// Package wsauth provides authentication and session-token management for
// the web service. Protocol surface (challenge nonce + SHA256-based response
// and SHA256(password+salt) hashes) is kept compatible with the existing
// browser front-end and users.json format. Internal token representation is
// deliberately different from the C++ counterpart — opaque random hex strings
// keyed into an in-memory map — to avoid leaking the proprietary token format.
package wsauth

import (
	"crypto/rand"
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"errors"
	"os"
	"path/filepath"
	"sort"
	"sync"
	"time"
)

// Default knobs. Override via SetDefaults at startup if needed.
const (
	DefaultTokenExpire = 24 * time.Hour
	nonceBytes         = 16 // 32 hex chars
	tokenBytes         = 32 // 64 hex chars
	saltBytes          = 8  // 16 hex chars
)

// ErrInvalidToken is returned when a token is unknown or expired.
var ErrInvalidToken = errors.New("invalid or expired token")

// User is the credentials record for one web account.
type User struct {
	Username     string
	PasswordHash string // SHA256(password+salt) in lowercase hex
	Salt         string // empty for admin (matches C++ convention)
	Role         string // "admin" or "viewer"
	// AllowedGroups restricts which device groups this user can view. Empty
	// slice = no device access (admin role is treated as "all" elsewhere
	// without consulting this list). Matches the C++ WebUser.allowed_groups
	// field one-for-one so users.json is interchangeable between the two
	// servers.
	AllowedGroups []string
}

// Session is the authenticated state attached to a valid token.
type Session struct {
	Username  string
	Role      string
	ExpiresAt time.Time
}

// Authenticator owns the user table and the active token map. It is safe to
// use from multiple goroutines.
type Authenticator struct {
	mu          sync.RWMutex
	users       map[string]*User    // username -> user
	tokens      map[string]*Session // token -> session
	tokenExpire time.Duration
	// usersFile is the persistence target for non-admin accounts (admin
	// lives in env/master-password only and is never written out, matching
	// the C++ SaveUsers behavior). Empty disables persistence.
	usersFile string
}

// New returns an empty Authenticator. Call AddUser to populate.
func New() *Authenticator {
	return &Authenticator{
		users:       make(map[string]*User),
		tokens:      make(map[string]*Session),
		tokenExpire: DefaultTokenExpire,
	}
}

// SetTokenExpire overrides the default session lifetime.
func (a *Authenticator) SetTokenExpire(d time.Duration) {
	if d <= 0 {
		return
	}
	a.mu.Lock()
	a.tokenExpire = d
	a.mu.Unlock()
}

// AddUser registers a user. PasswordHash should already be
// SHA256(password+salt) in lowercase hex; pass empty Salt to mirror the
// admin-style "no salt" convention used by the C++ side.
func (a *Authenticator) AddUser(u User) {
	if u.Username == "" {
		return
	}
	a.mu.Lock()
	a.users[u.Username] = &u
	a.mu.Unlock()
}

// AddAdminFromPlainPassword is a convenience for the bootstrap admin: salt is
// empty (matching the C++ admin record), hash is SHA256(password).
func (a *Authenticator) AddAdminFromPlainPassword(username, plainPassword string) {
	a.AddUser(User{
		Username:     username,
		PasswordHash: ComputeSHA256(plainPassword),
		Salt:         "",
		Role:         "admin",
	})
}

// CreateUser registers a new account, persists the user table, and returns
// nil on success. Mirrors the C++ CWebService::CreateUser semantics:
//   - role must be "admin" or "viewer"
//   - "admin" is reserved for the bootstrap account and cannot be created
//     via this API
//   - duplicate usernames are rejected
//
// Password is hashed with a fresh per-user salt before being stored.
func (a *Authenticator) CreateUser(username, plainPassword, role string, allowedGroups []string) error {
	if username == "" || plainPassword == "" {
		return errors.New("username and password required")
	}
	if username == "admin" {
		return errors.New("'admin' is reserved")
	}
	if role != "admin" && role != "viewer" {
		return errors.New("role must be 'admin' or 'viewer'")
	}

	salt, err := NewSalt()
	if err != nil {
		return err
	}
	u := User{
		Username:      username,
		PasswordHash:  HashPassword(plainPassword, salt),
		Salt:          salt,
		Role:          role,
		AllowedGroups: append([]string(nil), allowedGroups...),
	}

