66 lines
No EOL
2 KiB
Scheme
66 lines
No EOL
2 KiB
Scheme
#lang sicp
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(define (make-leaf symbol weight) (list 'leaf symbol weight))
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(define (leaf? object) (eq? (car object) 'leaf))
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(define (symbol-leaf x) (cadr x))
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(define (weight-leaf x) (caddr x))
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(define (make-code-tree left right)
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(list left
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right
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(append (symbols left) (symbols right))
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(+ (weight left) (weight right))))
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(define (left-branch tree) (car tree))
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(define (right-branch tree) (cadr tree))
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(define (symbols tree)
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(if (leaf? tree)
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(list (symbol-leaf tree))
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(caddr tree)))
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(define (weight tree)
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(if (leaf? tree)
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(weight-leaf tree)
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(cadddr tree)))
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(define (decode bits tree)
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(define (decode-1 bits current-branch)
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(if (null? bits)
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'()
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(let ((next-branch
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(choose-branch (car bits) current-branch)))
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(if (leaf? next-branch)
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(cons (symbol-leaf next-branch)
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(decode-1 (cdr bits) tree))
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(decode-1 (cdr bits) next-branch)))))
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(decode-1 bits tree))
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(define (choose-branch bit branch)
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(cond ((= bit 0) (left-branch branch))
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((= bit 1) (right-branch branch))
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(else (error "bad bit: CHOOSE-BRANCH" bit))))
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(define (adjoin-set x set)
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(cond ((null? set) (list x))
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((< (weight x) (weight (car set))) (cons x set))
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(else (cons (car set)
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(adjoin-set x (cdr set))))))
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(define (make-leaf-set pairs)
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(if (null? pairs)
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'()
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(let ((pair (car pairs)))
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(adjoin-set (make-leaf (car pair) ; symbol
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(cadr pair)) ; frequency
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(make-leaf-set (cdr pairs))))))
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; actual exercise
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(define (generate-huffman-tree pairs)
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(successive-merge (make-leaf-set pairs)))
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(define (successive-merge tree-set)
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(if (null? (cdr tree-set)) ; (= (length tree-list) 1)
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(car tree-set)
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(let ((tree1 (car tree-set))
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(tree2 (cadr tree-set)))
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(successive-merge (adjoin-set (make-code-tree tree1 tree2)
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(cddr tree-set)))))) |