Refactor code structure for improved readability and maintainability

resolver/pool.go

@@ -0,0 +1,48 @@
+package resolver
+
+import "dns-helper/platform"
+
+// BuildResolverPool constructs the ordered list of DNS resolver addresses for
+// the given ServerMode and discovered NetworkInfo.
+//
+// Modes:
+//   - "default"  → local DNS servers + bootstrap resolvers (deduplicated, local first)
+//   - "local"    → only the local DNS servers from NetworkInfo.DNSServers
+//   - "gateway"  → only NetworkInfo.Gateway
+//   - "explicit" → only ServerMode.ExplicitAddr
+//
+// Returns nil for unknown modes. Callers should validate that the pool is
+// non-empty before proceeding (e.g. gateway mode with no discovered gateway).
+func BuildResolverPool(mode ServerMode, info platform.NetworkInfo) []string {
+	switch mode.Mode {
+	case "default":
+		seen := make(map[string]bool, len(info.DNSServers)+len(BootstrapResolvers))
+		pool := make([]string, 0, len(info.DNSServers)+len(BootstrapResolvers))
+
+		for _, r := range info.DNSServers {
+			if !seen[r] {
+				seen[r] = true
+				pool = append(pool, r)
+			}
+		}
+		for _, r := range BootstrapResolvers {
+			if !seen[r] {
+				seen[r] = true
+				pool = append(pool, r)
+			}
+		}
+		return pool
+
+	case "local":
+		return info.DNSServers
+
+	case "gateway":
+		return []string{info.Gateway}
+
+	case "explicit":
+		return []string{mode.ExplicitAddr}
+
+	default:
+		return nil
+	}
+}