// Package config loads the Notification Service process configuration from
// environment variables.
package config

import (
	"crypto/tls"
	"fmt"
	"log/slog"
	"net"
	netmail "net/mail"
	"net/url"
	"os"
	"strconv"
	"strings"
	"time"

	"galaxy/notification/internal/telemetry"
)

const (
	shutdownTimeoutEnvVar = "NOTIFICATION_SHUTDOWN_TIMEOUT"
	logLevelEnvVar        = "NOTIFICATION_LOG_LEVEL"

	internalHTTPAddrEnvVar              = "NOTIFICATION_INTERNAL_HTTP_ADDR"
	internalHTTPReadHeaderTimeoutEnvVar = "NOTIFICATION_INTERNAL_HTTP_READ_HEADER_TIMEOUT"
	internalHTTPReadTimeoutEnvVar       = "NOTIFICATION_INTERNAL_HTTP_READ_TIMEOUT"
	internalHTTPIdleTimeoutEnvVar       = "NOTIFICATION_INTERNAL_HTTP_IDLE_TIMEOUT"

	redisAddrEnvVar             = "NOTIFICATION_REDIS_ADDR"
	redisUsernameEnvVar         = "NOTIFICATION_REDIS_USERNAME"
	redisPasswordEnvVar         = "NOTIFICATION_REDIS_PASSWORD"
	redisDBEnvVar               = "NOTIFICATION_REDIS_DB"
	redisTLSEnabledEnvVar       = "NOTIFICATION_REDIS_TLS_ENABLED"
	redisOperationTimeoutEnvVar = "NOTIFICATION_REDIS_OPERATION_TIMEOUT"

	intentsStreamEnvVar              = "NOTIFICATION_INTENTS_STREAM"
	intentsReadBlockTimeoutEnvVar    = "NOTIFICATION_INTENTS_READ_BLOCK_TIMEOUT"
	gatewayClientEventsStreamEnvVar  = "NOTIFICATION_GATEWAY_CLIENT_EVENTS_STREAM"
	gatewayClientEventsStreamMaxEnvVar = "NOTIFICATION_GATEWAY_CLIENT_EVENTS_STREAM_MAX_LEN"
	mailDeliveryCommandsStreamEnvVar = "NOTIFICATION_MAIL_DELIVERY_COMMANDS_STREAM"

	pushRetryMaxAttemptsEnvVar  = "NOTIFICATION_PUSH_RETRY_MAX_ATTEMPTS"
	emailRetryMaxAttemptsEnvVar = "NOTIFICATION_EMAIL_RETRY_MAX_ATTEMPTS"
	routeLeaseTTLEnvVar         = "NOTIFICATION_ROUTE_LEASE_TTL"
	routeBackoffMinEnvVar       = "NOTIFICATION_ROUTE_BACKOFF_MIN"
	routeBackoffMaxEnvVar       = "NOTIFICATION_ROUTE_BACKOFF_MAX"
	deadLetterTTLEnvVar         = "NOTIFICATION_DEAD_LETTER_TTL"
	recordTTLEnvVar             = "NOTIFICATION_RECORD_TTL"
	idempotencyTTLEnvVar        = "NOTIFICATION_IDEMPOTENCY_TTL"

	userServiceBaseURLEnvVar = "NOTIFICATION_USER_SERVICE_BASE_URL"
	userServiceTimeoutEnvVar = "NOTIFICATION_USER_SERVICE_TIMEOUT"

	adminEmailsGeoReviewRecommendedEnvVar      = "NOTIFICATION_ADMIN_EMAILS_GEO_REVIEW_RECOMMENDED"
	adminEmailsGameGenerationFailedEnvVar      = "NOTIFICATION_ADMIN_EMAILS_GAME_GENERATION_FAILED"
	adminEmailsLobbyRuntimePausedAfterEnvVar   = "NOTIFICATION_ADMIN_EMAILS_LOBBY_RUNTIME_PAUSED_AFTER_START"
	adminEmailsLobbyApplicationSubmittedEnvVar = "NOTIFICATION_ADMIN_EMAILS_LOBBY_APPLICATION_SUBMITTED"

