// Package settings is the runtime-settings service: the small set of
// switches an administrator can flip without restarting the server.
//
// Each setting has a value in the config file and, optionally, a value in
// the server_settings table. The stored value wins when the config allows
// the setting to be toggled at runtime; otherwise the file is authoritative
// and the stored value is ignored (but kept, so that re-enabling the toggle
// restores the administrator's last choice).
//
// Readers get a cached value — these are consulted on every federated
// request — and components that must react to a change subscribe.
package settings

import (
	"context"
	"errors"
	"fmt"
	"log/slog"
	"strconv"
	"sync"

	"github.com/dariuszjaniak/warp/server/internal/storage"
)

// Setting keys as stored in server_settings.
const (
	KeyFederationEnabled   = "federation.enabled"
	KeyRegistrationEnabled = "registration.enabled"
)

// ErrNotToggleable is returned when a setting is pinned by the config file.
var ErrNotToggleable = errors.New("setting is not toggleable at runtime")

// Defaults come from the config file.
type Defaults struct {
	FederationEnabled            bool
	FederationAllowRuntimeToggle bool

	RegistrationEnabled            bool
	RegistrationAllowRuntimeToggle bool
}

// Snapshot is a consistent view of every runtime setting.
type Snapshot struct {
	FederationEnabled   bool
	RegistrationEnabled bool
}

// Service caches the runtime settings and persists changes.
type Service struct {
	store    storage.SettingsStore
	log      *slog.Logger
	defaults Defaults

	mu        sync.RWMutex
	current   Snapshot
	listeners []func(Snapshot)

	// writeMu serialises Set calls; see set.
	writeMu sync.Mutex
}

// New loads the persisted settings and returns a service. A stored value
// that cannot be parsed is logged and ignored rather than failing the boot:
// an unreadable switch should not keep the server down.
func New(ctx context.Context, store storage.SettingsStore, defaults Defaults, log *slog.Logger) (*Service, error) {
	if log == nil {
		log = slog.Default()
	}
	s := &Service{
		store:    store,
		log:      log,
		defaults: defaults,
		current: Snapshot{
			FederationEnabled:   defaults.FederationEnabled,
			RegistrationEnabled: defaults.RegistrationEnabled,
		},
	}

	stored, err := store.GetAll(ctx)
	if err != nil {
		return nil, fmt.Errorf("load server settings: %w", err)
	}
	if defaults.FederationAllowRuntimeToggle {
		s.current.FederationEnabled = s.boolOr(stored, KeyFederationEnabled, defaults.FederationEnabled)
	}
	if defaults.RegistrationAllowRuntimeToggle {
		s.current.RegistrationEnabled = s.boolOr(stored, KeyRegistrationEnabled, defaults.RegistrationEnabled)
	}

	s.log.Info("runtime settings loaded",
		"federation_enabled", s.current.FederationEnabled,
		"registration_enabled", s.current.RegistrationEnabled)
	return s, nil
}

func (s *Service) boolOr(stored map[string]string, key string, fallback bool) bool {
	raw, ok := stored[key]
	if !ok {
		return fallback
	}
	v, err := strconv.ParseBool(raw)
	if err != nil {
		s.log.Warn("ignoring unparseable stored setting", "key", key, "value", raw, "error", err)
		return fallback
	}
	return v
}

// Snapshot returns the current values.
func (s *Service) Snapshot() Snapshot {
	s.mu.RLock()
	defer s.mu.RUnlock()
	return s.current
}

// FederationEnabled reports whether the federation surface is live. It is
// called on every federation request, so it only takes a read lock.
func (s *Service) FederationEnabled() bool {
	s.mu.RLock()
	defer s.mu.RUnlock()
	return s.current.FederationEnabled
}

// RegistrationEnabled reports whether open registration is accepted.
func (s *Service) RegistrationEnabled() bool {
	s.mu.RLock()
	defer s.mu.RUnlock()
	return s.current.RegistrationEnabled
}

// FederationToggleable reports whether SetFederationEnabled is allowed.
func (s *Service) FederationToggleable() bool { return s.defaults.FederationAllowRuntimeToggle }

// RegistrationToggleable reports whether SetRegistrationEnabled is allowed.
func (s *Service) RegistrationToggleable() bool { return s.defaults.RegistrationAllowRuntimeToggle }

// SetFederationEnabled persists and applies the federation switch.
func (s *Service) SetFederationEnabled(ctx context.Context, enabled bool) error {
	if !s.defaults.FederationAllowRuntimeToggle {
		return fmt.Errorf("federation.enabled: %w (federation.allow_runtime_toggle is false)", ErrNotToggleable)
	}
	return s.set(ctx, KeyFederationEnabled, enabled, func(snap *Snapshot) { snap.FederationEnabled = enabled })
}

// SetRegistrationEnabled persists and applies the registration switch.
func (s *Service) SetRegistrationEnabled(ctx context.Context, enabled bool) error {
	if !s.defaults.RegistrationAllowRuntimeToggle {
		return fmt.Errorf("registration.enabled: %w (registration.allow_runtime_toggle is false)", ErrNotToggleable)
	}
	return s.set(ctx, KeyRegistrationEnabled, enabled, func(snap *Snapshot) { snap.RegistrationEnabled = enabled })
}

// set writes the store first: if persistence fails the in-memory value must
// not change, or a restart would silently undo what the admin API reported
// as done.
func (s *Service) set(ctx context.Context, key string, value bool, apply func(*Snapshot)) error {
	// writeMu serialises the whole write path, so two concurrent toggles
	// cannot persist in one order and notify subscribers in another.
	s.writeMu.Lock()
	defer s.writeMu.Unlock()

	if err := s.store.Set(ctx, key, strconv.FormatBool(value)); err != nil {
		return err
	}

	s.mu.Lock()
	before := s.current
	apply(&s.current)
	after := s.current
	listeners := make([]func(Snapshot), len(s.listeners))
	copy(listeners, s.listeners)
	s.mu.Unlock()

	if before == after {
		return nil
	}
	s.log.Info("runtime setting changed", "key", key, "value", value)
	// Listeners are called outside s.mu: a federation module starting or
	// stopping can take a while and must not block readers.
	for _, fn := range listeners {
		fn(after)
	}
	return nil
}

// Subscribe registers fn to be called after any setting changes. It is
// called with the new snapshot, never concurrently with itself.
func (s *Service) Subscribe(fn func(Snapshot)) {
	s.mu.Lock()
	defer s.mu.Unlock()
	s.listeners = append(s.listeners, fn)
}