// Package config loads runtime settings from the environment.
//
// Environment variables rather than a config file, for two reasons: the DSN
// differs per machine and you are syncing the repo across two, and a file with
// credentials in it is one `git add .` away from being public. The bot uses
// config.json because its token is the only secret; the backend holds database
// credentials and key material.
package config

import (
	"errors"
	"fmt"
	"log/slog"
	"os"
	"strconv"
	"strings"
	"time"
)

type Config struct {
	// Addr is the listen address. Bind to localhost in production and put a
	// reverse proxy in front for TLS.
	Addr string

	// DSN for Postgres. Required.
	DSN string

	// MaxConns caps the pgx pool. Postgres forks a backend per connection, so
	// this is a real resource, not a client-side nicety.
	MaxConns int32

	// RedisAddr enables the shared rate limiter. Empty falls back to an
	// in-process limiter, which is correct for a single instance and wrong for
	// several - each would keep its own counters.
	RedisAddr string
	RedisDB   int

	// EndUserHeader carries the caller's identifier for whoever triggered the
	// request, for per-user rate limiting. Empty disables it.
	EndUserHeader string

	LogLevel slog.Level
	LogJSON  bool

	ReadHeaderTimeout time.Duration
	ShutdownTimeout   time.Duration
}

func Load() (Config, error) {
	c := Config{
		Addr:              env("HAMMY_ADDR", "127.0.0.1:8080"),
		DSN:               os.Getenv("HAMMY_DSN"),
		RedisAddr:         os.Getenv("HAMMY_REDIS_ADDR"),
		EndUserHeader:     env("HAMMY_ENDUSER_HEADER", "X-Hammy-User"),
		ReadHeaderTimeout: 10 * time.Second,
		ShutdownTimeout:   20 * time.Second,
		LogJSON:           boolEnv("HAMMY_LOG_JSON", false),
	}

	if c.DSN == "" {
		return Config{}, errors.New("config: HAMMY_DSN is not set")
	}

	n, err := intEnv("HAMMY_MAX_CONNS", 10)
	if err != nil {
		return Config{}, err
	}
	c.MaxConns = int32(n)

	if c.RedisDB, err = intEnv("HAMMY_REDIS_DB", 0); err != nil {
		return Config{}, err
	}

	if c.LogLevel, err = levelEnv("HAMMY_LOG_LEVEL", slog.LevelInfo); err != nil {
		return Config{}, err
	}

	if c.ReadHeaderTimeout, err = durEnv("HAMMY_READ_HEADER_TIMEOUT", c.ReadHeaderTimeout); err != nil {
		return Config{}, err
	}

	if c.ShutdownTimeout, err = durEnv("HAMMY_SHUTDOWN_TIMEOUT", c.ShutdownTimeout); err != nil {
		return Config{}, err
	}

	return c, nil
}

// Redacted is safe to log at startup. The DSN carries a password.
func (c Config) Redacted() string {
	redis := c.RedisAddr
	if redis == "" {
		redis = "(in-process limiter)"
	}

	return fmt.Sprintf("addr=%s db=%s redis=%s max_conns=%d level=%s",
		c.Addr, redactDSN(c.DSN), redis, c.MaxConns, c.LogLevel)
}

// redactDSN keeps the shape of the connection string and drops the password.
func redactDSN(dsn string) string {
	at := strings.LastIndex(dsn, "@")
	if at < 0 {
		return "(set)"
	}

	scheme := strings.Index(dsn, "://")
	if scheme < 0 {
		return "(set)"
	}

	creds := dsn[scheme+3 : at]

	user := creds
	if colon := strings.Index(creds, ":"); colon >= 0 {
		user = creds[:colon]
	}

	return dsn[:scheme+3] + user + ":***" + dsn[at:]
}

func env(key, def string) string {
	if v := os.Getenv(key); v != "" {
		return v
	}

	return def
}

func intEnv(key string, def int) (int, error) {
	v := os.Getenv(key)
	if v == "" {
		return def, nil
	}

	n, err := strconv.Atoi(v)
	if err != nil {
		return 0, fmt.Errorf("config: %s: %w", key, err)
	}

	return n, nil
}

func boolEnv(key string, def bool) bool {
	v := os.Getenv(key)
	if v == "" {
		return def
	}

	b, err := strconv.ParseBool(v)
	if err != nil {
		return def
	}

	return b
}

func durEnv(key string, def time.Duration) (time.Duration, error) {
	v := os.Getenv(key)
	if v == "" {
		return def, nil
	}

	d, err := time.ParseDuration(v)
	if err != nil {
		return 0, fmt.Errorf("config: %s: %w", key, err)
	}

	return d, nil
}

func levelEnv(key string, def slog.Level) (slog.Level, error) {
	v := os.Getenv(key)
	if v == "" {
		return def, nil
	}

	var l slog.Level
	if err := l.UnmarshalText([]byte(v)); err != nil {
		return 0, fmt.Errorf("config: %s: %w", key, err)
	}

	return l, nil
}