	otelServiceNameEnvVar                 = "OTEL_SERVICE_NAME"
	otelTracesExporterEnvVar              = "OTEL_TRACES_EXPORTER"
	otelMetricsExporterEnvVar             = "OTEL_METRICS_EXPORTER"
	otelExporterOTLPProtocolEnvVar        = "OTEL_EXPORTER_OTLP_PROTOCOL"
	otelExporterOTLPTracesProtocolEnvVar  = "OTEL_EXPORTER_OTLP_TRACES_PROTOCOL"
	otelExporterOTLPMetricsProtocolEnvVar = "OTEL_EXPORTER_OTLP_METRICS_PROTOCOL"
	otelStdoutTracesEnabledEnvVar         = "NOTIFICATION_OTEL_STDOUT_TRACES_ENABLED"
	otelStdoutMetricsEnabledEnvVar        = "NOTIFICATION_OTEL_STDOUT_METRICS_ENABLED"

	defaultShutdownTimeout       = 5 * time.Second
	defaultLogLevel              = "info"
	defaultInternalHTTPAddr      = ":8092"
	defaultReadHeaderTimeout     = 2 * time.Second
	defaultReadTimeout           = 10 * time.Second
	defaultIdleTimeout           = time.Minute
	defaultRedisDB               = 0
	defaultRedisOperationTimeout = 250 * time.Millisecond

	defaultIntentsStream              = "notification:intents"
	defaultIntentsReadBlockTimeout    = 2 * time.Second
	defaultGatewayClientEventsStream  = "gateway:client-events"
	defaultGatewayClientEventsStreamMaxLen int64 = 1024
	defaultMailDeliveryCommandsStream = "mail:delivery_commands"

	defaultPushRetryMaxAttempts  = 3
	defaultEmailRetryMaxAttempts = 7
	defaultRouteLeaseTTL         = 5 * time.Second
	defaultRouteBackoffMin       = time.Second
	defaultRouteBackoffMax       = 5 * time.Minute
	defaultDeadLetterTTL         = 720 * time.Hour
	defaultRecordTTL             = 720 * time.Hour
	defaultIdempotencyTTL        = 168 * time.Hour

	defaultUserServiceTimeout = time.Second
	defaultOTelServiceName    = "galaxy-notification"

	otelExporterNone         = "none"
	otelExporterOTLP         = "otlp"
	otelProtocolHTTPProtobuf = "http/protobuf"
	otelProtocolGRPC         = "grpc"
)

// Config stores the full Notification Service process configuration.
type Config struct {
	// ShutdownTimeout bounds graceful shutdown of every long-lived component.
	ShutdownTimeout time.Duration

	// Logging configures the process-wide structured logger.
	Logging LoggingConfig

	// InternalHTTP configures the private probe HTTP listener.
	InternalHTTP InternalHTTPConfig

	// Redis configures the shared Redis client used by the process.
	Redis RedisConfig

	// Streams stores the stable stream names reserved for notification ingress
	// and downstream publication.
	Streams StreamsConfig

	// IntentsReadBlockTimeout stores the maximum Redis Streams blocking read
	// window used by the intent consumer.
	IntentsReadBlockTimeout time.Duration

	// Retry stores the frozen retry and retention settings.
	Retry RetryConfig

	// UserService configures the trusted user-enrichment dependency.
	UserService UserServiceConfig

	// AdminRouting stores the type-specific configured administrator email
	// lists.
	AdminRouting AdminRoutingConfig

	// Telemetry configures the process-wide OpenTelemetry runtime.
	Telemetry TelemetryConfig
}

// LoggingConfig configures the process-wide structured logger.
type LoggingConfig struct {
	// Level stores the process log level accepted by log/slog.
	Level string
}

// InternalHTTPConfig configures the private probe HTTP listener.
type InternalHTTPConfig struct {
	// Addr stores the TCP listen address.
	Addr string

	// ReadHeaderTimeout bounds request-header reading.
	ReadHeaderTimeout time.Duration

	// ReadTimeout bounds reading one request.
	ReadTimeout time.Duration

	// IdleTimeout bounds how long keep-alive connections stay open.
	IdleTimeout time.Duration
}

// Validate reports whether cfg stores a usable internal HTTP listener
// configuration.
func (cfg InternalHTTPConfig) Validate() error {
	switch {
	case strings.TrimSpace(cfg.Addr) == "":
		return fmt.Errorf("internal HTTP addr must not be empty")
	case !isTCPAddr(cfg.Addr):
		return fmt.Errorf("internal HTTP addr %q must use host:port form", cfg.Addr)
	case cfg.ReadHeaderTimeout <= 0:
		return fmt.Errorf("internal HTTP read header timeout must be positive")
	case cfg.ReadTimeout <= 0:
		return fmt.Errorf("internal HTTP read timeout must be positive")
	case cfg.IdleTimeout <= 0:
		return fmt.Errorf("internal HTTP idle timeout must be positive")
	default:
		return nil
	}
}

