/**
 * LCR
 * -- compute a leader in a ring
 * from N. Lynch, p. 477
 */

type Pid;
type Status;
const unique UNKNOWN, CHOSEN, REPORTED: Status;

var u: [Pid] int;
var status: [Pid] Status;
const next: [Pid] Pid;

procedure Main ()
{
	var candidate: Pid;
	var neighbor: Pid;
	
	// identifiers are unique
	assume (forall p, q: Pid :: p != q ==> u[p] != u[q]);
	assume (forall p: Pid :: status[p] == UNKNOWN);
	
	// any number of nodes can start the leader electing
	while (*) {
		havoc candidate;
		havoc neighbor;
		assume candidate != neighbor;
		assume next[candidate] == neighbor;
		call {:async} leader (neighbor, u[candidate]);
	}
	return;
}

procedure leader (this: Pid, max: int)
{
	if (max > u[this]) {		
		// pass the higher numbers
		call {:async} leader(next[this], max);
		
	} else if (max == u[this]) {
		// win when we get our number back
		status[this] := CHOSEN;
		
	} else {
		// do nothing
	}
	return;
}