// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: PolyForm-Noncommercial-1.0.0

// Package users is a file-backed set of admin users stored as a TOML
// [[users]] array. The first account is created by the admin first-run
// wizard, never by an implicit identity in the configuration.
package users

import (
	"errors"
	"fmt"
	"log/slog"
	"os"
	"slices"
	"sync"

	"sourcedock.dev/petrbalvin/interpres/v2"
	"sourcedock.dev/petrbalvin/volumen/internal/i18n"
	"sourcedock.dev/petrbalvin/volumen/internal/identifiers"
	"sourcedock.dev/petrbalvin/volumen/internal/password"
	"sourcedock.dev/petrbalvin/volumen/internal/tomlfile"
	"sourcedock.dev/petrbalvin/volumen/internal/web"
)

// Roles accepted for user accounts.
var Roles = []string{"admin", "author"}

// The reasons a user change can be refused, so a caller can report which
// one happened rather than guessing from a nil result.
var (
	// ErrNameTaken is returned when another account already has the name.
	ErrNameTaken = errors.New("that username is taken")
	// ErrEmptyName is returned when the name is blank.
	ErrEmptyName = errors.New("the username must not be empty")
	// ErrNoSuchUser is returned when the account is not in the file.
	ErrNoSuchUser = errors.New("no such user")
	// ErrLastAdmin is returned when the change would remove the last
	// admin, or the last account.
	ErrLastAdmin = errors.New("the last admin cannot be removed or demoted")
	// ErrUsersExist is returned by AddFirst when the file already
	// holds accounts: the first-run wizard may create exactly one.
	ErrUsersExist = errors.New("accounts already exist")
)

// DefaultRole is assigned when an unknown role is requested.
const DefaultRole = "author"

// User is one admin/author account.
type User struct {
	Username         string
	PasswordHash     string
	Role             string
	Name             string
	FediverseCreator string

	// Orcid is the account's ORCID iD, the author identity that
	// pre-fills a post's orcid field and names the person in citations.
	Orcid string

	Photo string

	// Language is the admin interface language the account picked
	// ("en" or "cs"); empty inherits the site language.
	Language string

	// Theme is the admin colour scheme the account picked; empty
	// inherits the default scheme.
	Theme string

	// TotpSecret is the account's authenticator secret, base32; empty
	// means the second factor is off. TotpStep is the highest time
	// step already answered, the replay floor.
	TotpSecret string
	TotpStep   int64

	// Recovery holds the SHA-256 digests of the one-time codes that
	// open the account when the authenticator is lost.
	Recovery []string
}

// Users is the file-backed collection of admin/authors. Read-modify-
// write transactions are serialised with an internal lock; an mtime
// snapshot of the file invalidates the cache on out-of-band edits.
type Users struct {
	path string

	mu        sync.Mutex
	cached    []*User
	snapshot  fileSnapshot
	haveCache bool

	lock sync.Mutex
}

type fileSnapshot struct {
	present bool
	mtime   int64
	size    int64
}

// New opens the users file.
func New(path string) *Users {
	return &Users{path: path}
}

// All returns every account. A missing file holds no accounts; a file
// that exists but cannot be read or parsed yields no accounts either,
// which fails closed: the server never serves an identity the operator
// cannot account for.
func (u *Users) All() []*User {
	u.mu.Lock()
	defer u.mu.Unlock()
	loaded, err := u.loadLocked()
	if err != nil {
		slog.Error("users: cannot read the users file", "path", u.path, "error", err)
		return nil
	}
	return cloneList(loaded)
}

// Any reports whether at least one user exists.
func (u *Users) Any() bool { return len(u.All()) > 0 }

// Find returns a copy of the user with the given username, or nil.
func (u *Users) Find(username string) *User {
	for _, user := range u.All() {
		if user.Username == username {
			return user
		}
	}
	return nil
}

// Authenticate verifies the password and returns the user on success.
// An unknown username is verified against a dummy hash as well, so the
// response time does not reveal whether the account exists.
func (u *Users) Authenticate(username, secret string) *User {
	user := u.Find(username)
	if user == nil {
		password.Verify(secret, password.Dummy())
		return nil
	}
	if password.Verify(secret, user.PasswordHash) {
		return user
	}
	return nil
}