// RedisConfig configures the shared Redis client and its connection settings.
type RedisConfig struct {
	// Addr stores the Redis network address.
	Addr string

	// Username stores the optional Redis ACL username.
	Username string

	// Password stores the optional Redis ACL password.
	Password string

	// DB stores the Redis logical database index.
	DB int

	// TLSEnabled reports whether TLS must be used for Redis connections.
	TLSEnabled bool

	// OperationTimeout bounds one Redis round trip including the startup PING.
	OperationTimeout time.Duration
}

// TLSConfig returns the conservative TLS configuration used by the Redis
// client when TLSEnabled is true.
func (cfg RedisConfig) TLSConfig() *tls.Config {
	if !cfg.TLSEnabled {
		return nil
	}

	return &tls.Config{MinVersion: tls.VersionTLS12}
}

// Validate reports whether cfg stores a usable Redis configuration.
func (cfg RedisConfig) Validate() error {
	switch {
	case strings.TrimSpace(cfg.Addr) == "":
		return fmt.Errorf("redis addr must not be empty")
	case !isTCPAddr(cfg.Addr):
		return fmt.Errorf("redis addr %q must use host:port form", cfg.Addr)
	case cfg.DB < 0:
		return fmt.Errorf("redis db must not be negative")
	case cfg.OperationTimeout <= 0:
		return fmt.Errorf("redis operation timeout must be positive")
	default:
		return nil
	}
}

// StreamsConfig stores the stable Redis Stream names used by Notification
// Service.
type StreamsConfig struct {
	// Intents stores the ingress intent stream.
	Intents string

	// GatewayClientEvents stores the downstream Gateway client-events stream.
	GatewayClientEvents string

	// GatewayClientEventsStreamMaxLen bounds the downstream Gateway
	// client-events stream with approximate trimming.
	GatewayClientEventsStreamMaxLen int64

	// MailDeliveryCommands stores the downstream Mail Service command stream.
	MailDeliveryCommands string
}

// Validate reports whether cfg stores usable stream names.
func (cfg StreamsConfig) Validate() error {
	switch {
	case strings.TrimSpace(cfg.Intents) == "":
		return fmt.Errorf("intents stream must not be empty")
	case strings.TrimSpace(cfg.GatewayClientEvents) == "":
		return fmt.Errorf("gateway client-events stream must not be empty")
	case cfg.GatewayClientEventsStreamMaxLen <= 0:
		return fmt.Errorf("gateway client-events stream max len must be positive")
	case strings.TrimSpace(cfg.MailDeliveryCommands) == "":
		return fmt.Errorf("mail delivery-commands stream must not be empty")
	default:
		return nil
	}
}

// RetryConfig stores the frozen retry budgets, backoff settings, and retention
// periods used by the service.
type RetryConfig struct {
	// PushMaxAttempts stores the route retry budget for the `push` channel.
	PushMaxAttempts int

	// EmailMaxAttempts stores the route retry budget for the `email` channel.
	EmailMaxAttempts int

	// RouteLeaseTTL stores the temporary route-lease lifetime used to avoid
	// duplicate publication across replicas.
	RouteLeaseTTL time.Duration

	// RouteBackoffMin stores the minimum retry backoff.
	RouteBackoffMin time.Duration

	// RouteBackoffMax stores the maximum retry backoff.
	RouteBackoffMax time.Duration

	// DeadLetterTTL stores the retention period for dead-letter and malformed
	// intent records.
	DeadLetterTTL time.Duration

	// RecordTTL stores the retention period for notification and route records.
	RecordTTL time.Duration

	// IdempotencyTTL stores the retention period for idempotency records.
	IdempotencyTTL time.Duration
}

// Validate reports whether cfg stores usable retry and retention settings.
func (cfg RetryConfig) Validate() error {
	switch {
	case cfg.PushMaxAttempts <= 0:
		return fmt.Errorf("push retry max attempts must be positive")
	case cfg.EmailMaxAttempts <= 0:
		return fmt.Errorf("email retry max attempts must be positive")
	case cfg.RouteLeaseTTL <= 0:
		return fmt.Errorf("route lease ttl must be positive")
	case cfg.RouteBackoffMin <= 0:
		return fmt.Errorf("route backoff min must be positive")
	case cfg.RouteBackoffMax <= 0:
		return fmt.Errorf("route backoff max must be positive")
	case cfg.RouteBackoffMin > cfg.RouteBackoffMax:
		return fmt.Errorf("route backoff min must not exceed route backoff max")
	case cfg.DeadLetterTTL <= 0:
		return fmt.Errorf("dead-letter ttl must be positive")
	case cfg.RecordTTL <= 0:
		return fmt.Errorf("record ttl must be positive")
	case cfg.IdempotencyTTL <= 0:
		return fmt.Errorf("idempotency ttl must be positive")
	default:
		return nil
	}
}

