package config

import (
	"flag"
	"fmt"
	"net/url"
	"os"
	"strings"
	"time"

	"github.com/knadh/koanf/parsers/toml/v2"
	"github.com/knadh/koanf/providers/confmap"
	"github.com/knadh/koanf/providers/file"
	"github.com/knadh/koanf/v2"
)

type Config struct {
	MQTT struct {
		URL      string `koanf:"url"`
		ClientID string `koanf:"client_id"`
		Username string `koanf:"username"`
		Password string `koanf:"password"`
		DeviceID string `koanf:"device_id"`
	} `koanf:"mqtt"`
	App struct {
		ConnectTimeout string `koanf:"connect_timeout"`
	} `koanf:"app"`
}

// Keep duration input as text; validate/parse it explicitly after merging.
func (c Config) ConnectTimeout() time.Duration {
	d, _ := time.ParseDuration(c.App.ConnectTimeout) // Load already validated it.
	return d
}

func Load(args []string) (Config, error) {
	var result Config
	flags := flag.NewFlagSet("mqtt5-learning", flag.ContinueOnError)
	filename := flags.String("config", "configs/lab.toml", "TOML file")
	broker := flags.String("broker", "", "broker URL override")
	clientID := flags.String("client-id", "", "client ID override")
	username := flags.String("username", "", "lab username override")
	deviceID := flags.String("device-id", "", "device identity override")
	timeout := flags.String("connect-timeout", "", "startup deadline override")
	if err := flags.Parse(args); err != nil {
		return result, err
	}
	if flags.NArg() != 0 {
		return result, fmt.Errorf("unexpected positional arguments")
	}

	k := koanf.New(".")
	defaults := map[string]any{
		"mqtt.url":       "mqtt://127.0.0.1:18883",
		"mqtt.client_id": "lab-backend",
		"mqtt.username":  "", "mqtt.password": "",
		"mqtt.device_id":      "device-001",
		"app.connect_timeout": "10s",
	}
	if err := k.Load(confmap.Provider(defaults, "."), nil); err != nil {
		return result, fmt.Errorf("defaults: %w", err)
	}
	// A requested/missing config file is an error, not a silent fallback.
	if err := k.Load(file.Provider(*filename), toml.Parser()); err != nil {
		return result, fmt.Errorf("TOML: %w", err)
	}

	// Explicit mapping preserves underscores inside names such as client_id.
	mapping := map[string]string{
		"TEST_MQTT5_MQTT_URL":            "mqtt.url",
		"TEST_MQTT5_MQTT_CLIENT_ID":      "mqtt.client_id",
		"TEST_MQTT5_MQTT_USERNAME":       "mqtt.username",
		"TEST_MQTT5_MQTT_DEVICE_ID":      "mqtt.device_id",
		"TEST_MQTT5_APP_CONNECT_TIMEOUT": "app.connect_timeout",
	}
	for name, key := range mapping {
		if value, exists := os.LookupEnv(name); exists {
			if err := k.Set(key, value); err != nil {
				return result, err
			}
		}
	}
	overrides := map[string]string{}
	flags.Visit(func(f *flag.Flag) {
		switch f.Name {
		case "broker":
			overrides["mqtt.url"] = *broker
		case "client-id":
			overrides["mqtt.client_id"] = *clientID
		case "username":
			overrides["mqtt.username"] = *username
		case "device-id":
			overrides["mqtt.device_id"] = *deviceID
		case "connect-timeout":
			overrides["app.connect_timeout"] = *timeout
		}
	})
	for key, value := range overrides {
		if err := k.Set(key, value); err != nil {
			return result, err
		}
	}
	if err := k.Unmarshal("", &result); err != nil {
		return result, fmt.Errorf("decode configuration: %w", err)
	}
	if err := result.Validate(); err != nil {
		return result, err
	}
	return result, nil
}

func (c Config) Validate() error {
	u, err := url.Parse(c.MQTT.URL)
	if err != nil || u == nil || u.Hostname() == "" || u.Port() == "" {
		return fmt.Errorf("mqtt.url must contain a valid host and explicit port")
	}
	// Early lab only. TLS configuration is deliberately introduced later.
	if u.Scheme != "mqtt" || u.User != nil || u.RawQuery != "" || u.Fragment != "" || u.Path != "" {
		return fmt.Errorf("early lab requires mqtt://host:port without credentials or extra URL components")
	}
	if u.Hostname() != "127.0.0.1" && u.Hostname() != "localhost" {
		return fmt.Errorf("plaintext learning broker must be on loopback")
	}
	if strings.TrimSpace(c.MQTT.ClientID) == "" {
		return fmt.Errorf("mqtt.client_id is required")
	}
	if strings.TrimSpace(c.MQTT.Username) == "" {
		return fmt.Errorf("mqtt.username is required")
	}
	if strings.TrimSpace(c.MQTT.Password) == "" {
		return fmt.Errorf("mqtt.password is required in the TOML configuration")
	}
	if !validDeviceID(c.MQTT.DeviceID) {
		return fmt.Errorf("device ID must be 1-128 ASCII letters, digits, dots, underscores, or hyphens")
	}
	d, err := time.ParseDuration(c.App.ConnectTimeout)
	if err != nil || d <= 0 || d > time.Minute {
		return fmt.Errorf("connect timeout must be a duration greater than zero and at most 1m")
	}
	return nil
}

// validDeviceID mirrors the topic contract's single-segment ID rules so
// configuration fails before the process tries to build MQTT topics.
func validDeviceID(value string) bool {
	if value == "" || len(value) > 128 {
		return false
	}
	for _, r := range value {
		if !(r >= 'a' && r <= 'z' || r >= 'A' && r <= 'Z' || r >= '0' && r <= '9' || strings.ContainsRune("._-", r)) {
			return false
		}
	}
	return true
}