Implement divergent EC chain in simulations

So far simulations used the same EC chain across all participants after
the first instance. This approach was simple but not representative of
the real-world where the EC chain may diverge across multiple subsets
of participants.

The changes here implement a comprehensive set of APIs that allow a set
of EC generation strategies to mix-and-match and replicate complex EC
chain evolution through iterations of a simulation. The chain evolution
is controllable by instance by actor ID. This allows us to simulate
fluctuating EC evolution patterns throughout a simulation. The changes
include an initial set of EC generators that cover:

* Fixed EC for all instances and participants.
* Random EC per instance per participant.
* Random EC per instance but uniform across all participants.
* Common prefix EC per instance with probabilistic length
* Majority common prefix across a set of participants with probabilistic
  length

The EC generators can be aggregated and set per number of participants,
which enables simulation of a flexible set of scenarios including
evolving majority common prefix.

Fixes #115

gpbft/chain.go

@@ -71,8 +71,8 @@ func (c ECChain) BaseChain() ECChain {
 	return ECChain{c[0]}
 }
 
-func (c ECChain) Extend(tip TipSet) ECChain {
-	return append(c, tip)
+func (c ECChain) Extend(tip ...TipSet) ECChain {
+	return append(c, tip...)
 }
 
 // Returns a chain with suffix (after the base) truncated to a maximum length.

sim/cmd/f3sim/f3sim.go

@@ -35,19 +35,11 @@ func main() {
 			log.Panicf("failed to instantiate log normal latency model: %c\n", err)
 		}
 
-		tsg := sim.NewTipSetGenerator(uint64(seed))
-		baseChain, err := gpbft.NewChain(tsg.Sample())
-		if err != nil {
-			log.Fatalf("failed to generate base chain: %v\n", err)
-		}
-
 		options := []sim.Option{
-			sim.WithHonestParticipantCount(*participantCount),
 			sim.WithLatencyModel(latencyModel),
 			sim.WithECEpochDuration(30 * time.Second),
 			sim.WithECStabilisationDelay(0),
-			sim.WithTipSetGenerator(tsg),
-			sim.WithBaseChain(&baseChain),
+			sim.AddHonestParticipants(*participantCount, sim.NewUniformECChainGenerator(uint64(seed), 1, 10)),
 			sim.WithTraceLevel(*traceLevel),
 			sim.WithGpbftOptions(
 				gpbft.WithDelta(*delta),
@@ -60,19 +52,12 @@ func main() {
 		}
 
 		sm, err := sim.NewSimulation(options...)
-		if err != nil {
-			return
-		}
 		if err != nil {
 			log.Panicf("failed to instantiate simulation: %v\n", err)
 		}
 
-		// Same chain for everyone.
-		candidate := baseChain.Extend(tsg.Sample())
-		sm.SetChains(sim.ChainCount{Count: *participantCount, Chain: candidate})
-
 		if err := sm.Run(1, *maxRounds); err != nil {
-			sm.PrintResults()
+			sm.GetInstance(0).Print()
 			os.Exit(1)
 		}
 	}