// UserServiceConfig configures the trusted user-enrichment dependency.
type UserServiceConfig struct {
	// BaseURL stores the absolute base URL of the trusted User Service.
	BaseURL string

	// Timeout bounds one outbound User Service request.
	Timeout time.Duration
}

// Validate reports whether cfg stores a usable User Service configuration.
func (cfg UserServiceConfig) Validate() error {
	switch {
	case strings.TrimSpace(cfg.BaseURL) == "":
		return fmt.Errorf("user service base URL must not be empty")
	case !isAbsoluteHTTPURL(cfg.BaseURL):
		return fmt.Errorf("user service base URL %q must be an absolute http(s) URL", cfg.BaseURL)
	case cfg.Timeout <= 0:
		return fmt.Errorf("user service timeout must be positive")
	default:
		return nil
	}
}

// AdminRoutingConfig stores the type-specific configured administrator email
// lists.
type AdminRoutingConfig struct {
	// GeoReviewRecommended stores recipients for
	// `geo.review_recommended`.
	GeoReviewRecommended []string

	// GameGenerationFailed stores recipients for
	// `game.generation_failed`.
	GameGenerationFailed []string

	// LobbyRuntimePausedAfterStart stores recipients for
	// `lobby.runtime_paused_after_start`.
	LobbyRuntimePausedAfterStart []string

	// LobbyApplicationSubmitted stores recipients for public
	// `lobby.application.submitted` notifications.
	LobbyApplicationSubmitted []string
}

// Validate reports whether cfg stores valid normalized administrator email
// lists.
func (cfg AdminRoutingConfig) Validate() error {
	if err := validateNormalizedEmailList("geo.review_recommended", cfg.GeoReviewRecommended); err != nil {
		return err
	}
	if err := validateNormalizedEmailList("game.generation_failed", cfg.GameGenerationFailed); err != nil {
		return err
	}
	if err := validateNormalizedEmailList("lobby.runtime_paused_after_start", cfg.LobbyRuntimePausedAfterStart); err != nil {
		return err
	}
	if err := validateNormalizedEmailList("lobby.application.submitted", cfg.LobbyApplicationSubmitted); err != nil {
		return err
	}

	return nil
}

// TelemetryConfig configures the Notification Service OpenTelemetry runtime.
type TelemetryConfig struct {
	// ServiceName overrides the default OpenTelemetry service name.
	ServiceName string

	// TracesExporter selects the external traces exporter. Supported values are
	// `none` and `otlp`.
	TracesExporter string

	// MetricsExporter selects the external metrics exporter. Supported values
	// are `none` and `otlp`.
	MetricsExporter string

	// TracesProtocol selects the OTLP traces protocol when TracesExporter is
	// `otlp`.
	TracesProtocol string

	// MetricsProtocol selects the OTLP metrics protocol when MetricsExporter is
	// `otlp`.
	MetricsProtocol string

	// StdoutTracesEnabled enables the additional stdout trace exporter used for
	// local development and debugging.
	StdoutTracesEnabled bool

	// StdoutMetricsEnabled enables the additional stdout metric exporter used
	// for local development and debugging.
	StdoutMetricsEnabled bool
}

// Validate reports whether cfg contains a supported OpenTelemetry
// configuration.
func (cfg TelemetryConfig) Validate() error {
	return telemetry.ProcessConfig{
		ServiceName:          cfg.ServiceName,
		TracesExporter:       cfg.TracesExporter,
		MetricsExporter:      cfg.MetricsExporter,
		TracesProtocol:       cfg.TracesProtocol,
		MetricsProtocol:      cfg.MetricsProtocol,
		StdoutTracesEnabled:  cfg.StdoutTracesEnabled,
		StdoutMetricsEnabled: cfg.StdoutMetricsEnabled,
	}.Validate()
}

