関数型言語の勉強にSICPを読もう - (23) 2章 - データによる抽象の構築 - 2.3.4 (94-98ページ)

2.3.4 Huffman符号化木の話

Huffman符号、なんとも分かりやすい説明で感動。

これからは会話の随所に「Huffman符号がさ。」と散りばめよう。(嘘
SICPの該当箇所を伴侶の人に見せて読んでもらったら理解できたらしい。
IT系が苦手なのに恐るべし。

Huffman木をノートで書いてみる。ふむふむ。
おもしろいな。木ができる様子を視覚的に見せられたら良いんだけれどまぁ良いか。

```(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))

(define (weight tree)
(if (leaf? tree)
(weight-leaf 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))))```

コードの入力ばかりですが一応きちんと理解できているつもり。

問題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)
gosh> (A D A B B C A)```

※「SICPを読もう」の目次はこちら

Gerald Jay Sussman Julie Sussman Harold Abelson 和田 英一
ピアソンエデュケーション (2000/02)