33 lines
938 B
Racket
33 lines
938 B
Racket
|
#lang racket/base
|
||
|
|
;; Bags and Deltas (which are Bags where item-counts can be negative).
|
||
|
|
|
||
|
|
(provide make-bag
|
||
|
|
bag-change!
|
||
|
|
bag-ref
|
||
|
|
bag-clear!
|
||
|
|
in-bag)
|
||
|
|
|
||
|
|
;; A `(Bagof X)` is a `(MutableHash X Nat)`, where the `Nat` against
|
||
|
|
;; an `X` is its replication count in the bag.
|
||
|
|
;;
|
||
|
|
;; A `(Deltaof X)` is a `(MutableHash X Int)`, just like a `(Bagof X)`
|
||
|
|
;; except the replication counts can be negative.
|
||
|
|
|
||
|
|
(define make-bag make-hash)
|
||
|
|
|
||
|
|
(define (bag-change! b x delta)
|
||
|
|
(define old-count (bag-ref b x))
|
||
|
|
(define new-count (+ old-count delta))
|
||
|
|
(if (zero? new-count)
|
||
|
|
(begin (hash-remove! b x)
|
||
|
|
(if (zero? old-count) 'absent->absent 'present->absent))
|
||
|
|
(begin (hash-set! b x new-count)
|
||
|
|
(if (zero? old-count) 'absent->present 'present->present))))
|
||
|
|
|
||
|
|
(define (bag-ref b x)
|
||
|
|
(hash-ref b x 0))
|
||
|
|
|
||
|
|
(define bag-clear! hash-clear!)
|
||
|
|
|
||
|
|
(define-syntax-rule (in-bag piece ...) (in-hash piece ...))
|