// DefaultConfig returns the default Notification Service process
// configuration.
func DefaultConfig() Config {
	return Config{
		ShutdownTimeout: defaultShutdownTimeout,
		Logging: LoggingConfig{
			Level: defaultLogLevel,
		},
		InternalHTTP: InternalHTTPConfig{
			Addr:              defaultInternalHTTPAddr,
			ReadHeaderTimeout: defaultReadHeaderTimeout,
			ReadTimeout:       defaultReadTimeout,
			IdleTimeout:       defaultIdleTimeout,
		},
		Redis: RedisConfig{
			DB:               defaultRedisDB,
			OperationTimeout: defaultRedisOperationTimeout,
		},
		Streams: StreamsConfig{
			Intents:                        defaultIntentsStream,
			GatewayClientEvents:            defaultGatewayClientEventsStream,
			GatewayClientEventsStreamMaxLen: defaultGatewayClientEventsStreamMaxLen,
			MailDeliveryCommands:           defaultMailDeliveryCommandsStream,
		},
		IntentsReadBlockTimeout: defaultIntentsReadBlockTimeout,
		Retry: RetryConfig{
			PushMaxAttempts:  defaultPushRetryMaxAttempts,
			EmailMaxAttempts: defaultEmailRetryMaxAttempts,
			RouteLeaseTTL:    defaultRouteLeaseTTL,
			RouteBackoffMin:  defaultRouteBackoffMin,
			RouteBackoffMax:  defaultRouteBackoffMax,
			DeadLetterTTL:    defaultDeadLetterTTL,
			RecordTTL:        defaultRecordTTL,
			IdempotencyTTL:   defaultIdempotencyTTL,
		},
		UserService: UserServiceConfig{
			Timeout: defaultUserServiceTimeout,
		},
		Telemetry: TelemetryConfig{
			ServiceName:     defaultOTelServiceName,
			TracesExporter:  otelExporterNone,
			MetricsExporter: otelExporterNone,
		},
	}
}

// LoadFromEnv loads the Notification Service process configuration from
// environment variables, applying documented defaults where appropriate.
func LoadFromEnv() (Config, error) {
	cfg := DefaultConfig()

	var err error

	cfg.ShutdownTimeout, err = loadDurationEnvWithDefault(shutdownTimeoutEnvVar, cfg.ShutdownTimeout)
	if err != nil {
		return Config{}, fmt.Errorf("load notification config: %w", err)
	}

	cfg.Logging.Level = loadStringEnvWithDefault(logLevelEnvVar, cfg.Logging.Level)
	if err := validateLogLevel(cfg.Logging.Level); err != nil {
		return Config{}, fmt.Errorf("load notification config: %s: %w", logLevelEnvVar, err)
	}

	cfg.InternalHTTP.Addr = loadStringEnvWithDefault(internalHTTPAddrEnvVar, cfg.InternalHTTP.Addr)
	cfg.InternalHTTP.ReadHeaderTimeout, err = loadDurationEnvWithDefault(internalHTTPReadHeaderTimeoutEnvVar, cfg.InternalHTTP.ReadHeaderTimeout)
	if err != nil {
		return Config{}, fmt.Errorf("load notification config: %w", err)
	}
	cfg.InternalHTTP.ReadTimeout, err = loadDurationEnvWithDefault(internalHTTPReadTimeoutEnvVar, cfg.InternalHTTP.ReadTimeout)
	if err != nil {
		return Config{}, fmt.Errorf("load notification config: %w", err)
	}
	cfg.InternalHTTP.IdleTimeout, err = loadDurationEnvWithDefault(internalHTTPIdleTimeoutEnvVar, cfg.InternalHTTP.IdleTimeout)
	if err != nil {
		return Config{}, fmt.Errorf("load notification config: %w", err)
	}

	cfg.Redis.Addr = loadStringEnvWithDefault(redisAddrEnvVar, cfg.Redis.Addr)
	cfg.Redis.Username = os.Getenv(redisUsernameEnvVar)
	cfg.Redis.Password = os.Getenv(redisPasswordEnvVar)
	cfg.Redis.DB, err = loadIntEnvWithDefault(redisDBEnvVar, cfg.Redis.DB)
	if err != nil {
		return Config{}, fmt.Errorf("load notification config: %w", err)
	}
	cfg.Redis.TLSEnabled, err = loadBoolEnvWithDefault(redisTLSEnabledEnvVar, cfg.Redis.TLSEnabled)
	if err != nil {
		return Config{}, fmt.Errorf("load notification config: %w", err)
	}
	cfg.Redis.OperationTimeout, err = loadDurationEnvWithDefault(redisOperationTimeoutEnvVar, cfg.Redis.OperationTimeout)
	if err != nil {
		return Config{}, fmt.Errorf("load notification config: %w", err)
	}

