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- ;;;;
- ;;;; Prekode til innlevering 2a i INF2810 (V17): Prosedyrer for å jobbe med
- ;;;; Huffman-trær, fra SICP, Seksjon 2.3.4.
- ;;;;
-
- ;;; Merk at koden under gjør bruk av diverse innebygde kortformer for
- ;;; kjeder av car og cdr. F.eks er (cadr x) det samme som (car (cdr x)),
- ;;; og (caadr x) tilsvarer (car (car (cdr x))), osv.
-
-
-
- ;;;
- ;;; Abstraksjonsbarriere:
- ;;;
-
- (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)))
-
- ;;;
- ;;; Dekoding:
- ;;;
-
- (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)
- (if (= bit 0)
- (left-branch branch)
- (right-branch branch)))
-
-
- ;;;
- ;;; Sortering av node-lister:
- ;;;
-
- (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)
- (cadr pair))
- (make-leaf-set (cdr pairs))))))
-
- ;;;
- ;;; Diverse test-data:
- ;;;
-
- (define sample-tree
- (make-code-tree
- (make-leaf 'ninjas 8)
- (make-code-tree
- (make-leaf 'fight 5)
- (make-code-tree
- (make-leaf 'night 1)
- (make-leaf 'by 1)))))
-
- (define sample-code '(0 1 0 0 1 1 1 1 1 0))
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