	a.mu.Lock()
	if _, exists := a.users[username]; exists {
		a.mu.Unlock()
		return errors.New("user already exists")
	}
	a.users[username] = &u
	path := a.usersFile
	snapshot := a.snapshotPersistableLocked()
	a.mu.Unlock()

	if path != "" {
		return writeUsersFile(path, snapshot)
	}
	return nil
}

// DeleteUser removes a user account and persists the change. The bootstrap
// admin (always Role=="admin" with empty Salt) is protected: deleting it
// would lock everyone out of the UI with no way back in.
func (a *Authenticator) DeleteUser(username string) error {
	if username == "" || username == "admin" {
		return errors.New("cannot delete admin")
	}
	a.mu.Lock()
	if _, ok := a.users[username]; !ok {
		a.mu.Unlock()
		return errors.New("user not found")
	}
	delete(a.users, username)
	// Also drop any live sessions belonging to that user so they don't
	// outlive their account.
	for tok, s := range a.tokens {
		if s.Username == username {
			delete(a.tokens, tok)
		}
	}
	path := a.usersFile
	snapshot := a.snapshotPersistableLocked()
	a.mu.Unlock()

	if path != "" {
		return writeUsersFile(path, snapshot)
	}
	return nil
}

// ListUsers returns a stable, copied snapshot of all users in name order.
// PasswordHash and Salt are zeroed in the returned records so callers can
// JSON-marshal the result without leaking credentials.
func (a *Authenticator) ListUsers() []User {
	a.mu.RLock()
	defer a.mu.RUnlock()
	out := make([]User, 0, len(a.users))
	for _, u := range a.users {
		c := *u
		c.PasswordHash = ""
		c.Salt = ""
		out = append(out, c)
	}
	sort.Slice(out, func(i, j int) bool { return out[i].Username < out[j].Username })
	return out
}

// GetSalt returns the per-user salt. If the user does not exist, returns ("", false).
// Note: the C++ admin uses an empty salt — that is still considered "found"
// and the empty string is returned with ok=true.
func (a *Authenticator) GetSalt(username string) (string, bool) {
	a.mu.RLock()
	u, ok := a.users[username]
	a.mu.RUnlock()
	if !ok {
		return "", false
	}
	return u.Salt, true
}

// VerifyLogin checks a challenge-response login. The browser sends
// response = SHA256(passwordHash + nonce). On success the function mints a
// new session token, stores it, and returns (token, role, nil).
func (a *Authenticator) VerifyLogin(username, response, nonce string) (token, role string, err error) {
	a.mu.RLock()
	u, ok := a.users[username]
	expire := a.tokenExpire
	a.mu.RUnlock()
	if !ok {
		return "", "", errors.New("invalid credentials")
	}
	expected := ComputeSHA256(u.PasswordHash + nonce)
	if response != expected {
		return "", "", errors.New("invalid credentials")
	}

	token, err = randomHex(tokenBytes)
	if err != nil {
		return "", "", err
	}
	a.mu.Lock()
	a.tokens[token] = &Session{
		Username:  username,
		Role:      u.Role,
		ExpiresAt: time.Now().Add(expire),
	}
	a.mu.Unlock()
	return token, u.Role, nil
}

// usersFileEntry is the on-disk shape of one record in users.json. Field
// names match the C++ WebService SaveUsers output exactly so the two
// servers can share the file.
type usersFileEntry struct {
	Username      string   `json:"username"`
	PasswordHash  string   `json:"password_hash"`
	Salt          string   `json:"salt"`
	Role          string   `json:"role"`
	AllowedGroups []string `json:"allowed_groups"`
}

type usersFile struct {
	Users []usersFileEntry `json:"users"`
}

// SetUsersFile points the authenticator at a JSON file used to persist
// non-admin accounts and loads any existing entries. Calling it again with
// a different path is allowed but the previous file is not re-read on a
// nil-arg call — initialize once at startup. Missing files are not an
// error; they're treated as an empty user table.
func (a *Authenticator) SetUsersFile(path string) error {
	a.mu.Lock()
	a.usersFile = path
	a.mu.Unlock()
	if path == "" {
		return nil
	}
	data, err := os.ReadFile(path)
	if err != nil {
		if os.IsNotExist(err) {
			return nil
		}
		return err
	}
	if len(data) == 0 {
		return nil
	}
	var f usersFile
	if err := json.Unmarshal(data, &f); err != nil {
		return err
	}
	a.mu.Lock()
	defer a.mu.Unlock()
	for _, e := range f.Users {
		// Skip admin: it's always sourced from env/master password and
		// already injected via AddAdminFromPlainPassword.
		if e.Username == "" || e.Username == "admin" {
			continue
		}
		if e.PasswordHash == "" {
			continue
		}
		role := e.Role
		if role == "" {
			role = "viewer"
		}
		a.users[e.Username] = &User{
			Username:      e.Username,
			PasswordHash:  e.PasswordHash,
			Salt:          e.Salt,
			Role:          role,
			AllowedGroups: append([]string(nil), e.AllowedGroups...),
		}
	}
	return nil
}