// Add creates a user. It fails with ErrNameTaken, ErrEmptyName or the
// error the write returned, so a caller can say which of the three
// happened rather than repeating a guess.
func (u *Users) Add(username, secret, role string) (*User, error) {
	u.lock.Lock()
	defer u.lock.Unlock()
	users, err := u.reload()
	if err != nil {
		slog.Error("users: refusing to add, the users file is unreadable", "path", u.path, "error", err)
		return nil, err
	}
	if username == "" {
		return nil, ErrEmptyName
	}
	if findIn(users, username) != nil {
		return nil, ErrNameTaken
	}
	if !slices.Contains(Roles, role) {
		role = DefaultRole
	}
	hash, err := password.Hash(secret)
	if err != nil {
		slog.Warn("users: cannot hash password", "error", err)
		return nil, err
	}
	user := &User{Username: username, PasswordHash: hash, Role: role}
	if err := u.persist(append(cloneList(users), user)); err != nil {
		return nil, err
	}
	return user, nil
}

// AddFirst creates the very first admin account in one write: the
// language and the theme the first-run wizard picked are stored with
// it, so the founding record never exists half-set-up. It is refused
// with ErrUsersExist once any account exists, and the check sits under
// the store lock, so two claims arriving at once cannot both open an
// identity: one wins, the other is refused and goes to sign in.
func (u *Users) AddFirst(username, secret, language, theme, name string) (*User, error) {
	u.lock.Lock()
	defer u.lock.Unlock()
	users, err := u.reload()
	if err != nil {
		slog.Error("users: refusing the first account, the users file is unreadable", "path", u.path, "error", err)
		return nil, err
	}
	if len(users) > 0 {
		return nil, ErrUsersExist
	}
	if username == "" {
		return nil, ErrEmptyName
	}
	if findIn(users, username) != nil {
		return nil, ErrNameTaken
	}
	hash, err := password.Hash(secret)
	if err != nil {
		slog.Warn("users: cannot hash password", "error", err)
		return nil, err
	}
	user := &User{
		Username:     username,
		PasswordHash: hash,
		Role:         "admin",
		Language:     language,
		Theme:        theme,
		Name:         name,
	}
	if err := u.persist([]*User{user}); err != nil {
		return nil, err
	}
	return user, nil
}

// UpdateName sets the display name (empty clears it).
func (u *Users) UpdateName(username, name string) (*User, error) {
	return u.mutate(username, func(user *User) {
		if name == "" {
			user.Name = ""
			return
		}
		user.Name = name
	})
}

// UpdateFediverseCreator sets the fediverse handle (empty clears it).
func (u *Users) UpdateFediverseCreator(username, value string) (*User, error) {
	return u.mutate(username, func(user *User) {
		if value == "" {
			user.FediverseCreator = ""
			return
		}
		user.FediverseCreator = value
	})
}

// UpdatePhoto sets the profile photo URL (empty clears it).
func (u *Users) UpdatePhoto(username, photo string) (*User, error) {
	return u.mutate(username, func(user *User) { user.Photo = photo })
}

// UpdateOrcid sets the account's ORCID iD (empty clears it). Only a
// well-formed iD with a correct check digit is accepted; anything else
// is refused with an error rather than stored broken.
func (u *Users) UpdateOrcid(username, value string) (*User, error) {
	value = identifiers.NormalizeORCID(value)
	if value != "" && !identifiers.ValidORCID(value) {
		return nil, fmt.Errorf("invalid ORCID %q", value)
	}
	return u.mutate(username, func(user *User) { user.Orcid = value })
}

// UpdateLanguage stores the admin interface language the account
// picked. Only the shipped languages are accepted; anything else is
// refused with an error rather than silently resetting to the default.
func (u *Users) UpdateLanguage(username, lang string) (*User, error) {
	if !i18n.Valid(lang) {
		return nil, fmt.Errorf("unsupported language %q", lang)
	}
	return u.mutate(username, func(user *User) { user.Language = lang })
}

// UpdateTheme stores the admin colour scheme the account picked. Only
// the shipped schemes are accepted; anything else is refused with an
// error rather than silently resetting to the default.
func (u *Users) UpdateTheme(username, theme string) (*User, error) {
	if !web.ValidTheme(theme) {
		return nil, fmt.Errorf("unsupported colour scheme %q", theme)
	}
	return u.mutate(username, func(user *User) { user.Theme = theme })
}

// UpdatePassword re-hashes and stores a new password.
func (u *Users) UpdatePassword(username, secret string) (*User, error) {
	hash, err := password.Hash(secret)
	if err != nil {
		slog.Warn("users: cannot hash password", "error", err)
		return nil, err
	}
	return u.mutate(username, func(user *User) { user.PasswordHash = hash })
}

