Toy model

vchan.tla

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+------------------------------- MODULE vchan -------------------------------
+
+EXTENDS Naturals, Sequences   \* Library imports
+CONSTANT BufferSize
+Byte == 0..255
+MSG  == Seq(Byte)
+
+Take(m, i) == SubSeq(m, 1, i)
+Drop(m, i) == SubSeq(m, i + 1, Len(m))
+
+VARIABLES Got, Buffer, Sent
+vars == << Got, Buffer, Sent >> 
+
+\* Desired properties
+
+Integrity       == Take(Sent, Len(Got)) = Got
+
+AvailabilityNat == Nat
+Availability    == \A x \in AvailabilityNat :
+                    Len(Sent) = x ~> Len(Got) >= x
+
+\* Algorithm
+
+Read  == \E n \in 1..Len(Buffer) :
+          /\ Got' = Got \o Take(Buffer, n)
+          /\ Buffer' = Drop(Buffer, n)
+          /\ UNCHANGED Sent
+
+Write == \E m \in MSG :
+          /\ Buffer' = Buffer \o m
+          /\ Len(Buffer') <= BufferSize
+          /\ Sent' = Sent \o m
+          /\ UNCHANGED Got
+
+Next  == Read \/ Write
+
+Init  == /\ Sent   = << >>
+         /\ Buffer = << >>
+         /\ Got    = << >>
+
+Spec  == Init /\ [][Next]_vars /\ WF_vars(Read)
+
+BufferOK == Len(Buffer) <= BufferSize
+
+\* Model checking
+
+MSG_SEQ(max) == { [ x \in 1..N |-> Len(Sent) + x ] : N \in 1..max }
+
+=============================================================================
+