Chez Scheme Setup

October 3, 2017

; priqueue.ss

; pairing heaps

(define pq-empty '())
(define pq-empty? null?)

(define (pq-first pq)
  (if (null? pq)
      (error 'pq-first "can't extract minimum from null queue")
      (car pq)))

(define (pq-merge lt? p1 p2)
  (cond ((null? p1) p2)
        ((null? p2) p1)
        ((lt? (car p2) (car p1))
          (cons (car p2) (cons p1 (cdr p2))))
        (else (cons (car p1) (cons p2 (cdr p1))))))

(define (pq-insert lt? x pq)
  (pq-merge lt? (list x) pq))

(define (pq-merge-pairs lt? ps)
  (cond ((null? ps) '())
        ((null? (cdr ps)) (car ps))
        (else (pq-merge lt? (pq-merge lt? (car ps) (cadr ps))
                            (pq-merge-pairs lt? (cddr ps))))))

(define (pq-rest lt? pq)
  (if (null? pq)
      (error 'pq-rest "can't delete minimum from null queue")
      (pq-merge-pairs lt? (cdr pq))))

(define (list->pq lt? xs)
  (let loop ((xs xs) (pq pq-empty))
    (if (null? xs) pq
      (loop (cdr xs) (pq-insert lt? (car xs) pq)))))

(define (pq->list lt? pq)
  (let loop ((pq pq) (xs '()))
    (if (pq-empty? pq) (reverse xs)
      (loop (pq-rest lt? pq) (cons (pq-first pq) xs)))))

(define (pq-sort lt? xs)
  (pq->list lt? (list->pq lt? xs)))

; (display (pq-sort < '(3 7 8 1 2 9 6 4 5)))
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2 Responses to “Chez Scheme Setup”

  1. programmingpraxis said

    My apologies; the formatting got really messed up. I’ll work on fixing through the day today.

  2. chaw said

    I think the pairing heap implementation will not scale to beyond stack limits since it uses non-tail recursion. An easy fix would be to replace the recursive calls in pq-merge-pairs with a fold of pq-merge on pre-paired arguments.

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