package config

import (
	"errors"
	"flag"
	"fmt"
	"io"
	"net"
	"os"
	"strings"
)

// Config holds all runtime configuration. Each field can be set via a
// command-line flag (--<name>) or environment variable (ZDDC_<NAME>);
// the flag takes precedence when both are present.
type Config struct {
	Root        string   // --root / ZDDC_ROOT — served file tree (default: CWD)
	Addr        string   // --addr / ZDDC_ADDR — bind address (default :8443)
	TLSCert     string   // --tls-cert / ZDDC_TLS_CERT — PEM cert path; "none" = plain HTTP; empty = self-signed
	TLSKey      string   // --tls-key / ZDDC_TLS_KEY — PEM key path
	TLSMode     string   // computed: none/selfsigned/provided
	LogLevel    string   // --log-level / ZDDC_LOG_LEVEL — debug/info/warn/error (default info)
	IndexPath   string   // --index-path / ZDDC_INDEX_PATH — virtual archive prefix (default .archive)
	EmailHeader string   // --email-header / ZDDC_EMAIL_HEADER — auth header name (default X-Auth-Request-Email)
	CORSOrigins []string // --cors-origin / ZDDC_CORS_ORIGIN — comma-separated allowlist; default https://zddc.varasys.io; empty disables
	AccessLog   string   // --access-log / ZDDC_ACCESS_LOG — file path for tee'd JSON access log; empty = stderr only
}

// ErrHelpRequested is returned by Load when --help is passed; the caller
// should print Usage() to stderr and exit 0.
var ErrHelpRequested = errors.New("help requested")

// ErrVersionRequested is returned by Load when --version is passed; the
// caller should print version info and exit 0.
var ErrVersionRequested = errors.New("version requested")

// Load reads configuration from CLI flags + environment variables.
//
// Precedence (highest → lowest): command-line flag, environment variable,
// hard-coded default. Special-cases:
//   - --root / ZDDC_ROOT default to the current working directory if both
//     are unset, so an operator can `cd /srv/zddc && zddc-server` with
//     zero config.
//   - --version and --help return distinguished sentinel errors; the caller
//     handles printing and exit. Pass nil for args to skip flag parsing
//     entirely (used by tests that set state via env vars only).
//
// Standard usage from main.go:
//
//	cfg, err := config.Load(os.Args[1:])
//	if errors.Is(err, config.ErrHelpRequested) { os.Exit(0) }
//	if errors.Is(err, config.ErrVersionRequested) { /* print versions */ ; os.Exit(0) }
//	if err != nil { ... }
func Load(args []string) (Config, error) {
	fs := flag.NewFlagSet("zddc-server", flag.ContinueOnError)
	// Discard flag's own error output; we wrap and return our own.
	fs.SetOutput(io.Discard)

	rootFlag := fs.String("root", os.Getenv("ZDDC_ROOT"),
		"Path to the served file tree. Default: ZDDC_ROOT or the current directory.")
	addrFlag := fs.String("addr", getEnv("ZDDC_ADDR", ":8443"),
		"Listen address (host:port). Default: ZDDC_ADDR or :8443.")
	tlsCertFlag := fs.String("tls-cert", os.Getenv("ZDDC_TLS_CERT"),
		"Path to a PEM TLS certificate. \"none\" disables TLS (plain HTTP). Empty means self-signed.")
	tlsKeyFlag := fs.String("tls-key", os.Getenv("ZDDC_TLS_KEY"),
		"Path to the matching PEM TLS private key.")
	logLevelFlag := fs.String("log-level", getEnv("ZDDC_LOG_LEVEL", "info"),
		"Log level: debug, info, warn, error.")
	indexPathFlag := fs.String("index-path", getEnv("ZDDC_INDEX_PATH", ".archive"),
		"URL segment that triggers the virtual archive index (default \".archive\").")
	emailHeaderFlag := fs.String("email-header", getEnv("ZDDC_EMAIL_HEADER", "X-Auth-Request-Email"),
		"HTTP header carrying the authenticated user's email.")
	corsOriginFlag := fs.String("cors-origin", "",
		"Comma-separated CORS allowlist. Empty = CORS disabled. Default: ZDDC_CORS_ORIGIN or https://zddc.varasys.io.")
	insecureDirectFlag := fs.Bool("insecure-direct", os.Getenv("ZDDC_INSECURE_DIRECT") == "1",
		"Allow plain HTTP on non-loopback addresses (only safe behind an authenticating proxy).")
	accessLogFlag := fs.String("access-log", os.Getenv("ZDDC_ACCESS_LOG"),
		"Tee structured access logs to this file (JSON, size-rotated). Empty = stderr only.")
	helpFlag := fs.Bool("help", false, "Print this help and exit.")
	versionFlag := fs.Bool("version", false, "Print version info and exit.")

	if args != nil {
		if err := fs.Parse(args); err != nil {
			if errors.Is(err, flag.ErrHelp) {
				return Config{}, ErrHelpRequested
			}
			return Config{}, err
		}
	}
	if *helpFlag {
		return Config{}, ErrHelpRequested
	}
	if *versionFlag {
		return Config{}, ErrVersionRequested
	}

	// CORS has special semantics: "unset" → default origin list; "set to
	// empty" → CORS disabled. The flag default is "" so we can't tell unset
	// from explicit-empty via the flag alone — fs.Visit catches explicit
	// flag use, and os.LookupEnv catches explicit env-var use.
	corsFlagSet := false
	if args != nil {
		fs.Visit(func(f *flag.Flag) {
			if f.Name == "cors-origin" {
				corsFlagSet = true
			}
		})
	}

	cfg := Config{
		Root:        *rootFlag,
		Addr:        *addrFlag,
		TLSCert:     *tlsCertFlag,
		TLSKey:      *tlsKeyFlag,
		LogLevel:    *logLevelFlag,
		IndexPath:   *indexPathFlag,
		EmailHeader: *emailHeaderFlag,
		CORSOrigins: resolveCORS(corsFlagSet, *corsOriginFlag),
		AccessLog:   *accessLogFlag,
	}

