// Package config is responsible for one thing only: reading configuration
// from environment variables and handing back a single, typed Config struct
// that the rest of the app can use.
//
// Why centralize this instead of calling os.Getenv() all over the codebase?
//   - One place to see every setting the app needs.
//   - One place to define sane defaults for local development.
//   - Easy to swap the *source* later (e.g. read from a file, from Vault,
//     from AWS Secrets Manager) without touching any other package.
package config

import (
	"os"
	"strings"
)

// Config holds every piece of runtime configuration the application needs.
// It is built once, in main(), and then passed down (by value - it's a
// small, read-only struct) into whichever package needs it: the router,
// the database connector, the session manager, the OAuth config, etc.
type Config struct {
	// Port is the TCP port the HTTP server listens on.
	Port string
	// Env distinguishes "development" from "production". Currently used
	// to decide whether the session cookie requires HTTPS (Secure flag).
	Env string

	// --- MySQL connection settings ---
	DBHost     string
	DBPort     string
	DBUser     string
	DBPassword string
	DBName     string

	// --- Redis connection settings (used for session storage) ---
	RedisAddr string

	// --- Google OAuth2 settings ---
	GoogleClientID     string
	GoogleClientSecret string
	GoogleRedirectURL  string

	// AllowedOrigins is the CORS allowlist: which frontend origins are
	// permitted to call this API from browser JavaScript.
	AllowedOrigins []string
}

// Load reads every setting from the process environment, falling back to
// sensible local-development defaults when a variable isn't set. In
// production you would set all of these explicitly (e.g. via
// docker-compose "environment:", a systemd unit, or your orchestrator's
// secret/config mechanism) rather than relying on the defaults.
func Load() Config {
	return Config{
		Port: getEnv("PORT", "8080"),
		Env:  getEnv("ENV", "development"),

		DBHost:     getEnv("DB_HOST", "127.0.0.1"),
		DBPort:     getEnv("DB_PORT", "3306"),
		DBUser:     getEnv("DB_USER", "root"),
		DBPassword: getEnv("DB_PASSWORD", "devpass"),
		DBName:     getEnv("DB_NAME", "go_simple_api"),

		RedisAddr: getEnv("REDIS_ADDR", "127.0.0.1:6379"),

		GoogleClientID:     getEnv("GOOGLE_CLIENT_ID", ""),
		GoogleClientSecret: getEnv("GOOGLE_CLIENT_SECRET", ""),
		GoogleRedirectURL:  getEnv("GOOGLE_REDIRECT_URL", "http://localhost:8080/auth/google/callback"),

		// ALLOWED_ORIGINS is a comma-separated list, e.g.:
		//   ALLOWED_ORIGINS=http://localhost:3000,https://myapp.com
		// strings.Split on a single-value default still works fine and
		// yields a one-element slice.
		AllowedOrigins: strings.Split(getEnv("ALLOWED_ORIGINS", "http://localhost:3000"), ","),
	}
}

// getEnv reads a single environment variable, returning fallback if it's
// unset or empty. It's unexported (lowercase) because nothing outside this
// package needs to read raw env vars directly - everyone else should go
// through the Config struct instead.
func getEnv(key, fallback string) string {
	if v := os.Getenv(key); v != "" {
		return v
	}
	return fallback
}