tick: dedupe rule-by-name map building (V-581)

stopFinishedAlarms and repeatableRules each built their own
map[string]rule (one keyed to loop.Rule, one to bool) from t.rules on
every call. Factored into rulesByName(), one map[string]loop.Rule both
callers read.

cmd/mavend/tick.go

@@ -319,10 +319,7 @@ func (t *tickLoop) stopFinishedAlarms(ctx context.Context, keys []string, state
 	if len(keys) == 0 {
 		return nil
 	}
-	byName := make(map[string]loop.Rule, len(t.rules))
-	for _, r := range t.rules {
-		byName[r.Name] = r
-	}
+	byName := t.rulesByName()
 	live := keys[:0:0]
 	for _, key := range keys {
 		outcome := ""
@@ -381,19 +378,27 @@ func (t *tickLoop) repeatableRules(keys []string) []string {
 	if len(keys) == 0 {
 		return nil
 	}
-	wired := make(map[string]bool, len(t.rules))
-	for _, r := range t.rules {
-		wired[r.Name] = true
-	}
+	wired := t.rulesByName()
 	out := keys[:0:0]
 	for _, k := range keys {
-		if wired[k] {
+		if _, ok := wired[k]; ok {
 			out = append(out, k)
 		}
 	}
 	return out
 }
 
+// rulesByName indexes the wired rule set by name, for the two lookups above
+// that only care whether a key is still wired (repeatableRules) or need the
+// rule itself (stopFinishedAlarms).
+func (t *tickLoop) rulesByName() map[string]loop.Rule {
+	byName := make(map[string]loop.Rule, len(t.rules))
+	for _, r := range t.rules {
+		byName[r.Name] = r
+	}
+	return byName
+}
+
 // cachePhrase keeps the latest phrased nudge per rule for the sev4-repeat
 // path. writing under a mutex; the repeat path reads under the same. the
 // cache is bounded by the rule count (≤ ~30 per spec) so eviction is not a