// Package authn owns browser-operator identities and credential verification.
// Authorization policy remains in authz; this package only proves who signed
// in. Operator records live in a small embedded bbolt database so a deployment
// never needs to carry a reusable password hash in its environment.
package authn

import (
	"crypto/subtle"
	"encoding/json"
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"strings"
	"time"
	"unicode"
	"unicode/utf8"

	bolt "go.etcd.io/bbolt"
	"golang.org/x/crypto/bcrypt"
)

const (
	DatabaseFile      = "auth.db"
	MinimumPassword   = 10
	maximumPassword   = 72 // bcrypt rejects passwords longer than 72 bytes.
	maximumUsername   = 128
	databaseOpenWait  = 2 * time.Second
	databaseFileMode  = 0600
	databaseDirectory = 0700
)

var (
	usersBucket           = []byte("operator_users")
	ErrInvalidCredentials = errors.New("invalid username or password")
	ErrUsernameExists     = errors.New("username already exists")
	// Unknown users still take a bcrypt comparison. The hash is generated once
	// at process start with the same cost used for real records so the login
	// response does not disclose whether an account exists.
	dummyPasswordHash = func() []byte {
		hash, err := bcrypt.GenerateFromPassword([]byte("orchestra-invalid-login-sentinel"), bcrypt.DefaultCost)
		if err != nil {
			panic(err)
		}
		return hash
	}()
)

type User struct {
	Username  string    `json:"username"`
	CreatedAt time.Time `json:"created_at"`
	UpdatedAt time.Time `json:"updated_at"`
}

type storedUser struct {
	User
	PasswordHash string `json:"password_hash"`
}

type Store struct {
	db *bolt.DB
}

func Path(dataDir string) string { return filepath.Join(dataDir, DatabaseFile) }

func Open(path string) (*Store, error) {
	if strings.TrimSpace(path) == "" {
		return nil, errors.New("auth database path is required")
	}
	if err := os.MkdirAll(filepath.Dir(path), databaseDirectory); err != nil {
		return nil, fmt.Errorf("create auth database directory: %w", err)
	}
	db, err := bolt.Open(path, databaseFileMode, &bolt.Options{Timeout: databaseOpenWait})
	if err != nil {
		return nil, fmt.Errorf("open auth database: %w", err)
	}
	s := &Store{db: db}
	if err := db.Update(func(tx *bolt.Tx) error {
		_, err := tx.CreateBucketIfNotExists(usersBucket)
		return err
	}); err != nil {
		_ = db.Close()
		return nil, fmt.Errorf("initialize auth database: %w", err)
	}
	return s, nil
}

func (s *Store) Close() error {
	if s == nil || s.db == nil {
		return nil
	}
	return s.db.Close()
}

func normalizedUsername(username string) string {
	return strings.ToLower(strings.TrimSpace(username))
}

func ValidateUsername(username string) error {
	username = strings.TrimSpace(username)
	if username == "" {
		return errors.New("username is required")
	}
	if len(username) > maximumUsername {
		return fmt.Errorf("username must be at most %d bytes", maximumUsername)
	}
	if !utf8.ValidString(username) {
		return errors.New("username must be valid UTF-8")
	}
	for _, r := range username {
		if unicode.IsControl(r) {
			return errors.New("username must not contain control characters")
		}
	}
	return nil
}

func ValidatePassword(password string) error {
	if len(password) < MinimumPassword {
		return fmt.Errorf("password must be at least %d characters", MinimumPassword)
	}
	if len([]byte(password)) > maximumPassword {
		return fmt.Errorf("password must be at most %d bytes", maximumPassword)
	}
	return nil
}

func decodeUser(raw []byte) (storedUser, error) {
	var user storedUser
	if err := json.Unmarshal(raw, &user); err != nil {
		return storedUser{}, err
	}
	return user, nil
}

func (s *Store) Count() (int, error) {
	count := 0
	err := s.db.View(func(tx *bolt.Tx) error {
		count = tx.Bucket(usersBucket).Stats().KeyN
		return nil
	})
	return count, err
}

func (s *Store) Users() ([]User, error) {
	users := []User{}
	err := s.db.View(func(tx *bolt.Tx) error {
		return tx.Bucket(usersBucket).ForEach(func(_, raw []byte) error {
			stored, err := decodeUser(raw)
			if err != nil {
				return err
			}
			users = append(users, stored.User)
			return nil
		})
	})
	return users, err
}

func (s *Store) User(username string) (User, error) {
	var out User
	err := s.db.View(func(tx *bolt.Tx) error {
		raw := tx.Bucket(usersBucket).Get([]byte(normalizedUsername(username)))
		if raw == nil {
			return ErrInvalidCredentials
		}
		stored, err := decodeUser(raw)
		if err == nil {
			out = stored.User
		}
		return err
	})
	return out, err
}

