연습문제 3.47, 3.48

[준경군]

; 3.47 - a
(define (make-semaphore-mtx maximal)
(let ((count maximal)
(mutex (make-mutex)))
(define (the-sema m)
(cond ((eq? m 'release)
(mutex 'acquire)
(unless (= count maximal)
(set! count (+ 1 count)))
(mutex 'release))
((eq? m 'acquire)
(cond ((> count 0)
(mutex 'acquire)
(set! count (- count 1))
(mutex 'release))
(else
(the-sema 'acquire))))
(else
(error "Unknown request -- " m))))
the-sema))

; 3.47 - b
(define (loop-test-and-set! cell)
(if (test-and-set! cell)
(loop-test-and-set! cell)))

(define (make-semaphore-ts maximal)
(let ((count maximal)
(guard (list #f)))
(define (the-sema m)
(cond ((eq? m 'release)
(loop-test-and-set! guard)
(unless (= count maximal)
(set! count (+ 1 count)))
(clear! guard))
((eq? m 'acquire)
(cond
((> count 0)
(loop-test-and-set! guard)
(set! count (- count 1))
(clear! guard))
(else
(the-sema 'acquire))))
(else
(error "Unknown request -- " m))))
the-sema))

; 3.48
(define (make-account number balance)
(define (withdraw amount)
(if (>= balance amount)
(begin (set! balance (- balance amount))
balance)
"Insufficient funds"))
(define (deposit amount)
(set! balance (+ balance amount))
balance)
(let ((balance-serializer (make-serializer)))
(define (dispatch m)
(cond ((eq? m 'withdraw) withdraw)
((eq? m 'deposit) deposit)
((eq? m 'number) number)
((eq? m 'balance) balance)
((eq? m 'serializer) balance-serializer)
(else (error "Unknown request -- MAKE-ACCOUNT"
m))))
dispatch))

(define (serialized-exchange account1 account2)
(let ((serializer1 (account1 'serializer))
(serializer2 (account2 'serializer)))
(if (< (account1 'number) (account2 'number))
((serializer2 (serializer1 exchange))
account1 account2)
((serializer1 (serializer2 exchange))
account1 account2))))