1 1 1 0) (encode '(SHA NA NA NA NA NA NA NA NA) rock-tree) ; => (1 1 0 0 0 0 0 0 0 0 0)
2.71
(encode '(A B C D E) (generate-huffman-tree '((A 16) (B 8) (C 4) (D 2) (E 1)))) ; => (1 0 1 0 0 1 0 0 0 1 0 0 0 0) (encode '(A B C D E F G H I J) (generate-huffman-tree '((A 512) (B 256) (C 128) (D 64) (E 32) (F 16) (G 8) (H 4) (I 2) (J 1)))) ; => (1 0 1 0 0 1 0 0 0 1 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0)