	cfg.Streams.Intents = loadStringEnvWithDefault(intentsStreamEnvVar, cfg.Streams.Intents)
	cfg.Streams.GatewayClientEvents = loadStringEnvWithDefault(gatewayClientEventsStreamEnvVar, cfg.Streams.GatewayClientEvents)
	cfg.Streams.GatewayClientEventsStreamMaxLen, err = loadInt64EnvWithDefault(gatewayClientEventsStreamMaxEnvVar, cfg.Streams.GatewayClientEventsStreamMaxLen)
	if err != nil {
		return Config{}, fmt.Errorf("load notification config: %w", err)
	}
	cfg.Streams.MailDeliveryCommands = loadStringEnvWithDefault(mailDeliveryCommandsStreamEnvVar, cfg.Streams.MailDeliveryCommands)
	cfg.IntentsReadBlockTimeout, err = loadDurationEnvWithDefault(intentsReadBlockTimeoutEnvVar, cfg.IntentsReadBlockTimeout)
	if err != nil {
		return Config{}, fmt.Errorf("load notification config: %w", err)
	}

	cfg.Retry.PushMaxAttempts, err = loadIntEnvWithDefault(pushRetryMaxAttemptsEnvVar, cfg.Retry.PushMaxAttempts)
	if err != nil {
		return Config{}, fmt.Errorf("load notification config: %w", err)
	}
	cfg.Retry.EmailMaxAttempts, err = loadIntEnvWithDefault(emailRetryMaxAttemptsEnvVar, cfg.Retry.EmailMaxAttempts)
	if err != nil {
		return Config{}, fmt.Errorf("load notification config: %w", err)
	}
	cfg.Retry.RouteLeaseTTL, err = loadDurationEnvWithDefault(routeLeaseTTLEnvVar, cfg.Retry.RouteLeaseTTL)
	if err != nil {
		return Config{}, fmt.Errorf("load notification config: %w", err)
	}
	cfg.Retry.RouteBackoffMin, err = loadDurationEnvWithDefault(routeBackoffMinEnvVar, cfg.Retry.RouteBackoffMin)
	if err != nil {
		return Config{}, fmt.Errorf("load notification config: %w", err)
	}
	cfg.Retry.RouteBackoffMax, err = loadDurationEnvWithDefault(routeBackoffMaxEnvVar, cfg.Retry.RouteBackoffMax)
	if err != nil {
		return Config{}, fmt.Errorf("load notification config: %w", err)
	}
	cfg.Retry.DeadLetterTTL, err = loadDurationEnvWithDefault(deadLetterTTLEnvVar, cfg.Retry.DeadLetterTTL)
	if err != nil {
		return Config{}, fmt.Errorf("load notification config: %w", err)
	}
	cfg.Retry.RecordTTL, err = loadDurationEnvWithDefault(recordTTLEnvVar, cfg.Retry.RecordTTL)
	if err != nil {
		return Config{}, fmt.Errorf("load notification config: %w", err)
	}
	cfg.Retry.IdempotencyTTL, err = loadDurationEnvWithDefault(idempotencyTTLEnvVar, cfg.Retry.IdempotencyTTL)
	if err != nil {
		return Config{}, fmt.Errorf("load notification config: %w", err)
	}

	cfg.UserService.BaseURL = normalizeBaseURL(loadStringEnvWithDefault(userServiceBaseURLEnvVar, cfg.UserService.BaseURL))
	cfg.UserService.Timeout, err = loadDurationEnvWithDefault(userServiceTimeoutEnvVar, cfg.UserService.Timeout)
	if err != nil {
		return Config{}, fmt.Errorf("load notification config: %w", err)
	}

	cfg.AdminRouting.GeoReviewRecommended, err = loadEmailListEnv(adminEmailsGeoReviewRecommendedEnvVar, cfg.AdminRouting.GeoReviewRecommended)
	if err != nil {
		return Config{}, fmt.Errorf("load notification config: %w", err)
	}
	cfg.AdminRouting.GameGenerationFailed, err = loadEmailListEnv(adminEmailsGameGenerationFailedEnvVar, cfg.AdminRouting.GameGenerationFailed)
	if err != nil {
		return Config{}, fmt.Errorf("load notification config: %w", err)
	}
	cfg.AdminRouting.LobbyRuntimePausedAfterStart, err = loadEmailListEnv(adminEmailsLobbyRuntimePausedAfterEnvVar, cfg.AdminRouting.LobbyRuntimePausedAfterStart)
	if err != nil {
		return Config{}, fmt.Errorf("load notification config: %w", err)
	}
	cfg.AdminRouting.LobbyApplicationSubmitted, err = loadEmailListEnv(adminEmailsLobbyApplicationSubmittedEnvVar, cfg.AdminRouting.LobbyApplicationSubmitted)
	if err != nil {
		return Config{}, fmt.Errorf("load notification config: %w", err)
	}

