// Package config holds two distinct kinds of configuration.
//
// Bootstrap holds the startup-critical values that must be known before the
// database is open: where the database lives, which addresses to listen on and
// the initial administrator. It comes from CLI flags and environment
// variables.
//
// Settings holds everything else. It lives in SQLite, is editable from the web
// UI and can mostly be changed without restarting.
package config

import (
	"errors"
	"flag"
	"fmt"
	"net"
	"os"
	"path/filepath"
	"strconv"
	"strings"
)

// Bootstrap is the startup configuration.
type Bootstrap struct {
	DBPath        string
	HTTPAddr      string
	DNSUDPAddr    string
	DNSTCPAddr    string
	AdminUsername string
	AdminPassword string
	LogLevel      string
	LogFormat     string

	// dnsAddrSet records whether --dns was given, so that a stored setting is
	// only overridden when the operator explicitly asked for it.
	dnsAddrSet  bool
	httpAddrSet bool
}

// Default values used when neither a flag nor an environment variable is set.
const (
	DefaultDBPath   = "./data/dns.db"
	DefaultHTTPAddr = "127.0.0.1:8080"
	DefaultDNSAddr  = "0.0.0.0:53"
)

// Environment variable names.
const (
	EnvDBPath        = "VIBEDNS_DB_PATH"
	EnvHTTPAddr      = "VIBEDNS_HTTP_ADDR"
	EnvDNSAddr       = "VIBEDNS_DNS_ADDR"
	EnvAdminUsername = "VIBEDNS_ADMIN_USERNAME"
	EnvAdminPassword = "VIBEDNS_ADMIN_PASSWORD"
	EnvLogLevel      = "VIBEDNS_LOG_LEVEL"
	EnvLogFormat     = "VIBEDNS_LOG_FORMAT"
)

// DefaultBootstrap returns the built-in defaults with environment overrides
// applied.
func DefaultBootstrap() Bootstrap {
	b := Bootstrap{
		DBPath:        envOr(EnvDBPath, DefaultDBPath),
		HTTPAddr:      envOr(EnvHTTPAddr, DefaultHTTPAddr),
		AdminUsername: envOr(EnvAdminUsername, "admin"),
		AdminPassword: os.Getenv(EnvAdminPassword),
		LogLevel:      envOr(EnvLogLevel, "info"),
		LogFormat:     envOr(EnvLogFormat, "text"),
	}
	dnsAddr := envOr(EnvDNSAddr, DefaultDNSAddr)
	b.DNSUDPAddr = dnsAddr
	b.DNSTCPAddr = dnsAddr
	if _, ok := os.LookupEnv(EnvDNSAddr); ok {
		b.dnsAddrSet = true
	}
	if _, ok := os.LookupEnv(EnvHTTPAddr); ok {
		b.httpAddrSet = true
	}
	return b
}

// BindFlags registers the bootstrap flags on fs.
func (b *Bootstrap) BindFlags(fs *flag.FlagSet) {
	fs.StringVar(&b.DBPath, "db", b.DBPath, "path to the SQLite database file")
	fs.StringVar(&b.HTTPAddr, "http", b.HTTPAddr, "management HTTP listen address")
	fs.StringVar(&b.DNSUDPAddr, "dns", b.DNSUDPAddr, "DNS listen address for both UDP and TCP")
	fs.StringVar(&b.LogLevel, "log-level", b.LogLevel, "log level: debug, info, warn, error")
	fs.StringVar(&b.LogFormat, "log-format", b.LogFormat, "log format: text or json")
	fs.StringVar(&b.AdminUsername, "admin-username", b.AdminUsername,
		"administrator username created on first run")
}

// NoteFlagsSet records which addressing flags were explicitly provided so that
// stored settings are respected otherwise.
func (b *Bootstrap) NoteFlagsSet(fs *flag.FlagSet) {
	fs.Visit(func(f *flag.Flag) {
		switch f.Name {
		case "dns":
			b.dnsAddrSet = true
			b.DNSTCPAddr = b.DNSUDPAddr
		case "http":
			b.httpAddrSet = true
		}
	})
}

// DNSAddrOverridden reports whether the DNS listen address was given on the
// command line or in the environment.
func (b Bootstrap) DNSAddrOverridden() bool { return b.dnsAddrSet }

// HTTPAddrOverridden reports whether the HTTP listen address was overridden.
func (b Bootstrap) HTTPAddrOverridden() bool { return b.httpAddrSet }

// Validate checks the bootstrap configuration and returns an actionable error.
func (b Bootstrap) Validate() error {
	if strings.TrimSpace(b.DBPath) == "" {
		return errors.New("database path must not be empty")
	}
	if !filepath.IsAbs(b.DBPath) {
		if _, err := filepath.Abs(b.DBPath); err != nil {
			return fmt.Errorf("database path %q cannot be resolved: %w", b.DBPath, err)
		}
	}
	for label, addr := range map[string]string{
		"management HTTP address": b.HTTPAddr,
		"DNS UDP address":         b.DNSUDPAddr,
		"DNS TCP address":         b.DNSTCPAddr,
	} {
		if err := validateListenAddr(addr); err != nil {
			return fmt.Errorf("%s: %w", label, err)
		}
	}
	switch strings.ToLower(b.LogLevel) {
	case "debug", "info", "warn", "error":
	default:
		return fmt.Errorf("log level %q must be one of debug, info, warn, error", b.LogLevel)
	}
	switch strings.ToLower(b.LogFormat) {
	case "text", "json":
	default:
		return fmt.Errorf("log format %q must be text or json", b.LogFormat)
	}
	if b.AdminUsername != "" {
		if err := ValidateUsername(b.AdminUsername); err != nil {
			return err
		}
	}
	return nil
}

// validateListenAddr accepts "host:port" with an optional empty host.
func validateListenAddr(addr string) error {
	if strings.TrimSpace(addr) == "" {
		return errors.New("must not be empty")
	}
	host, port, err := net.SplitHostPort(addr)
	if err != nil {
		return fmt.Errorf("%q is not a valid host:port address", addr)
	}
	p, err := strconv.Atoi(port)
	if err != nil || p < 1 || p > 65535 {
		return fmt.Errorf("%q has an invalid port", addr)
	}
	if host != "" && net.ParseIP(host) == nil {
		// Allow host names for the HTTP listener; reject obvious nonsense.
		if strings.ContainsAny(host, " \t/\\") {
			return fmt.Errorf("%q has an invalid host", addr)
		}
	}
	return nil
}

// ValidateUsername enforces a conservative username policy.
func ValidateUsername(u string) error {
	if len(u) < 2 || len(u) > 64 {
		return errors.New("username must be between 2 and 64 characters")
	}
	for _, r := range u {
		switch {
		case r >= 'a' && r <= 'z', r >= 'A' && r <= 'Z', r >= '0' && r <= '9':
		case r == '.', r == '-', r == '_', r == '@':
		default:
			return errors.New("username may contain only letters, digits and the characters . - _ @")
		}
	}
	return nil
}

func envOr(key, def string) string {
	if v, ok := os.LookupEnv(key); ok && strings.TrimSpace(v) != "" {
		return v
	}
	return def
}