// Rename changes the username. It fails with ErrEmptyName, ErrNameTaken
// or ErrNoSuchUser when the new name is blank, taken or the current
// account is missing, and with the write error when the file cannot be
// written.
func (u *Users) Rename(currentName, newName string) (*User, error) {
	if newName == "" {
		return nil, ErrEmptyName
	}
	u.lock.Lock()
	defer u.lock.Unlock()
	users, err := u.reload()
	if err != nil {
		slog.Error("users: refusing to rename, the users file is unreadable", "path", u.path, "error", err)
		return nil, err
	}
	if findIn(users, newName) != nil {
		return nil, ErrNameTaken
	}
	users = cloneList(users)
	user := findIn(users, currentName)
	if user == nil {
		return nil, ErrNoSuchUser
	}
	user.Username = newName
	if err := u.persist(users); err != nil {
		return nil, err
	}
	return user, nil
}

// cloneList deep-copies the user list so mutations never write to
// objects a concurrent reader may hold.
func cloneList(users []*User) []*User {
	out := make([]*User, len(users))
	for i, user := range users {
		clone := *user
		out[i] = &clone
	}
	return out
}

// SetRole changes the role, refusing to demote the last admin.
func (u *Users) SetRole(username, role string) (*User, error) {
	if !slices.Contains(Roles, role) {
		return nil, fmt.Errorf("unknown role %q", role)
	}
	u.lock.Lock()
	defer u.lock.Unlock()
	users, err := u.reload()
	if err != nil {
		slog.Error("users: refusing to change a role, the users file is unreadable", "path", u.path, "error", err)
		return nil, err
	}
	users = cloneList(users)
	user := findIn(users, username)
	if user == nil {
		return nil, ErrNoSuchUser
	}
	if user.Role == "admin" && role != "admin" && adminCount(users) <= 1 {
		return nil, ErrLastAdmin
	}
	user.Role = role
	if err := u.persist(users); err != nil {
		return nil, err
	}
	return user, nil
}

// Delete removes a user, refusing to remove the last user or the last
// admin. Returns the removed username, or "".
func (u *Users) Delete(username string) (string, error) {
	u.lock.Lock()
	defer u.lock.Unlock()
	users, err := u.reload()
	if err != nil {
		slog.Error("users: refusing to delete, the users file is unreadable", "path", u.path, "error", err)
		return "", err
	}
	users = cloneList(users)
	target := findIn(users, username)
	if target == nil {
		return "", ErrNoSuchUser
	}
	if len(users) <= 1 || (target.Role == "admin" && adminCount(users) <= 1) {
		return "", ErrLastAdmin
	}
	remaining := make([]*User, 0, len(users)-1)
	for _, user := range users {
		if user.Username != username {
			remaining = append(remaining, user)
		}
	}
	if err := u.persist(remaining); err != nil {
		return "", err
	}
	return username, nil
}

func (u *Users) mutate(username string, apply func(*User)) (*User, error) {
	u.lock.Lock()
	defer u.lock.Unlock()
	users, err := u.reload()
	if err != nil {
		slog.Error("users: refusing to update, the users file is unreadable", "path", u.path, "error", err)
		return nil, err
	}
	users = cloneList(users)
	user := findIn(users, username)
	if user == nil {
		return nil, ErrNoSuchUser
	}
	apply(user)
	if err := u.persist(users); err != nil {
		return nil, err
	}
	return user, nil
}

func findIn(users []*User, username string) *User {
	for _, user := range users {
		if user.Username == username {
			return user
		}
	}
	return nil
}

func adminCount(users []*User) int {
	count := 0
	for _, user := range users {
		if user.Role == "admin" {
			count++
		}
	}
	return count
}

// Health reports why the users file cannot be read, or nil when it is
// fine or absent. A diagnostic uses it to say that accounts are
// unreachable rather than reporting a count of zero.
func (u *Users) Health() error {
	u.mu.Lock()
	defer u.mu.Unlock()
	if _, err := u.loadLocked(); err != nil {
		return err
	}
	return nil
}

// Invalidate drops the cache so the next read re-reads the file.
func (u *Users) Invalidate() {
	u.mu.Lock()
	defer u.mu.Unlock()
	u.cached = nil
	u.snapshot = fileSnapshot{}
	u.haveCache = false
}

