package config

import (
	"os"
	"strconv"
)

type DatabaseConfig struct {
	Host     string
	Name     string
	User     string
	Password string
	Port     string
}

type GoogleConfig struct {
	ClientID     string
	ClientSecret string
	RedirectURL  string
}

type JwtConfig struct {
	Secret              string
	AccessTokenDuration int
}

type Config struct {
	Database DatabaseConfig
	Google   GoogleConfig
	Jwt      JwtConfig
}

func LoadConfig() (*Config, error) {
	config := &Config{
		Database: DatabaseConfig{
			Host:     getEnv("DB_HOST", ""),
			Name:     getEnv("DB_NAME", ""),
			User:     getEnv("DB_USER", ""),
			Password: getEnv("DB_PASSWORD", ""),
			Port:     getEnv("DB_PORT", ""),
		},
		Google: GoogleConfig{
			ClientID:     getEnv("GOOGLE_CLIENT_ID", ""),
			ClientSecret: getEnv("GOOGLE_CLIENT_SECRET", ""),
			RedirectURL:  getEnv("GOOGLE_REDIRECT_URL", ""),
		},
		Jwt: JwtConfig{
			Secret:              getEnv("JWT_SECRET", ""),
			AccessTokenDuration: getEnvAsInt("JWT_ACCESS_TOKEN_DURATION", 0),
		},
	}

	return config, nil
}

func getEnv(key, fallback string) string {
	if value, ok := os.LookupEnv(key); ok {
		return value
	}

	return fallback
}

func getEnvAsInt(key string, fallback int) int {
	strValue := getEnv(key, "")
	if value, err := strconv.Atoi(strValue); err == nil {
		return value
	}

	return fallback
}

使用感としては以下

config, err := config.LoadConfig()

if err != nil {
  return nil, fmt.Errorf("could not load config: %w", err)
}

// データベース接続を作成
db, err := NewDatabaseConnection(DatabaseConfig{
  Host:     config.Database.Host,
  Name:     config.Database.Name,
  User:     config.Database.User,
  Password: config.Database.Password,
  Port:     config.Database.Port,
})