SICPの2.3.4 Example: Huffman Encoding TreesあたりをPythonでやってみた
scheme
;2.3.4 Example: Huffman Encoding Trees ;Representing Huffman trees (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))) ;The decoding procedure (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)))) ;Sets of weighted elements (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))))))
python
#2.3.4 Example: Huffman Encoding Trees #Representing Huffman trees def make_leaf(symbol, weight): return ['leaf', symbol, weight] def leaf_q(object): return object == 'leaf' def symbol_leaf(x): return x[1] def weight_leaf(x): return x[2] def make_code_tree(left, right): return [left, right, symbols(left) + symbols(right), \ weight(left) + weight(right)] def left_branch(tree): return tree[0] def right_branch(tree): return tree[1] def symbols(tree): if leaf_q(tree): return [symbol_leaf(tree)] return tree[2] def weight(tree): if leaf_q(tree): return weight_leaf(tree) return tree[3] #The decoding procedure def decode_f(bits, tree): def decode_1(bits, current_branch): if not bits: return [] next_branch = choose-branch(bits[0], current_branch) if leaf_q(next_branch): return [symbol_leaf(next_branch), decode_1(bits[1:], tree)] return decode_1(bits[1:], next_branch) return decode_1(bits, tree) def choose_branch(bits, branch): if bit == 0: return left_branch(branch) if bit == 1: return right_branch(branch) else: print "error bad bit -- CHOOSE-BRANCCH", bit #Sets of weighted elements def adjoin_set(x, set): if not set: return [x] if weight(x) < weight(set[0]): return [x, set] else: return [set[0], adjoin_set(x, set[1:])] def make_leaf_set(pairs): if not pairs: return [] pair = pairs[0] return adjoin_set(make_leaf(pair[0], pair[1]), make_leaf_set(pairs[1:]))
いまひとつ理解できなかった(つдT)
練習問題やりなおしてみようかな...