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

import (
	"fmt"
	"os"
)

// 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
}

// 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) {
	c := Config{
		HTTPAddr: envOr("BACKEND_HTTP_ADDR", defaultHTTPAddr),
		LogLevel: envOr("BACKEND_LOG_LEVEL", defaultLogLevel),
	}
	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")
	}
	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
}