counted-set.scm

(define-library (counted-set)

  (export make-counted-set
	  counted-set-add
	  counted-set-count
	  counted-set-unique-keys)

  (import (scheme base)
	  (lists))

  (begin
    (define (make-counted-set) '())

    (define (counted-set-add s key amount)
      (let* ((existing (assq key s))
             (count (if existing
			(+ (cdr existing) amount)
			amount))
             (new-head (cons key count))
             (rest (filter (lambda (entry) (not (eq? key (car entry)))) s)))
	(cons new-head rest)))

    (define (counted-set-count s key)
      (let ((existing (assq key s)))
	(if existing (cdr existing) 0)))

    (define (counted-set-unique-keys s)
      (length s))
    ))