-------------------------------------------------------------------------------
Mohamed G. Gouda                                        CS 356
Summer      2007                                        Homework 1 Solution
-------------------------------------------------------------------------------

1. (3 Points):

process p[0]

var ready : boolean  {init. true}
par g     : 1..3

begin
     ready -> send rqst to p[1]; ready := false

[]   rcv rply from p[g] -> ready := true
end

process p[i : 1..3]

begin
     rcv rqst from p[i-1] ->
                            if i<3 -> send rqst to p[i+1]
                            [] true -> send rply to p[0]
                            fi
end

-------------------------------------------------------------------------------

2. (3 Points):

* Add the following two actions to process p[0]:

    rcv error from p[g] -> skip
[]  timeout G -> ready := true

Where G is the following protocol predicate: 
    ~ready ^ ( #ch.p[0].p[1] + #ch.p[1].p[2] +
               #ch.p[2].p[3] + #ch.p[1].p[0] +
               #ch.p[2].p[0] + #ch.p[3].p[0] = 0)

* Add the following action to process p[i : 1..3]

rcv error from p[i-1] -> skip             

-------------------------------------------------------------------------------

2. (4 Points):

process p[i : 0..n-1]

inp N : set {g| p[g] is a neigbor of p[i]}
var S : boolean,
    e : boolean,
    g : N
par j : N

begin
     ~s ^ ~e -> s, g := true, any; send rqst to p[g]
[]   rcv no from p[g] -> s := false
[]   rcv yes from p[g] -> e:= true
[]   s ^ e -> send done to p[g]; s, e := false, false
[]   rcv done from p[g] -> e:= false
[]   rcv rqst from p[j] -> if s V e -> send no to p[j]
                           [] ~s ^ ~e -> send yes to p[j]; e := true
                           fi
end