From d429f3a7dbe8fc8cc43ebe565b6130b1cfce4ea1 Mon Sep 17 00:00:00 2001
From: xengineering <me@xengineering.eu>
Date: Sat, 20 Dec 2025 14:15:58 +0100
Subject: Add StartupConfiguration.Validate()

This method makes it easy to validate a configuration.

A call of it is now embedded into the StartupConfiguration.FromJSON()
method which should always be the lowest level function to parse
configurations.

Thus configurations can usually be trusted.
---
 config.go      | 93 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 config_test.go |  5 ++++
 2 files changed, 97 insertions(+), 1 deletion(-)

diff --git a/config.go b/config.go
index edd6f4b..7d3cab8 100644
--- a/config.go
+++ b/config.go
@@ -3,9 +3,33 @@ package main
 import (
 	_ "embed"
 	"encoding/json"
+	"fmt"
 	"log"
+	"net"
+	"net/url"
+	"regexp"
+	"strconv"
+	"time"
 )
 
+const (
+	MQTT_BROKER_REGEX       = `^tcp://127\.0\.0\.1:\d+$`
+	MQTT_CLIENT_ID_REGEX    = `^[0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ]{1,23}$`
+	MQTT_TOPIC_PREFIX_REGEX = `^[a-zA-Z0-9]{1,20}$`
+)
+
+var (
+	mqttBrokerRegexp      *regexp.Regexp
+	mqttClientIDRegexp    *regexp.Regexp
+	mqttTopicPrefixRegexp *regexp.Regexp
+)
+
+func init() {
+	mqttBrokerRegexp = regexp.MustCompile(MQTT_BROKER_REGEX)
+	mqttClientIDRegexp = regexp.MustCompile(MQTT_CLIENT_ID_REGEX)
+	mqttTopicPrefixRegexp = regexp.MustCompile(MQTT_TOPIC_PREFIX_REGEX)
+}
+
 //go:embed configs/default.json
 var defaultConfig []byte
 
@@ -25,8 +49,75 @@ type StartupConfig struct {
 	Homematic HomematicConfig `json:"homematic"`
 }
 
+func (sc StartupConfig) Validate() error {
+	if !mqttBrokerRegexp.MatchString(sc.MQTT.Broker) {
+		return fmt.Errorf(
+			"mqtt/broker configuration '%s' does not match regular expression '%s'.",
+			sc.MQTT.Broker,
+			MQTT_BROKER_REGEX,
+		)
+	}
+
+	if !mqttClientIDRegexp.MatchString(sc.MQTT.ClientID) {
+		return fmt.Errorf(
+			"mqtt/client-id configuration '%s' does not match regular expression '%s'.",
+			sc.MQTT.ClientID,
+			MQTT_CLIENT_ID_REGEX,
+		)
+	}
+
+	if !mqttTopicPrefixRegexp.MatchString(sc.MQTT.TopicPrefix) {
+		return fmt.Errorf(
+			"mqtt/topic-prefix configuration '%s' does not match regular expression '%s'.",
+			sc.MQTT.TopicPrefix,
+			MQTT_TOPIC_PREFIX_REGEX,
+		)
+	}
+
+	cu, err := url.Parse(sc.Homematic.CCU)
+	if err != nil {
+		return fmt.Errorf("homematic/ccu configuration '%s' cannot be parsed as URL: %v", sc.Homematic.CCU, err)
+	}
+
+	if cu.Scheme != `http` {
+		return fmt.Errorf("homematic/ccu configuration '%s' must use scheme 'http'.", sc.Homematic.CCU)
+	}
+
+	host, portstring, err := net.SplitHostPort(cu.Host)
+	if err != nil {
+		return fmt.Errorf("homematic/ccu configuration '%s' can't be split into hostname and port: %v", sc.Homematic.CCU, err)
+	}
+
+	ip := net.ParseIP(host)
+	if ip == nil {
+		return fmt.Errorf("homematic/ccu configuration '%s' must use a plain IP address as host.", sc.Homematic.CCU)
+	}
+
+	_, err = strconv.Atoi(portstring)
+	if err != nil {
+		return fmt.Errorf("homematic/ccu configuration '%s' must use a numeric port: %v", sc.Homematic.CCU, err)
+	}
+
+	_, err = time.ParseDuration(sc.Homematic.PollingPeriod)
+	if err != nil {
+		return fmt.Errorf("homematic/polling-period configuration '%s' could not be parsed to duration: %v", sc.Homematic.PollingPeriod, err)
+	}
+
+	return nil
+}
+
 func (sc *StartupConfig) FromJSON(data []byte) error {
-	return json.Unmarshal(data, sc)
+	err := json.Unmarshal(data, sc)
+	if err != nil {
+		return fmt.Errorf("Failed to unmarshal configuration: %w", err)
+	}
+
+	err = sc.Validate()
+	if err != nil {
+		return fmt.Errorf("Could not validate configuration: %w", err)
+	}
+
+	return nil
 }
 
 func GetStartupConfig() StartupConfig {
diff --git a/config_test.go b/config_test.go
index c568a34..0972d5d 100644
--- a/config_test.go
+++ b/config_test.go
@@ -11,4 +11,9 @@ func TestDefaultConfig(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Failed parsing default config from JSON: %v", err)
 	}
+
+	err = config.Validate()
+	if err != nil {
+		t.Fatalf("Failed to validate default config: %v", err)
+	}
 }
-- 
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