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/* Dolev, Klawe & Rodeh for leader election in unidirectional ring
* `An O(n log n) unidirectional distributed algorithm for extrema
* finding in a circle,' J. of Algs, Vol 3. (1982), pp. 245-260
*/
#define N 5 /* nr of processes (use 5 for demos) */
#define I 3 /* node given the smallest number */
#define L 10 /* size of buffer (>= 2*N) */
mtype = { one, two, winner };
chan q[N] = [L] of { mtype, byte};
byte nr_leaders = 0;
proctype node (chan inp, out; byte mynumber)
{ bit Active = 1, know_winner = 0;
byte nr, maximum = mynumber, neighbourR;
xr inp;
xs out;
printf("MSC: %d\n", mynumber);
out!one(mynumber);
end: do
:: inp?one(nr) ->
if
:: Active ->
if
:: nr != maximum ->
out!two(nr);
neighbourR = nr
:: else ->
/* Raynal p.39: max is greatest number */
assert(nr == N);
know_winner = 1;
out!winner,nr;
fi
:: else ->
out!one(nr)
fi
:: inp?two(nr) ->
if
:: Active ->
if
:: neighbourR > nr && neighbourR > maximum ->
maximum = neighbourR;
out!one(neighbourR)
:: else ->
Active = 0
fi
:: else ->
out!two(nr)
fi
:: inp?winner,nr ->
if
:: nr != mynumber ->
printf("MSC: LOST\n");
:: else ->
printf("MSC: LEADER\n");
nr_leaders++;
assert(nr_leaders == 1)
fi;
if
:: know_winner
:: else -> out!winner,nr
fi;
break
od
}
init {
byte proc;
atomic {
proc = 1;
do
:: proc <= N ->
run node (q[proc-1], q[proc%N], (N+I-proc)%N+1);
proc++
:: proc > N ->
break
od
}
}
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