package app

import (
	"encoding/json"
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"strings"
)

func transportMaybeMigrateSingBoxDNSConfig(client TransportClient) (string, error) {
	if client.Kind != TransportClientSingBox {
		return "", nil
	}
	if !transportSingBoxDNSMigrationEnabled(client) {
		return "", nil
	}
	path := transportSingBoxConfigPath(client)
	if path == "" {
		return "", nil
	}
	data, err := os.ReadFile(path)
	if err != nil {
		return "", fmt.Errorf("read singbox config failed: %w", err)
	}
	var root map[string]any
	if err := json.Unmarshal(data, &root); err != nil {
		return "", fmt.Errorf("parse singbox config failed: %w", err)
	}

	changed, warnings := transportMigrateSingBoxDNSConfigMap(root)
	if !changed {
		if len(warnings) == 0 {
			return "", nil
		}
		return "", errors.New(strings.Join(warnings, "; "))
	}

	backup := path + ".legacy-dns.bak"
	if _, err := os.Stat(backup); os.IsNotExist(err) {
		if writeErr := os.WriteFile(backup, data, 0o644); writeErr != nil {
			return "", fmt.Errorf("write backup failed: %w", writeErr)
		}
	}

	out, err := json.MarshalIndent(root, "", "  ")
	if err != nil {
		return "", fmt.Errorf("marshal migrated config failed: %w", err)
	}
	tmp := path + ".tmp"
	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
		return "", fmt.Errorf("ensure config dir failed: %w", err)
	}
	if err := os.WriteFile(tmp, append(out, '\n'), 0o644); err != nil {
		return "", fmt.Errorf("write migrated config failed: %w", err)
	}
	if err := os.Rename(tmp, path); err != nil {
		return "", fmt.Errorf("replace migrated config failed: %w", err)
	}

	msg := "singbox dns migrated to new server format"
	if len(warnings) > 0 {
		return msg, errors.New(strings.Join(warnings, "; "))
	}
	return msg, nil
}