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;;;; "priorque.scm" priority queues for Scheme.
;;; Copyright (C) 1992, 1993, 1994, 1995, 1997 Aubrey Jaffer.
;
;Permission to copy this software, to redistribute it, and to use it
;for any purpose is granted, subject to the following restrictions and
;understandings.
;
;1. Any copy made of this software must include this copyright notice
;in full.
;
;2. I have made no warrantee or representation that the operation of
;this software will be error-free, and I am under no obligation to
;provide any services, by way of maintenance, update, or otherwise.
;
;3. In conjunction with products arising from the use of this
;material, there shall be no use of my name in any advertising,
;promotional, or sales literature without prior written consent in
;each case.
;;; Algorithm from:
;;; Introduction to Algorithms by T. Cormen, C. Leiserson, R. Rivest.
;;; 1989 MIT Press.
(require 'record)
;; Record type.
(define heap:rtd (make-record-type "heap" '(array size heap<?)))
;; Constructor.
(define heap:make-heap
(let ((cstr (record-constructor heap:rtd)))
(lambda (pred<?)
(cstr (make-vector 4) 0 pred<?))))
;; Reference an element.
(define heap:ref
(let ((ra (record-accessor heap:rtd 'array)))
(lambda (a i)
(vector-ref (ra a) (+ -1 i)))))
;; Set an element.
(define heap:set!
(let ((ra (record-accessor heap:rtd 'array)))
(lambda (a i v)
(vector-set! (ra a) (+ -1 i) v))))
;; Exchange two elements.
(define heap:exchange
(let ((aa (record-accessor heap:rtd 'array)))
(lambda (a i j)
(set! i (+ -1 i))
(set! j (+ -1 j))
(let* ((ra (aa a))
(tmp (vector-ref ra i)))
(vector-set! ra i (vector-ref ra j))
(vector-set! ra j tmp)))))
;; Get length.
(define heap:length (record-accessor heap:rtd 'size))
(define heap:heap<? (record-accessor heap:rtd 'heap<?))
(define heap:set-size!
(let ((aa (record-accessor heap:rtd 'array))
(am (record-modifier heap:rtd 'array))
(sm (record-modifier heap:rtd 'size)))
(lambda (a s)
(let ((ra (aa a)))
(if (> s (vector-length ra))
(let ((nra (make-vector (+ s (quotient s 2)))))
(do ((i (+ -1 (vector-length ra)) (+ -1 i)))
((negative? i) (am a nra))
(vector-set! nra i (vector-ref ra i)))))
(sm a s)))))
(define (heap:parent i) (quotient i 2))
(define (heap:left i) (* 2 i))
(define (heap:right i) (+ 1 (* 2 i)))
(define (heap:heapify a i)
(let* ((l (heap:left i))
(r (heap:right i))
(largest (if (and (<= l (heap:length a))
((heap:heap<? a) (heap:ref a i) (heap:ref a l)))
l
i)))
(cond ((and (<= r (heap:length a))
((heap:heap<? a) (heap:ref a largest) (heap:ref a r)))
(set! largest r)))
(cond ((not (= largest i))
(heap:exchange a i largest)
(heap:heapify a largest)))))
(define (heap:insert! a key)
(define i (+ 1 (heap:length a)))
(heap:set-size! a i)
(do ()
((not (and (> i 1)
((heap:heap<? a) (heap:ref a (heap:parent i)) key))))
(heap:set! a i (heap:ref a (heap:parent i)))
(set! i (heap:parent i)))
(heap:set! a i key))
(define (heap:extract-max! a)
(if (< (heap:length a) 1)
(slib:error "heap underflow" a))
(let ((max (heap:ref a 1)))
(heap:set! a 1 (heap:ref a (heap:length a)))
(heap:set-size! a (+ -1 (heap:length a)))
(heap:heapify a 1)
max))
;;
;; Externals.
;;
(define make-heap heap:make-heap)
(define heap-insert! heap:insert!)
(define heap-extract-max! heap:extract-max!)
(define heap-length heap:length)
(define (heap:test)
(require 'debug)
(let ((heap #f))
(set! heap (make-heap char>?))
(heap-insert! heap #\A)
(heap-insert! heap #\Z)
(heap-insert! heap #\G)
(heap-insert! heap #\B)
(heap-insert! heap #\G)
(heap-insert! heap #\Q)
(heap-insert! heap #\S)
(heap-insert! heap #\R)
(do ((i 7 (+ -1 i)))
((negative? i))
(write (heap-extract-max! heap)) (newline))))
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