// Package config loads MyDay's configuration from `config.yaml` into a typed
// struct and validates it fail-closed (SPEC-003 §1, D-010/D-035).
//
// Settings live in the config file, not the database (D-035): infra · secrets ·
// domain · mail · locale. The real `config.yaml` is gitignored (D-010); the
// committed `config.example.yaml` is used as a development fallback so a fresh
// checkout can still start (in setup mode, loopback-only).
//
// Fail-closed rule (SPEC-003 §1): in PRODUCTIVE mode (`setup: false`) the
// process refuses to start if a required secret is missing or weak. Setup mode
// tolerates the placeholder values in the example, because the operator has not
// provisioned the instance yet — and setup mode is loopback-only (D-025).
//
// Spec: docs/specs/spec-003-core-door.md §1, §7.
package config

import (
	"bytes"
	"crypto/rand"
	"encoding/base64"
	"errors"
	"fmt"
	"io/fs"
	"os"
	"path/filepath"
	"strings"
	"time"

	"gopkg.in/yaml.v3"
)

// FileName is the productive configuration file (gitignored — D-010).
const FileName = "config.yaml"

// ExampleFileName is the committed template, used as a development fallback.
const ExampleFileName = "config.example.yaml"

// Secret-length policy. The TOTP key must be exactly totpKeyLen bytes because
// it feeds AES-256-GCM (mirrors auth.KeySize); the other secrets are signing /
// derivation keys and must be at least minSecretLen bytes.
const (
	minSecretLen = 32
	totpKeyLen   = 32
)

// Config is the typed shape of `config.yaml` / `config.example.yaml`.
type Config struct {
	// Setup forces setup mode (D-026). Together with "no user exists" it makes
	// the instance serve only /setup, loopback-only (D-025, SPEC-005 §3).
	Setup bool `yaml:"setup"`

	// AdminRecovery is the config approval flag for admin recovery (D-033).
	// Without it nothing happens (dual control).
	AdminRecovery bool `yaml:"admin_recovery"`

	Server   Server          `yaml:"server"`
	App      App             `yaml:"app"`
	Database Database        `yaml:"database"`
	Secrets  Secrets         `yaml:"secrets"`
	Mail     Mail            `yaml:"mail"`
	Modules  map[string]bool `yaml:"modules"`
	// Enrollment bounds the invite window (A1). SPEC-006 refuses enrollment
	// outside it.
	Enrollment Enrollment `yaml:"enrollment"`

	// source is the path actually read (config.yaml or the example fallback).
	source string
}

// Server holds the listener, its public base URL, and the trusted proxies.
type Server struct {
	Listen string `yaml:"listen"`
	// BaseURL is used in invite / recovery links and decides whether cookies
	// are Secure (https).
	BaseURL string `yaml:"base_url"`
	// TrustedProxies are the peers whose X-Forwarded-For is honoured (A6).
	TrustedProxies []string `yaml:"trusted_proxies"`
}

// App holds locale and timezone.
type App struct {
	Locale   string `yaml:"locale"`
	Timezone string `yaml:"timezone"`
}

// Database points at the SQLite file (D-012).
type Database struct {
	Path string `yaml:"path"`
}

// Secrets holds the symmetric key material (D-010). Never commit real values.
// The wizard generates a strong value for any field left at a placeholder
// (R-6); the startup path does the same in setup mode (M1s).
type Secrets struct {
	CookieSecret      string `yaml:"cookie_secret"`
	CSRFKey           string `yaml:"csrf_key"`
	TOTPEncryptionKey string `yaml:"totp_encryption_key"` // A9
}

// Mail holds the SMTP settings used for invites and recovery.
type Mail struct {
	Host     string `yaml:"host"`
	Port     int    `yaml:"port"`
	Username string `yaml:"username"`
	Password string `yaml:"password"`
	From     string `yaml:"from"`
}