// snapshotPersistableLocked builds the on-disk slice while a.mu is held by
// the caller. Admin records are excluded — they live in env, not in the file.
func (a *Authenticator) snapshotPersistableLocked() []usersFileEntry {
	out := make([]usersFileEntry, 0, len(a.users))
	for _, u := range a.users {
		if u.Username == "admin" {
			continue
		}
		groups := append([]string{}, u.AllowedGroups...)
		out = append(out, usersFileEntry{
			Username:      u.Username,
			PasswordHash:  u.PasswordHash,
			Salt:          u.Salt,
			Role:          u.Role,
			AllowedGroups: groups,
		})
	}
	sort.Slice(out, func(i, j int) bool { return out[i].Username < out[j].Username })
	return out
}

// writeUsersFile writes the user list atomically: encode to a temp file in
// the same directory, fsync, rename — so a crash mid-write can't leave the
// service starting up with a half-written users.json next time.
func writeUsersFile(path string, entries []usersFileEntry) error {
	data, err := json.MarshalIndent(usersFile{Users: entries}, "", "  ")
	if err != nil {
		return err
	}
	dir := filepath.Dir(path)
	if dir != "" {
		if err := os.MkdirAll(dir, 0o755); err != nil {
			return err
		}
	}
	tmp, err := os.CreateTemp(dir, "users-*.json.tmp")
	if err != nil {
		return err
	}
	tmpName := tmp.Name()
	if _, err := tmp.Write(data); err != nil {
		_ = tmp.Close()
		_ = os.Remove(tmpName)
		return err
	}
	if err := tmp.Sync(); err != nil {
		_ = tmp.Close()
		_ = os.Remove(tmpName)
		return err
	}
	if err := tmp.Close(); err != nil {
		_ = os.Remove(tmpName)
		return err
	}
	if err := os.Rename(tmpName, path); err != nil {
		_ = os.Remove(tmpName)
		return err
	}
	return nil
}

// ValidateToken returns the session for a token or ErrInvalidToken. Expired
// tokens are removed lazily as they are looked up.
func (a *Authenticator) ValidateToken(token string) (*Session, error) {
	a.mu.RLock()
	s, ok := a.tokens[token]
	a.mu.RUnlock()
	if !ok {
		return nil, ErrInvalidToken
	}
	if time.Now().After(s.ExpiresAt) {
		a.mu.Lock()
		delete(a.tokens, token)
		a.mu.Unlock()
		return nil, ErrInvalidToken
	}
	return s, nil
}

// RevokeToken removes a token from the active set. No-op for unknown tokens.
func (a *Authenticator) RevokeToken(token string) {
	a.mu.Lock()
	delete(a.tokens, token)
	a.mu.Unlock()
}

// NewNonce returns a fresh challenge nonce (hex string). Each WS connection
// should receive exactly one nonce, consumed by a single login attempt.
func NewNonce() (string, error) {
	return randomHex(nonceBytes)
}

// NewSalt returns a fresh per-user salt (hex string).
func NewSalt() (string, error) {
	return randomHex(saltBytes)
}

// ComputeSHA256 returns the lowercase-hex SHA256 of s.
func ComputeSHA256(s string) string {
	sum := sha256.Sum256([]byte(s))
	return hex.EncodeToString(sum[:])
}

// HashPassword computes the stored hash for a (password, salt) pair using
// the same scheme as the existing C++ users.json: SHA256(password + salt).
func HashPassword(password, salt string) string {
	return ComputeSHA256(password + salt)
}

func randomHex(n int) (string, error) {
	b := make([]byte, n)
	if _, err := rand.Read(b); err != nil {
		return "", err
	}
	return hex.EncodeToString(b), nil
}