	cfg.Telemetry.ServiceName = loadStringEnvWithDefault(otelServiceNameEnvVar, cfg.Telemetry.ServiceName)
	cfg.Telemetry.TracesExporter = normalizeExporterValue(loadStringEnvWithDefault(otelTracesExporterEnvVar, cfg.Telemetry.TracesExporter))
	cfg.Telemetry.MetricsExporter = normalizeExporterValue(loadStringEnvWithDefault(otelMetricsExporterEnvVar, cfg.Telemetry.MetricsExporter))
	cfg.Telemetry.TracesProtocol = loadOTLPProtocol(
		os.Getenv(otelExporterOTLPTracesProtocolEnvVar),
		os.Getenv(otelExporterOTLPProtocolEnvVar),
		cfg.Telemetry.TracesExporter,
	)
	cfg.Telemetry.MetricsProtocol = loadOTLPProtocol(
		os.Getenv(otelExporterOTLPMetricsProtocolEnvVar),
		os.Getenv(otelExporterOTLPProtocolEnvVar),
		cfg.Telemetry.MetricsExporter,
	)
	cfg.Telemetry.StdoutTracesEnabled, err = loadBoolEnvWithDefault(otelStdoutTracesEnabledEnvVar, cfg.Telemetry.StdoutTracesEnabled)
	if err != nil {
		return Config{}, fmt.Errorf("load notification config: %w", err)
	}
	cfg.Telemetry.StdoutMetricsEnabled, err = loadBoolEnvWithDefault(otelStdoutMetricsEnabledEnvVar, cfg.Telemetry.StdoutMetricsEnabled)
	if err != nil {
		return Config{}, fmt.Errorf("load notification config: %w", err)
	}

	if err := cfg.Validate(); err != nil {
		return Config{}, err
	}

	return cfg, nil
}

// Validate reports whether cfg contains a consistent Notification Service
// process configuration.
func (cfg Config) Validate() error {
	switch {
	case cfg.ShutdownTimeout <= 0:
		return fmt.Errorf("load notification config: %s must be positive", shutdownTimeoutEnvVar)
	case strings.TrimSpace(cfg.Redis.Addr) == "":
		return fmt.Errorf("load notification config: %s must not be empty", redisAddrEnvVar)
	case strings.TrimSpace(cfg.UserService.BaseURL) == "":
		return fmt.Errorf("load notification config: %s must not be empty", userServiceBaseURLEnvVar)
	}

	if err := cfg.InternalHTTP.Validate(); err != nil {
		return fmt.Errorf("load notification config: %s", err)
	}
	if err := cfg.Redis.Validate(); err != nil {
		return fmt.Errorf("load notification config: %s", err)
	}
	if err := cfg.Streams.Validate(); err != nil {
		return fmt.Errorf("load notification config: %s", err)
	}
	if cfg.IntentsReadBlockTimeout <= 0 {
		return fmt.Errorf("load notification config: %s must be positive", intentsReadBlockTimeoutEnvVar)
	}
	if err := cfg.Retry.Validate(); err != nil {
		return fmt.Errorf("load notification config: %s", err)
	}
	if err := cfg.UserService.Validate(); err != nil {
		return fmt.Errorf("load notification config: %s", err)
	}
	if err := cfg.AdminRouting.Validate(); err != nil {
		return fmt.Errorf("load notification config: %s", err)
	}
	if err := cfg.Telemetry.Validate(); err != nil {
		return fmt.Errorf("load notification config: %w", err)
	}

	return nil
}

func loadStringEnvWithDefault(name string, value string) string {
	if raw, ok := os.LookupEnv(name); ok {
		return strings.TrimSpace(raw)
	}

	return value
}

func loadDurationEnvWithDefault(name string, value time.Duration) (time.Duration, error) {
	raw, ok := os.LookupEnv(name)
	if !ok {
		return value, nil
	}

