// Package config loads and validates the backend's runtime configuration from
// the process environment.
package config

import (
	"fmt"
	"os"
	"strconv"
	"time"

	"scrabble/backend/internal/account"
	"scrabble/backend/internal/game"
	"scrabble/backend/internal/lobby"
	"scrabble/backend/internal/postgres"
	"scrabble/backend/internal/robot"
	"scrabble/backend/internal/telemetry"
)

// Config holds the backend's runtime configuration.
type Config struct {
	// HTTPAddr is the listen address of the HTTP listener (host:port).
	HTTPAddr string
	// GRPCAddr is the listen address of the gRPC push listener (host:port) that
	// streams live events to the gateway.
	GRPCAddr string
	// LogLevel is the zap log level: "debug", "info", "warn" or "error".
	LogLevel string
	// Postgres configures the primary database pool.
	Postgres postgres.Config
	// Telemetry configures the OpenTelemetry providers.
	Telemetry telemetry.Config
	// Game configures the game subsystem (dictionaries, sweeper, live-game cache).
	Game game.Config
	// Lobby configures matchmaking robot substitution (wait window, reaper cadence).
	Lobby lobby.Config
	// Robot configures the robot opponent driver (scan cadence).
	Robot robot.Config
	// SMTP configures the email relay used for confirm-codes. An empty Host
	// selects the development log mailer (the code is logged, not sent).
	SMTP account.SMTPConfig
}

// Defaults applied when the corresponding environment variable is unset.
const (
	defaultHTTPAddr = ":8080"
	defaultGRPCAddr = ":9090"
	defaultLogLevel = "info"
)

// Load reads the configuration from the environment, applies defaults for unset
// variables, and validates the result.
func Load() (Config, error) {
	pg := postgres.DefaultConfig()
	pg.DSN = os.Getenv("BACKEND_POSTGRES_DSN")
	var err error
	if pg.MaxOpenConns, err = envInt("BACKEND_POSTGRES_MAX_OPEN_CONNS", pg.MaxOpenConns); err != nil {
		return Config{}, err
	}
	if pg.MaxIdleConns, err = envInt("BACKEND_POSTGRES_MAX_IDLE_CONNS", pg.MaxIdleConns); err != nil {
		return Config{}, err
	}
	if pg.ConnMaxLifetime, err = envDuration("BACKEND_POSTGRES_CONN_MAX_LIFETIME", pg.ConnMaxLifetime); err != nil {
		return Config{}, err
	}
	if pg.OperationTimeout, err = envDuration("BACKEND_POSTGRES_OPERATION_TIMEOUT", pg.OperationTimeout); err != nil {
		return Config{}, err
	}

	tel := telemetry.DefaultConfig()
	tel.ServiceName = envOr("BACKEND_SERVICE_NAME", tel.ServiceName)
	tel.TracesExporter = envOr("BACKEND_OTEL_TRACES_EXPORTER", tel.TracesExporter)
	tel.MetricsExporter = envOr("BACKEND_OTEL_METRICS_EXPORTER", tel.MetricsExporter)

	gm := game.DefaultConfig()
	gm.DictDir = envOr("BACKEND_DICT_DIR", gm.DictDir)
	gm.DictVersion = envOr("BACKEND_DICT_VERSION", gm.DictVersion)
	if gm.TimeoutSweepInterval, err = envDuration("BACKEND_GAME_TIMEOUT_SWEEP_INTERVAL", gm.TimeoutSweepInterval); err != nil {
		return Config{}, err
	}
	if gm.CacheTTL, err = envDuration("BACKEND_GAME_CACHE_TTL", gm.CacheTTL); err != nil {
		return Config{}, err
	}

	lb := lobby.DefaultConfig()
	if lb.RobotWait, err = envDuration("BACKEND_LOBBY_ROBOT_WAIT", lb.RobotWait); err != nil {
		return Config{}, err
	}
	if lb.ReaperInterval, err = envDuration("BACKEND_LOBBY_REAPER_INTERVAL", lb.ReaperInterval); err != nil {
		return Config{}, err
	}

	rb := robot.DefaultConfig()
	if rb.DriveInterval, err = envDuration("BACKEND_ROBOT_DRIVE_INTERVAL", rb.DriveInterval); err != nil {
		return Config{}, err
	}

	smtp := account.SMTPConfig{
		Host:     os.Getenv("BACKEND_SMTP_HOST"),
		Port:     envOr("BACKEND_SMTP_PORT", "587"),
		Username: os.Getenv("BACKEND_SMTP_USERNAME"),
		Password: os.Getenv("BACKEND_SMTP_PASSWORD"),
		From:     envOr("BACKEND_SMTP_FROM", "no-reply@localhost"),
	}

	c := Config{
		HTTPAddr:  envOr("BACKEND_HTTP_ADDR", defaultHTTPAddr),
		GRPCAddr:  envOr("BACKEND_GRPC_ADDR", defaultGRPCAddr),
		LogLevel:  envOr("BACKEND_LOG_LEVEL", defaultLogLevel),
		Postgres:  pg,
		Telemetry: tel,
		Game:      gm,
		Lobby:     lb,
		Robot:     rb,
		SMTP:      smtp,
	}
	if err := c.validate(); err != nil {
		return Config{}, err
	}
	return c, nil
}

// validate reports whether the configuration values are acceptable.
func (c Config) validate() error {
	switch c.LogLevel {
	case "debug", "info", "warn", "error":
	default:
		return fmt.Errorf("config: invalid BACKEND_LOG_LEVEL %q", c.LogLevel)
	}
	if c.HTTPAddr == "" {
		return fmt.Errorf("config: BACKEND_HTTP_ADDR must not be empty")
	}
	if c.GRPCAddr == "" {
		return fmt.Errorf("config: BACKEND_GRPC_ADDR must not be empty")
	}
	if err := c.Postgres.Validate(); err != nil {
		return fmt.Errorf("config: %w (set BACKEND_POSTGRES_DSN)", err)
	}
	if err := c.Telemetry.Validate(); err != nil {
		return fmt.Errorf("config: %w", err)
	}
	if err := c.Game.Validate(); err != nil {
		return fmt.Errorf("config: %w (set BACKEND_DICT_DIR)", err)
	}
	if err := c.Lobby.Validate(); err != nil {
		return fmt.Errorf("config: %w", err)
	}
	if err := c.Robot.Validate(); err != nil {
		return fmt.Errorf("config: %w", err)
	}
	return nil
}

// envOr returns the value of the environment variable named key, or fallback
// when the variable is unset or empty.
func envOr(key, fallback string) string {
	if v := os.Getenv(key); v != "" {
		return v
	}
	return fallback
}

// envInt parses the environment variable named key as an int, returning
// fallback when it is unset and an error when it is set but malformed.
func envInt(key string, fallback int) (int, error) {
	v := os.Getenv(key)
	if v == "" {
		return fallback, nil
	}
	n, err := strconv.Atoi(v)
	if err != nil {
		return 0, fmt.Errorf("config: %s: %w", key, err)
	}
	return n, nil
}

// envDuration parses the environment variable named key as a Go duration,
// returning fallback when it is unset and an error when it is set but malformed.
func envDuration(key string, fallback time.Duration) (time.Duration, error) {
	v := os.Getenv(key)
	if v == "" {
		return fallback, nil
	}
	d, err := time.ParseDuration(v)
	if err != nil {
		return 0, fmt.Errorf("config: %s: %w", key, err)
	}
	return d, nil
}