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Mohamed G. Gouda                                        CS 356
Summer 2006                                             Midterm 3
Solution
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1. 

process p[i:0..n-1]
inp     N: set{g|p[g] is a neighbor of p[i]}
var     up: array[N] of boolean,
        b: 0..1
        rcvchk: array[N] of boolean {init. false}
par     g: N

begin

    rcv chk(b) from p[g] ->
           if b=0 -> up[g] := false
           [] b=1 -> up[g] := true
           fi; rcvchk[g] := true

    [] rcvchk[g] -> send chk(1) to p[g]; rcvchk[g] := false       

    [] rcv error from p[g] -> skip

    [] timeout ~rcvchk[g].p[i] ^~rcvchk[i].p[g]^(#ch.p[i].p[g] + #ch.p[g].p[i] = 0)
               ->
               if i<g -> send chk(0) to p[g]
               [] i>=g -> skip
               fi; up[g] :=false

end

2.

RTMSG :: if i!=x ^ j!=0 ^ up[rtb[0]]  -> send data(x,y) to p[rtb[0]]
         [] i!=x ^ j!=0 ^ ~up[rtb[0]] -> {nonreachable} skip
         [] i!=x ^ j=0  ^ up[1-i,0]   -> send data(x,y) to p[1-i,0]
         [] i!=x ^ j=0  ^ ~up[1-i,0]  -> {nonreachable} skip
         [] i=x  ^ j!=y ^ up[rtb[y]]  -> send data(x,y) to p[rtb[y]]
         [] i=x  ^ j!=y ^ up[rtb[y]]  -> {nonreachable} skip
         [] i=x  ^ j=y                -> {arrived} skip
         fi