// SetPassword creates an operator or replaces that operator's password. It is
// intended for the local orchestra-user command; browser changes use Update,
// which also proves the current password.
func (s *Store) SetPassword(username, password string) (User, bool, error) {
	username = strings.TrimSpace(username)
	if err := ValidateUsername(username); err != nil {
		return User{}, false, err
	}
	if err := ValidatePassword(password); err != nil {
		return User{}, false, err
	}
	hash, err := bcrypt.GenerateFromPassword([]byte(password), bcrypt.DefaultCost)
	if err != nil {
		return User{}, false, err
	}
	return s.putHash(username, string(hash), false)
}

// ImportBcrypt is the one-time compatibility bridge from the old environment
// credential. It only creates the named user when the database is empty.
func (s *Store) ImportBcrypt(username, passwordHash string) (bool, error) {
	username = strings.TrimSpace(username)
	if err := ValidateUsername(username); err != nil {
		return false, err
	}
	if _, err := bcrypt.Cost([]byte(passwordHash)); err != nil {
		return false, fmt.Errorf("legacy web password hash must be bcrypt: %w", err)
	}
	_, created, err := s.putHash(username, passwordHash, true)
	return created, err
}

func (s *Store) putHash(username, passwordHash string, onlyIfEmpty bool) (User, bool, error) {
	now := time.Now().UTC()
	key := []byte(normalizedUsername(username))
	var out User
	created := false
	err := s.db.Update(func(tx *bolt.Tx) error {
		bucket := tx.Bucket(usersBucket)
		if onlyIfEmpty && bucket.Stats().KeyN != 0 {
			return nil
		}
		stored := storedUser{User: User{Username: username, CreatedAt: now, UpdatedAt: now}, PasswordHash: passwordHash}
		if raw := bucket.Get(key); raw != nil {
			current, err := decodeUser(raw)
			if err != nil {
				return err
			}
			stored.CreatedAt = current.CreatedAt
		} else {
			created = true
		}
		encoded, err := json.Marshal(stored)
		if err != nil {
			return err
		}
		out = stored.User
		return bucket.Put(key, encoded)
	})
	return out, created, err
}

func (s *Store) Authenticate(username, password string) (User, error) {
	var stored storedUser
	found := false
	err := s.db.View(func(tx *bolt.Tx) error {
		raw := tx.Bucket(usersBucket).Get([]byte(normalizedUsername(username)))
		if raw == nil {
			return nil
		}
		var err error
		stored, err = decodeUser(raw)
		found = err == nil
		return err
	})
	if err != nil {
		return User{}, err
	}
	hash := dummyPasswordHash
	if found {
		hash = []byte(stored.PasswordHash)
	}
	passwordOK := bcrypt.CompareHashAndPassword(hash, []byte(password)) == nil
	usernameOK := found && subtle.ConstantTimeCompare(
		[]byte(normalizedUsername(username)),
		[]byte(normalizedUsername(stored.Username)),
	) == 1
	if !passwordOK || !usernameOK {
		return User{}, ErrInvalidCredentials
	}
	return stored.User, nil
}

// Update changes the authenticated operator's username and/or password. The
// current password is required even though the endpoint also requires a live
// browser session, protecting an unattended unlocked browser.
func (s *Store) Update(currentUsername, currentPassword, newUsername, newPassword string) (User, error) {
	current, err := s.Authenticate(currentUsername, currentPassword)
	if err != nil {
		return User{}, err
	}
	newUsername = strings.TrimSpace(newUsername)
	if newUsername == "" {
		newUsername = current.Username
	}
	if err := ValidateUsername(newUsername); err != nil {
		return User{}, err
	}

	var newHash string
	if newPassword != "" {
		if err := ValidatePassword(newPassword); err != nil {
			return User{}, err
		}
		hash, err := bcrypt.GenerateFromPassword([]byte(newPassword), bcrypt.DefaultCost)
		if err != nil {
			return User{}, err
		}
		newHash = string(hash)
	}

	oldKey := []byte(normalizedUsername(current.Username))
	newKey := []byte(normalizedUsername(newUsername))
	var out User
	err = s.db.Update(func(tx *bolt.Tx) error {
		bucket := tx.Bucket(usersBucket)
		raw := bucket.Get(oldKey)
		if raw == nil {
			return ErrInvalidCredentials
		}
		stored, err := decodeUser(raw)
		if err != nil {
			return err
		}
		// Refuse a stale credential update if another password change landed
		// between Authenticate and this write transaction.
		if bcrypt.CompareHashAndPassword([]byte(stored.PasswordHash), []byte(currentPassword)) != nil {
			return ErrInvalidCredentials
		}
		if !bytesEqual(oldKey, newKey) && bucket.Get(newKey) != nil {
			return ErrUsernameExists
		}
		stored.Username = newUsername
		stored.UpdatedAt = time.Now().UTC()
		if newHash != "" {
			stored.PasswordHash = newHash
		}
		encoded, err := json.Marshal(stored)
		if err != nil {
			return err
		}
		if !bytesEqual(oldKey, newKey) {
			if err := bucket.Delete(oldKey); err != nil {
				return err
			}
		}
		if err := bucket.Put(newKey, encoded); err != nil {
			return err
		}
		out = stored.User
		return nil
	})
	return out, err
}

func bytesEqual(a, b []byte) bool {
	return len(a) == len(b) && subtle.ConstantTimeCompare(a, b) == 1
}