package auth

import (
	"context"
	"database/sql"
	"errors"
	"fmt"
	"regexp"
	"strings"
	"time"
)

var (
	// ErrUserExists means the username is taken (case-insensitive).
	ErrUserExists = errors.New("auth: username already taken")
	// ErrUserNotFound means no user matched.
	ErrUserNotFound = errors.New("auth: user not found")
	// ErrInvalidUsername means the username failed validation.
	ErrInvalidUsername = errors.New("auth: invalid username")
	// ErrInvalidCredentials is returned for both unknown users and bad
	// passwords, so callers cannot distinguish the two.
	ErrInvalidCredentials = errors.New("auth: invalid credentials")
)

// usernamePattern keeps usernames URL- and log-safe, and avoids the
// confusable-character mess of allowing arbitrary Unicode identifiers.
var usernamePattern = regexp.MustCompile(`^[a-zA-Z0-9][a-zA-Z0-9._-]{2,31}$`)

// User is an account. The password hash is never serialized to a client.
type User struct {
	ID           int64
	Username     string
	DisplayName  string
	PasswordHash string
	IsAdmin      bool
	CreatedAt    time.Time
}

// ValidateUsername enforces the username policy.
func ValidateUsername(username string) error {
	if !usernamePattern.MatchString(username) {
		return fmt.Errorf("%w: must be 3-32 characters, start with a letter or digit, and contain only letters, digits, dot, underscore or hyphen", ErrInvalidUsername)
	}
	return nil
}

// CreateUser hashes the password and inserts the account.
func CreateUser(ctx context.Context, db *sql.DB, username, displayName, password string, isAdmin bool) (User, error) {
	if err := ValidateUsername(username); err != nil {
		return User{}, err
	}
	if password == "" {
		return User{}, errors.New("auth: password must not be empty")
	}
	if displayName == "" {
		displayName = username
	}

	hash, err := HashPassword(password)
	if err != nil {
		return User{}, err
	}

	u := User{Username: username, DisplayName: displayName, IsAdmin: isAdmin}
	var created string
	err = db.QueryRowContext(ctx, `
		INSERT INTO users (username, display_name, password_hash, is_admin)
		VALUES (?, ?, ?, ?)
		RETURNING id, created_at`,
		username, displayName, hash, boolToInt(isAdmin),
	).Scan(&u.ID, &created)
	if err != nil {
		if isUniqueViolation(err) {
			return User{}, ErrUserExists
		}
		return User{}, fmt.Errorf("auth: create user: %w", err)
	}
	u.CreatedAt, _ = time.Parse(time.RFC3339Nano, created)
	u.PasswordHash = hash
	return u, nil
}

// GetUserByUsername does a case-insensitive lookup (the column is COLLATE NOCASE).
func GetUserByUsername(ctx context.Context, db *sql.DB, username string) (User, error) {
	return scanUser(db.QueryRowContext(ctx, userSelect+` WHERE username = ?`, username))
}

// GetUserByID loads one user by primary key.
func GetUserByID(ctx context.Context, db *sql.DB, id int64) (User, error) {
	return scanUser(db.QueryRowContext(ctx, userSelect+` WHERE id = ?`, id))
}

// ListUsers returns every account, oldest first.
func ListUsers(ctx context.Context, db *sql.DB) ([]User, error) {
	rows, err := db.QueryContext(ctx, userSelect+` ORDER BY id`)
	if err != nil {
		return nil, fmt.Errorf("auth: list users: %w", err)
	}
	defer rows.Close()

	var out []User
	for rows.Next() {
		u, err := scanUserRow(rows)
		if err != nil {
			return nil, err
		}
		out = append(out, u)
	}
	return out, rows.Err()
}

// CountUsers reports how many accounts exist. Used to decide whether to
// bootstrap the first admin.
func CountUsers(ctx context.Context, db *sql.DB) (int, error) {
	var n int
	err := db.QueryRowContext(ctx, `SELECT count(*) FROM users`).Scan(&n)
	return n, err
}

// SetPassword replaces a user's password and logs out all their sessions.
// Logging out everywhere is deliberate: a password change is often a response
// to a suspected compromise.
func SetPassword(ctx context.Context, db *sql.DB, userID int64, password string) error {
	if password == "" {
		return errors.New("auth: password must not be empty")
	}
	hash, err := HashPassword(password)
	if err != nil {
		return err
	}
	tx, err := db.BeginTx(ctx, nil)
	if err != nil {
		return err
	}
	defer tx.Rollback() //nolint:errcheck

	if _, err := tx.ExecContext(ctx, `UPDATE users SET password_hash = ? WHERE id = ?`, hash, userID); err != nil {
		return fmt.Errorf("auth: set password: %w", err)
	}
	if _, err := tx.ExecContext(ctx, `DELETE FROM sessions WHERE user_id = ?`, userID); err != nil {
		return fmt.Errorf("auth: clear sessions: %w", err)
	}
	return tx.Commit()
}

// ── Authentication ─────────────────────────────────────────────────────

// Authenticate verifies a username/password pair and returns the user.
//
// It hashes even when the user does not exist, so response time does not
// reveal whether a username is registered.
func Authenticate(ctx context.Context, db *sql.DB, username, password string) (User, error) {
	u, err := GetUserByUsername(ctx, db, username)
	if errors.Is(err, sql.ErrNoRows) {
		_, _ = HashPassword(password) // burn equivalent time
		return User{}, ErrInvalidCredentials
	}
	if err != nil {
		return User{}, err
	}

	ok, err := VerifyPassword(password, u.PasswordHash)
	if err != nil {
		// A malformed stored hash is an operational error, not a bad login.
		return User{}, fmt.Errorf("auth: verify password for %q: %w", u.Username, err)
	}
	if !ok {
		return User{}, ErrInvalidCredentials
	}
	return u, nil
}

