1;;;
2;;; srfi-124 - Ephemerons
3;;;
4;;;   Copyright (c) 2019-2020  Shiro Kawai  <shiro@acm.org>
5;;;
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7;;;   modification, are permitted provided that the following conditions
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9;;;
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16;;;
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32;;;
33
34;; NB: This is an *incomplete* emulation of ephemerons.  Notably,
35;; if a key has a strong reference from its datum, the key won't be
36;; collected even if it is not strongly referenced from elsewhere.
37;; The complete implementation would require to cooperate closely with GC.
38
39(define-module srfi-124
40  (use gauche.record)
41  (export ephemeron? make-ephemeron ephemeron-broken?
42          ephemeron-key ephemeron-datum
43          reference-barrier))
44(select-module srfi-124)
45
46(define-record-type <ephemeron> %make-ephemeron ephemeron?
47  (key-box %ephemeron-key-box)
48  (datum   ephemeron-datum %ephemeron-datum-set!))
49
50(define *broken-mark* (list 'broken))
51
52(define (make-ephemeron key datum)
53  (let1 v (make-weak-vector 1)
54    (weak-vector-set! v 0 key)
55    (%make-ephemeron v datum)))
56
57(define (ephemeron-broken? e)
58  (let1 k (weak-vector-ref (%ephemeron-key-box e) 0 *broken-mark*)
59    (and (eq? k *broken-mark*)
60         (begin (%ephemeron-datum-set! e #f) ; remove ref to datum
61                #t))))
62
63(define (ephemeron-key e)
64  (weak-vector-ref (%ephemeron-key-box e) 0 #f))
65
66(define (reference-barrier key) #t)
67
68
69
70
71
72