	parsed, err := time.ParseDuration(strings.TrimSpace(raw))
	if err != nil {
		return 0, fmt.Errorf("%s: %w", name, err)
	}

	return parsed, nil
}

func loadIntEnvWithDefault(name string, value int) (int, error) {
	raw, ok := os.LookupEnv(name)
	if !ok {
		return value, nil
	}

	parsed, err := strconv.Atoi(strings.TrimSpace(raw))
	if err != nil {
		return 0, fmt.Errorf("%s: %w", name, err)
	}

	return parsed, nil
}

func loadInt64EnvWithDefault(name string, value int64) (int64, error) {
	raw, ok := os.LookupEnv(name)
	if !ok {
		return value, nil
	}

	parsed, err := strconv.ParseInt(strings.TrimSpace(raw), 10, 64)
	if err != nil {
		return 0, fmt.Errorf("%s: %w", name, err)
	}

	return parsed, nil
}

func loadBoolEnvWithDefault(name string, value bool) (bool, error) {
	raw, ok := os.LookupEnv(name)
	if !ok {
		return value, nil
	}

	parsed, err := strconv.ParseBool(strings.TrimSpace(raw))
	if err != nil {
		return false, fmt.Errorf("%s: %w", name, err)
	}

	return parsed, nil
}

func loadEmailListEnv(name string, value []string) ([]string, error) {
	raw, ok := os.LookupEnv(name)
	if !ok {
		return append([]string(nil), value...), nil
	}

	return parseEmailList(name, raw)
}

func parseEmailList(name string, raw string) ([]string, error) {
	trimmed := strings.TrimSpace(raw)
	if trimmed == "" {
		return nil, nil
	}

	parts := strings.Split(trimmed, ",")
	addresses := make([]string, 0, len(parts))
	seen := make(map[string]struct{}, len(parts))
	for index, part := range parts {
		normalized, err := normalizeMailboxAddress(part)
		if err != nil {
			return nil, fmt.Errorf("%s[%d]: %w", name, index, err)
		}
		if _, ok := seen[normalized]; ok {
			continue
		}
		seen[normalized] = struct{}{}
		addresses = append(addresses, normalized)
	}

	return addresses, nil
}

func normalizeMailboxAddress(value string) (string, error) {
	trimmed := strings.TrimSpace(value)
	if trimmed == "" {
		return "", fmt.Errorf("email address must not be empty")
	}

	parsed, err := netmail.ParseAddress(trimmed)
	if err != nil {
		return "", fmt.Errorf("invalid email address %q: %w", trimmed, err)
	}
	if parsed.Name != "" {
		return "", fmt.Errorf("email address %q must not include a display name", trimmed)
	}

	return strings.ToLower(parsed.Address), nil
}

func validateNormalizedEmailList(name string, values []string) error {
	for index, value := range values {
		normalized, err := normalizeMailboxAddress(value)
		if err != nil {
			return fmt.Errorf("%s[%d]: %w", name, index, err)
		}
		if normalized != value {
			return fmt.Errorf("%s[%d]: email address must already be normalized", name, index)
		}
	}

	return nil
}

func validateLogLevel(value string) error {
	var level slog.Level
	return level.UnmarshalText([]byte(strings.TrimSpace(value)))
}

func normalizeExporterValue(value string) string {
	switch strings.TrimSpace(value) {
	case "", otelExporterNone:
		return otelExporterNone
	default:
		return strings.TrimSpace(value)
	}
}

func loadOTLPProtocol(primary string, fallback string, exporter string) string {
	protocol := strings.TrimSpace(primary)
	if protocol == "" {
		protocol = strings.TrimSpace(fallback)
	}
	if protocol == "" && exporter == otelExporterOTLP {
		return otelProtocolHTTPProtobuf
	}

	return protocol
}

func normalizeBaseURL(value string) string {
	trimmed := strings.TrimSpace(value)
	if trimmed == "" {
		return ""
	}

	return strings.TrimRight(trimmed, "/")
}

func isAbsoluteHTTPURL(value string) bool {
	parsed, err := url.Parse(strings.TrimSpace(value))
	if err != nil {
		return false
	}

	if parsed.Scheme != "http" && parsed.Scheme != "https" {
		return false
	}

	return parsed.Host != ""
}

func isTCPAddr(value string) bool {
	host, port, err := net.SplitHostPort(strings.TrimSpace(value))
	if err != nil {
		return false
	}

	if port == "" {
		return false
	}
	if host == "" {
		return true
	}

	return true
}