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

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

	"scrabble/backend/internal/postgres"
	"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
	// 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
}

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

	c := Config{
		HTTPAddr:  envOr("BACKEND_HTTP_ADDR", defaultHTTPAddr),
		LogLevel:  envOr("BACKEND_LOG_LEVEL", defaultLogLevel),
		Postgres:  pg,
		Telemetry: tel,
	}
	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 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)
	}
	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
}