solution to 4.48 and 4.49

diff --git a/src/sicp/ex4_48.rkt b/src/sicp/ex4_48.rkt
new file mode 100644 (file)
index 0000000..2163808
--- /dev/null
+++ b/src/sicp/ex4_48.rkt
@@ -0,0 +1,46 @@
+#lang racket
+
+#|
+
+by giving a unique number to each account, we are ordering the access to the account
+in a specific manner (say in the ascending order of the number). The reason why deadlock
+occurs is be cause of the concurrent access to the accounts in a specific order (Paul
+attempts to transfer a1 and a2 while Peter attempts to transfer a2 and a1. With the 
+suggested change, both will concurrently try to access a1 first but only one of them
+will get access while the other will wait until the serializer for a1 is released.
+
+|#
+
+(define (make-account-and-serializer 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 'balance) balance)
+            ((eq? m 'account-number) account-number)
+            ((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))
+        (number1 (account1 'account-number))
+        (number2 (account2 'account-number)))
+    (if (< number1 number2)
+        ((serializer2 (serializer1 exchange))
+         account1
+         account2)
+        ((serializer1 (serializer2 exchange))
+         account1
+         account2)
+))
\ No newline at end of file

diff --git a/src/sicp/ex4_49.rkt b/src/sicp/ex4_49.rkt
new file mode 100644 (file)
index 0000000..b326820
--- /dev/null
+++ b/src/sicp/ex4_49.rkt
@@ -0,0 +1,11 @@
+#lang racket
+
+#|
+
+Process 1 needs access to a shared resource depending on some runtime condition and needs
+another resource depending on another condition.
+
+Consider another similar process being executed concurrently. We cannot determine
+the order of the resource acquisition apriori in this case.
+
+|#
\ No newline at end of file