Quotas: carry what was used when a rule counts differently

model.QuotaCarry says what becomes of a quota's use when its rule
changes how it counts; the firewall's apply follows it, comparing with
the rules in force: per address to a total starts at the addresses' sum,
wider groups add up their addresses, and what can't be carried (a total
to per address, per source to per destination, finer groups) starts
again, with the engine warning quota_restarts on that change. Before,
per source to per destination kept the sources' use as destinations',
and regrouping left old entries behind.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/model/objects.go

@@ -274,6 +274,45 @@ func (r *Rule) Groups() (v4, v6 int) {
 	return v4, v6
 }
 
+// What becomes of what a rule's quota used when the rule changes
+// (QuotaCarry).
+const (
+	QuotaKept      = ""          // counted as before: carried as it is
+	QuotaSummed    = "summed"    // per address to a total: the addresses' sum
+	QuotaRegrouped = "regrouped" // per address in wider groups: added up per group
+	QuotaRestarts  = "restarts"  // can't be carried: starts again
+)
+
+// QuotaCarry says what becomes of what old's quota used when the rule
+// becomes new (both the same rule, with quotas), and for QuotaRestarts
+// why. The amount, over and the resets don't matter: only how it counts.
+func QuotaCarry(old, new *Rule) (carry, why string) {
+	if old == nil || new == nil || old.Quota == nil || new.Quota == nil {
+		return QuotaKept, ""
+	}
+	was, is := old.Quota.PerAddress(), new.Quota.PerAddress()
+	words := map[string]string{"saddr": "per source", "daddr": "per destination", "": "in total"}
+	switch {
+	case was == "" && is == "":
+		return QuotaKept, ""
+	case is == "":
+		return QuotaSummed, ""
+	case was == "":
+		return QuotaRestarts, "a total can't be split by address, so each address starts at nothing"
+	case was != is:
+		return QuotaRestarts, "what was counted " + words[was] + " can't be counted " + words[is]
+	}
+	o4, o6 := old.Groups()
+	n4, n6 := new.Groups()
+	switch {
+	case o4 == n4 && o6 == n6:
+		return QuotaKept, ""
+	case n4 > o4 || n6 > o6:
+		return QuotaRestarts, "addresses are grouped more finely now, and a group's use can't be split"
+	}
+	return QuotaRegrouped, ""
+}
+
 // PortForward publishes a service on an inside host: incoming connections
 // to a public address and port are sent on (DNAT) to To[:ToPort], let
 // through the forward chain, and, with Hairpin, also work from inside.