// reload re-reads the file under u.mu, dropping the cache first so the
// writer works from what is on disk right now. The caller must hold
// u.lock; taking u.mu inside is the same order persist uses, so the two
// locks never invert.
func (u *Users) reload() ([]*User, error) {
	u.mu.Lock()
	defer u.mu.Unlock()
	u.cached = nil
	u.snapshot = fileSnapshot{}
	u.haveCache = false
	return u.loadLocked()
}

// loadLocked returns the cached account list, rebuilding it when the
// file changed. It returns an error only when the file exists and
// cannot be read or parsed; a missing file is not an error. The caller
// must hold u.mu.
func (u *Users) loadLocked() ([]*User, error) {
	snapshot := u.buildSnapshot()
	if u.haveCache && snapshot == u.snapshot {
		return u.cached, nil
	}
	users, err := u.readFile()
	if err != nil {
		return nil, err
	}
	u.snapshot = snapshot
	u.cached = users
	u.haveCache = true
	return users, nil
}

func (u *Users) buildSnapshot() fileSnapshot {
	info, err := os.Stat(u.path)
	if err != nil {
		return fileSnapshot{}
	}
	return fileSnapshot{present: true, mtime: info.ModTime().UnixNano(), size: info.Size()}
}

// readFile parses the users file. A missing file yields no users and no
// error; an unreadable or unparsable file yields an error, which the
// caller must surface rather than treat as "no users".
func (u *Users) readFile() ([]*User, error) {
	raw, err := os.ReadFile(u.path)
	if err != nil {
		if errors.Is(err, os.ErrNotExist) {
			return nil, nil
		}
		return nil, fmt.Errorf("read %s: %w", u.path, err)
	}
	data, err := interpres.ParseMap(raw)
	if err != nil {
		return nil, fmt.Errorf("parse %s: %w", u.path, err)
	}
	var result []*User
	for _, entry := range tomlfile.Tables(data["users"]) {
		username := tomlfile.String(entry["username"])
		hash := tomlfile.String(entry["password_hash"])
		if username == "" {
			continue
		}
		role := tomlfile.String(entry["role"])
		if role == "" {
			role = DefaultRole
		}
		result = append(result, &User{
			Username:         username,
			PasswordHash:     hash,
			Role:             role,
			Name:             tomlfile.String(entry["name"]),
			FediverseCreator: tomlfile.String(entry["fediverse_creator"]),
			Orcid:            tomlfile.String(entry["orcid"]),
			Photo:            tomlfile.String(entry["photo"]),
			Language:         tomlfile.String(entry["language"]),
			Theme:            tomlfile.String(entry["theme"]),
			TotpSecret:       tomlfile.String(entry["totp_secret"]),
			TotpStep:         tomlfile.Int64(entry["totp_step"], 0),
			Recovery:         readTotpRecovery(entry),
		})
	}
	for _, user := range result {
		if password.NeedsRehash(user.PasswordHash) {
			slog.Warn("users: stored hash uses parameters the login rejects; the password must be reset out of band",
				"username", user.Username)
		}
	}
	return result, nil
}

func (u *Users) persist(users []*User) error {
	entries := make([]map[string]any, 0, len(users))
	for _, user := range users {
		entry := map[string]any{
			"username":      user.Username,
			"password_hash": user.PasswordHash,
			"role":          user.Role,
		}
		if user.Name != "" {
			entry["name"] = user.Name
		}
		if user.FediverseCreator != "" {
			entry["fediverse_creator"] = user.FediverseCreator
		}
		if user.Orcid != "" {
			entry["orcid"] = user.Orcid
		}
		if user.Photo != "" {
			entry["photo"] = user.Photo
		}
		if user.Language != "" {
			entry["language"] = user.Language
		}
		if user.Theme != "" {
			entry["theme"] = user.Theme
		}
		if user.TotpSecret != "" {
			entry["totp_secret"] = user.TotpSecret
			entry["totp_step"] = user.TotpStep
		}
		if len(user.Recovery) > 0 {
			recovery := make([]any, len(user.Recovery))
			for i, hash := range user.Recovery {
				recovery[i] = hash
			}
			entry["recovery"] = recovery
		}
		entries = append(entries, entry)
	}
	if err := tomlfile.Write(u.path, "users", entries); err != nil {
		slog.Error("users: cannot persist", "path", u.path, "error", err)
		return err
	}
	u.mu.Lock()
	u.cached = nil
	u.snapshot = fileSnapshot{}
	u.haveCache = false
	u.mu.Unlock()
	return nil
}