make p4 quadratic

score.go

@@ -219,7 +219,7 @@ func (ps *peerScore) score(p peer.ID) float64 {
 
 		// P4: invalid messages
 		// NOTE: the weight of P4 is negative (validated in TopicScoreParams.validate), so this detracts.
-		p4 := tstats.invalidMessageDeliveries
+		p4 := (tstats.invalidMessageDeliveries * tstats.invalidMessageDeliveries)
 		topicScore += p4 * topicParams.InvalidMessageDeliveriesWeight
 
 		// update score, mixing with topic weight

score_params.go

@@ -139,7 +139,8 @@ type TopicScoreParams struct {
 
 	// P4: invalid messages
 	// This is the number of invalid messages in the topic.
-	// The value of the parameter is a counter, decaying with InvalidMessageDeliveriesDecay.
+	// The value of the parameter is the square of the counter, decaying with
+	// InvalidMessageDeliveriesDecay.
 	// The weight of the parameter MUST be negative (or zero to disable).
 	InvalidMessageDeliveriesWeight, InvalidMessageDeliveriesDecay float64
 }