(sicp26)m2.67〜m2.69

2.68でmemqを使ってしまった。
(後でまじめにparseする方法を考える)

(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 ((= 0 bit) (left-branch branch))
        ((= 1 bit) (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))))))


;;m 2.67
;;such as ((A 4) (B 2) (C 1) (D 1))
(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))
(decode sample-message sample-tree)
;;	33:user> => (A D A B B C A)

;;2.68
;;木でなければerrorを返す必要がある。
;;最後に2.67の通信文と同じかテストする。

(define (encode message tree)
  (if (null? message)
      '()
      (append (encode-symbol (car message) tree)
              (encode (cdr message) tree))))

;;木を自力でparseするのは面倒だなー
;;left-branch , right-branchがあることを忘れていた。
;;sybolsがあることを忘れていた。
(symbols (left-branch sample-tree))
(define (encode-symbol sym tree)
    (if (leaf? tree)
        (if (eq? sym (symbol-leaf tree))
            '()
            (error sym "is not defined"))
        (if (memq sym (left-branch tree))
            (cons 0 (encode-symbol sym (left-branch tree)))
            (cons 1 (encode-symbol sym (right-branch tree))))))

(encode '(A D A B B C A) sample-tree)
;;user> => (0 1 1 0 0 1 0 1 0 1 1 1 0)
(equal? (encode '(A D A B B C A) sample-tree)
     sample-message)
;;user> => #t
;;すごい時間がかかりました><

;;2.69
(define (generate-huffman-tree pairs)
  (successive-marge (make-leaf-set pairs)))

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

;;問題の出現頻度と同じもの.
(let ((sample-pairs
       (zip (symbols sample-tree) '(4 2 1 1))))
  (print sample-tree)
  (print (generate-huffman-tree sample-pairs)))
115:user> ((leaf A 4) ((leaf B 2) ((leaf D 1) (leaf C 1) (D C) 2) (B D C) 4) (A B D C) 8)
((leaf A 4) ((leaf B 2) ((leaf C 1) (leaf D 1) (C D) 2) (B C D) 4) (A B C D) 8)
=> #<undef>