	// Default Root to the current working directory.
	if cfg.Root == "" {
		cwd, err := os.Getwd()
		if err != nil {
			return Config{}, fmt.Errorf("--root not set and could not determine current directory: %w", err)
		}
		cfg.Root = cwd
	}

	info, err := os.Stat(cfg.Root)
	if err != nil {
		return Config{}, fmt.Errorf("--root %q is not accessible: %w", cfg.Root, err)
	}
	if !info.IsDir() {
		return Config{}, fmt.Errorf("--root %q is not a directory", cfg.Root)
	}

	// Determine TLS mode.
	switch {
	case cfg.TLSCert == "none":
		cfg.TLSMode = "none"
	case cfg.TLSCert == "" && cfg.TLSKey == "":
		cfg.TLSMode = "selfsigned"
	default:
		cfg.TLSMode = "provided"
	}

	if cfg.TLSMode == "provided" && (cfg.TLSCert == "") != (cfg.TLSKey == "") {
		return Config{}, errors.New("--tls-cert and --tls-key must both be set or both be empty")
	}

	// Plain HTTP mode trusts the email header from any client. Only safe
	// behind an authenticating reverse proxy. Refuse to start when binding
	// plain HTTP to a non-loopback interface unless the operator has
	// explicitly acknowledged the deployment shape.
	if cfg.TLSMode == "none" && !isLoopbackAddr(cfg.Addr) && !*insecureDirectFlag {
		return Config{}, fmt.Errorf(
			"--tls-cert=none binds plain HTTP to %q which trusts %s headers from any client; "+
				"either use TLS (omit --tls-cert or supply a cert), bind to loopback (127.0.0.1: or [::1]:), "+
				"or pass --insecure-direct to confirm an authenticating reverse proxy is in front",
			cfg.Addr, cfg.EmailHeader)
	}

	return cfg, nil
}

// Usage prints the flag list to w (stderr is the conventional caller).
// Returned format mirrors `flag.PrintDefaults` plus a one-line summary.
func Usage(w io.Writer) {
	fmt.Fprintln(w, "Usage: zddc-server [flags]")
	fmt.Fprintln(w, "")
	fmt.Fprintln(w, "Each flag has an equivalent ZDDC_* environment variable; the flag wins on conflict.")
	fmt.Fprintln(w, "ZDDC_ROOT defaults to the current working directory.")
	fmt.Fprintln(w, "")
	fmt.Fprintln(w, "Flags:")
	fs := flag.NewFlagSet("zddc-server", flag.ContinueOnError)
	fs.SetOutput(w)
	// Re-register flags to populate Usage output (we discard the values).
	fs.String("root", "", "Path to the served file tree. Default: ZDDC_ROOT or the current directory.")
	fs.String("addr", ":8443", "Listen address (host:port). Default: ZDDC_ADDR or :8443.")
	fs.String("tls-cert", "", "Path to a PEM TLS certificate. \"none\" disables TLS. Empty = self-signed.")
	fs.String("tls-key", "", "Path to the matching PEM TLS private key.")
	fs.String("log-level", "info", "Log level: debug, info, warn, error.")
	fs.String("index-path", ".archive", "URL segment for the virtual archive index.")
	fs.String("email-header", "X-Auth-Request-Email", "HTTP header carrying the authenticated user's email.")
	fs.String("cors-origin", "", "Comma-separated CORS allowlist. Empty = CORS disabled.")
	fs.Bool("insecure-direct", false, "Allow plain HTTP on non-loopback addresses.")
	fs.String("access-log", "", "Tee structured access logs to this file (JSON, size-rotated). Empty = stderr only.")
	fs.Bool("help", false, "Print this help and exit.")
	fs.Bool("version", false, "Print version info and exit.")
	fs.PrintDefaults()
}

// resolveCORS implements the precedence rules for the CORS allowlist:
//   - flag explicitly set → use flag value (empty = disabled)
//   - else env var explicitly set → use env value (empty = disabled)
//   - else → default to the canonical upstream
func resolveCORS(flagSet bool, flagValue string) []string {
	if flagSet {
		return parseCSV(flagValue)
	}
	if v, ok := os.LookupEnv("ZDDC_CORS_ORIGIN"); ok {
		return parseCSV(v)
	}
	return []string{"https://zddc.varasys.io"}
}

// parseCSV splits a comma-separated list and trims whitespace. Empty
// returns nil (which the middleware treats as "CORS disabled").
func parseCSV(s string) []string {
	if s == "" {
		return nil
	}
	parts := strings.Split(s, ",")
	out := make([]string, 0, len(parts))
	for _, p := range parts {
		if t := strings.TrimSpace(p); t != "" {
			out = append(out, t)
		}
	}
	return out
}

// isLoopbackAddr reports whether addr binds only to a loopback interface.
// addr is in net.Listen form: "host:port", ":port", or "[ipv6]:port".
// ":port" means all interfaces, so it is NOT loopback.
func isLoopbackAddr(addr string) bool {
	host, _, err := net.SplitHostPort(addr)
	if err != nil {
		return false
	}
	if host == "" {
		return false
	}
	if host == "localhost" {
		return true
	}
	ip := net.ParseIP(host)
	if ip == nil {
		return false
	}
	return ip.IsLoopback()
}

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