// Enrollment holds the invite-window settings (A1).
type Enrollment struct {
	// Window is a Go duration string, e.g. "72h". Empty defaults to 72h.
	Window string `yaml:"window"`
}

// Sentinel errors.
var (
	// ErrListenRequired is returned when server.listen is empty.
	ErrListenRequired = errors.New("config: server.listen is required")
	// ErrDatabaseRequired is returned when database.path is empty.
	ErrDatabaseRequired = errors.New("config: database.path is required")
	// ErrBaseURLRequired is returned when server.base_url is empty in
	// productive mode (invite/recovery links need it).
	ErrBaseURLRequired = errors.New("config: server.base_url is required in productive mode")
	// ErrWeakSecret is returned when a required secret is missing or weak.
	ErrWeakSecret = errors.New("config: required secret is missing or weak")
	// ErrInvalidEnrollWindow is returned when enrollment.window is not a valid
	// positive Go duration.
	ErrInvalidEnrollWindow = errors.New("config: enrollment.window is not a valid duration")
)

// DefaultEnrollWindow is the code default for the bounded enrollment window
// (A1) when enrollment.window is unset.
const DefaultEnrollWindow = 72 * time.Hour

// EnrollWindow returns the configured enrollment window, defaulting to 72h.
func (c *Config) EnrollWindow() time.Duration {
	d, err := time.ParseDuration(strings.TrimSpace(c.Enrollment.Window))
	if err != nil || d <= 0 {
		return DefaultEnrollWindow
	}
	return d
}

// Load reads and validates the configuration at path.
//
// Development fallback (SPEC-003 §1): when path is the canonical `config.yaml`
// and it does not exist, the committed `config.example.yaml` in the same
// directory is used instead. Any other missing path is an error.
func Load(path string) (*Config, error) {
	used, data, err := read(path)
	if err != nil {
		return nil, err
	}

	dec := yaml.NewDecoder(bytes.NewReader(data))
	dec.KnownFields(true) // fail-closed: an unknown key is a configuration error

	var c Config
	if err := dec.Decode(&c); err != nil {
		return nil, fmt.Errorf("config: parse %s: %w", used, err)
	}
	c.source = used

	if err := c.Validate(); err != nil {
		return nil, fmt.Errorf("config: %s: %w", used, err)
	}
	return &c, nil
}

// Source returns the path the configuration was actually loaded from.
func (c *Config) Source() string { return c.source }

// Marshal renders c as the YAML document written to `config.yaml` (D-035). The
// wizard persists through it (SPEC-005 §4). The unexported source field is not
// part of the document.
func Marshal(c *Config) ([]byte, error) {
	if c == nil {
		return nil, errors.New("config: cannot marshal a nil config")
	}
	body, err := yaml.Marshal(c)
	if err != nil {
		return nil, fmt.Errorf("config: marshal: %w", err)
	}
	const header = "# MyDay configuration — written by MyDay (the /setup wizard or the Modules page, D-035).\n" +
		"# Settings live HERE, not in the database. This file holds SECRETS:\n" +
		"# it is gitignored — never commit it (D-010).\n\n"
	return append([]byte(header), body...), nil
}

// IsSecure reports whether the app is served over https (so cookies are Secure).
func (c *Config) IsSecure() bool {
	return strings.HasPrefix(strings.ToLower(strings.TrimSpace(c.Server.BaseURL)), "https://")
}

// GenerateSecret returns n cryptographically-random bytes, base64url-encoded
// without padding (R-6): the value carries no YAML-hostile characters and its
// encoded length is 4*ceil(n/3) — 24 bytes yield the 32 characters the TOTP
// sealer requires (auth.KeySize).
func GenerateSecret(n int) (string, error) {
	if n <= 0 {
		return "", errors.New("config: secret length must be positive")
	}
	b := make([]byte, n)
	if _, err := rand.Read(b); err != nil {
		return "", fmt.Errorf("config: secret entropy: %w", err)
	}
	return base64.RawURLEncoding.EncodeToString(b), nil
}