// ── Library grants ─────────────────────────────────────────────────────

// Grant is one user's access to one library.
type Grant struct {
	LibraryID   int64
	CanRead     bool
	CanDownload bool
}

// GrantLibrary upserts a user's access to a library.
func GrantLibrary(ctx context.Context, db *sql.DB, userID, libraryID int64, canRead, canDownload bool) error {
	_, err := db.ExecContext(ctx, `
		INSERT INTO library_grants (user_id, library_id, can_read, can_download)
		VALUES (?, ?, ?, ?)
		ON CONFLICT (user_id, library_id)
		DO UPDATE SET can_read = excluded.can_read, can_download = excluded.can_download`,
		userID, libraryID, boolToInt(canRead), boolToInt(canDownload))
	if err != nil {
		return fmt.Errorf("auth: grant library: %w", err)
	}
	return nil
}

// RevokeLibrary removes a user's access to a library.
func RevokeLibrary(ctx context.Context, db *sql.DB, userID, libraryID int64) error {
	_, err := db.ExecContext(ctx, `DELETE FROM library_grants WHERE user_id = ? AND library_id = ?`, userID, libraryID)
	if err != nil {
		return fmt.Errorf("auth: revoke library: %w", err)
	}
	return nil
}

// LibraryGrants lists a user's explicit grants.
func LibraryGrants(ctx context.Context, db *sql.DB, userID int64) ([]Grant, error) {
	rows, err := db.QueryContext(ctx,
		`SELECT library_id, can_read, can_download FROM library_grants WHERE user_id = ?`, userID)
	if err != nil {
		return nil, fmt.Errorf("auth: list grants: %w", err)
	}
	defer rows.Close()

	var out []Grant
	for rows.Next() {
		var g Grant
		var read, download int
		if err := rows.Scan(&g.LibraryID, &read, &download); err != nil {
			return nil, err
		}
		g.CanRead, g.CanDownload = read == 1, download == 1
		out = append(out, g)
	}
	return out, rows.Err()
}

// VisibleLibraryIDs returns the libraries a user may read.
//
// This is the single access-control chokepoint. Admins see every library;
// everyone else sees exactly their grants with can_read set. Every endpoint
// that returns media must filter through this or CanAccessLibrary.
func VisibleLibraryIDs(ctx context.Context, db *sql.DB, u User) ([]int64, error) {
	if u.IsAdmin {
		rows, err := db.QueryContext(ctx, `SELECT id FROM libraries ORDER BY id`)
		if err != nil {
			return nil, fmt.Errorf("auth: list all libraries: %w", err)
		}
		defer rows.Close()
		var out []int64
		for rows.Next() {
			var id int64
			if err := rows.Scan(&id); err != nil {
				return nil, err
			}
			out = append(out, id)
		}
		return out, rows.Err()
	}

	rows, err := db.QueryContext(ctx,
		`SELECT library_id FROM library_grants WHERE user_id = ? AND can_read = 1 ORDER BY library_id`, u.ID)
	if err != nil {
		return nil, fmt.Errorf("auth: list visible libraries: %w", err)
	}
	defer rows.Close()
	var out []int64
	for rows.Next() {
		var id int64
		if err := rows.Scan(&id); err != nil {
			return nil, err
		}
		out = append(out, id)
	}
	return out, rows.Err()
}

// CanAccessLibrary reports whether a user may read one library.
func CanAccessLibrary(ctx context.Context, db *sql.DB, u User, libraryID int64) (bool, error) {
	if u.IsAdmin {
		return true, nil
	}
	var n int
	err := db.QueryRowContext(ctx,
		`SELECT count(*) FROM library_grants WHERE user_id = ? AND library_id = ? AND can_read = 1`,
		u.ID, libraryID).Scan(&n)
	if err != nil {
		return false, fmt.Errorf("auth: check library access: %w", err)
	}
	return n > 0, nil
}

// ── helpers ────────────────────────────────────────────────────────────

const userSelect = `SELECT id, username, display_name, password_hash, is_admin, created_at FROM users`

type rowScanner interface {
	Scan(dest ...any) error
}

func scanUser(row rowScanner) (User, error) {
	u, err := scanUserRow(row)
	if errors.Is(err, sql.ErrNoRows) {
		return User{}, sql.ErrNoRows
	}
	return u, err
}

func scanUserRow(row rowScanner) (User, error) {
	var u User
	var isAdmin int
	var created string
	if err := row.Scan(&u.ID, &u.Username, &u.DisplayName, &u.PasswordHash, &isAdmin, &created); err != nil {
		return User{}, err
	}
	u.IsAdmin = isAdmin == 1
	u.CreatedAt, _ = time.Parse(time.RFC3339Nano, created)
	return u, nil
}

func boolToInt(b bool) int {
	if b {
		return 1
	}
	return 0
}

// isUniqueViolation sniffs the SQLite error for a uniqueness constraint.
// modernc.org/sqlite does not export a typed error for this, so string
// matching on the driver message is the pragmatic option.
func isUniqueViolation(err error) bool {
	if err == nil {
		return false
	}
	msg := strings.ToUpper(err.Error())
	return strings.Contains(msg, "UNIQUE CONSTRAINT FAILED") || strings.Contains(msg, "CONSTRAINT FAILED: UNIQUE")
}