// Validate applies the fail-closed rules (SPEC-003 §1).
func (c *Config) Validate() error {
	if strings.TrimSpace(c.Server.Listen) == "" {
		return ErrListenRequired
	}
	if strings.TrimSpace(c.Database.Path) == "" {
		return ErrDatabaseRequired
	}

	// The enrollment window (A1) is parsed in both modes: a typo is a config
	// error, never a silently-unbounded window.
	if w := strings.TrimSpace(c.Enrollment.Window); w != "" {
		d, err := time.ParseDuration(w)
		if err != nil || d <= 0 {
			return fmt.Errorf("%w: %q", ErrInvalidEnrollWindow, w)
		}
	}

	// Setup mode may run with placeholder secrets: the operator has not yet
	// provisioned the instance, and it is loopback-only (D-025). Productive
	// mode is strict.
	if c.Setup {
		return nil
	}

	if strings.TrimSpace(c.Server.BaseURL) == "" {
		return ErrBaseURLRequired
	}

	for _, s := range []struct{ name, value string }{
		{"secrets.cookie_secret", c.Secrets.CookieSecret},
		{"secrets.csrf_key", c.Secrets.CSRFKey},
		{"secrets.totp_encryption_key", c.Secrets.TOTPEncryptionKey},
	} {
		if err := checkSecret(s.name, s.value, minSecretLen); err != nil {
			return err
		}
	}
	// The TOTP key feeds AES-256-GCM, so it must be exactly 32 bytes.
	if n := len(c.Secrets.TOTPEncryptionKey); n != totpKeyLen {
		return fmt.Errorf("%w: secrets.totp_encryption_key must be exactly %d bytes, got %d",
			ErrWeakSecret, totpKeyLen, n)
	}
	return nil
}

// read loads the file, falling back to config.example.yaml for the canonical
// config.yaml only.
func read(path string) (used string, data []byte, err error) {
	data, err = os.ReadFile(path)
	if err == nil {
		return path, data, nil
	}
	if !errors.Is(err, fs.ErrNotExist) {
		return "", nil, fmt.Errorf("config: read %q: %w", path, err)
	}
	if filepath.Base(path) != FileName {
		return "", nil, fmt.Errorf("config: read %q: %w", path, err)
	}

	alt := filepath.Join(filepath.Dir(path), ExampleFileName)
	data, altErr := os.ReadFile(alt)
	if altErr != nil {
		return "", nil, fmt.Errorf("config: read %q (and fallback %q): %w", path, alt, err)
	}
	return alt, data, nil
}

// checkSecret rejects an empty, short, or placeholder secret value.
func checkSecret(name, value string, minLen int) error {
	if strings.TrimSpace(value) == "" {
		return fmt.Errorf("%w: %s is empty", ErrWeakSecret, name)
	}
	if len(value) < minLen {
		return fmt.Errorf("%w: %s is %d bytes, need at least %d", ErrWeakSecret, name, len(value), minLen)
	}
	if isPlaceholder(value) {
		return fmt.Errorf("%w: %s still holds a placeholder value", ErrWeakSecret, name)
	}
	return nil
}

// SecretIsPlaceholder reports whether a secret value is empty or still holds
// one of the shipped placeholder markers (R-6). The /setup wizard replaces such
// values with strong generated ones, so a first boot from config.example.yaml
// persists a viable productive configuration.
func SecretIsPlaceholder(value string) bool {
	return strings.TrimSpace(value) == "" || isPlaceholder(value)
}

// isPlaceholder detects the well-known placeholder markers shipped in the
// example so a copy that was never edited fails closed in productive mode.
func isPlaceholder(value string) bool {
	upper := strings.ToUpper(value)
	return strings.Contains(upper, "CHANGE_ME") ||
		strings.Contains(upper, "CHANGEME") ||
		strings.Contains(upper, "PLACEHOLDER")
}