// Package daemon implements the core event loop and state machine
// for automatic monitor layout switching.
//
// The daemon starts by determining the current hardware state (portable or
// docked) and applying the correct layout. It then subscribes to compositor
// hotplug events via the Backend and re-evaluates the layout after a
// debounce period. A polling fallback runs on a timer to catch missed events.
//
// Safety invariants:
//   - A layout with zero enabled monitors is never applied.
//   - Docked mode is never applied unless an external monitor is physically
//     connected.
//   - On shutdown, the portable layout is restored (configurable).
package daemon

import (
	"context"
	"fmt"
	"log/slog"
	"time"

	"monitor-lets-go/internal/backend"
	"monitor-lets-go/internal/config"
	"monitor-lets-go/internal/hook"
)

// Daemon orchestrates monitor switching.
type Daemon struct {
	backend    backend.Backend
	config     *config.Config
	hookRunner *hook.Runner
	logger     *slog.Logger
	state      backend.State
}

// New creates a Daemon with the given dependencies.
func New(b backend.Backend, cfg *config.Config, hr *hook.Runner, logger *slog.Logger) *Daemon {
	return &Daemon{
		backend:    b,
		config:     cfg,
		hookRunner: hr,
		logger:     logger,
		state:      backend.StateUnknown,
	}
}

// Run starts the daemon's event loop. It blocks until ctx is cancelled.
func (d *Daemon) Run(ctx context.Context) error {
	d.logger.Info("daemon starting", "backend", d.backend.Name())

	// Phase 1: determine and apply the initial state.
	d.applyInitialState(ctx)

	// Phase 2: subscribe to compositor events.
	events, _ := d.backend.Events(ctx)

	pollInterval := time.Duration(d.config.PollInterval)
	var pollTicker *time.Ticker
	var pollCh <-chan time.Time
	if pollInterval > 0 {
		pollTicker = time.NewTicker(pollInterval)
		pollCh = pollTicker.C
		defer pollTicker.Stop()
	}

	var debounceTimer *time.Timer
	var debounceCh <-chan time.Time

	for {
		select {
		case <-ctx.Done():
			d.onShutdown(context.Background())
			return ctx.Err()

		case evt, ok := <-events:
			if !ok {
				if ctx.Err() != nil {
					return ctx.Err()
				}
				d.logger.Error("event channel closed unexpectedly")
				return fmt.Errorf("backend event stream terminated")
			}

			d.logger.Debug("hotplug event",
				"type", evt.Type, "monitor", evt.MonitorName)

			// Reset the debounce timer on every event.
			if debounceTimer != nil {
				debounceTimer.Stop()
			}
			debounceTimer = time.NewTimer(time.Duration(d.config.Debounce))
			debounceCh = debounceTimer.C

		case <-debounceCh:
			debounceCh = nil
			debounceTimer = nil
			d.checkAndApply(ctx)

		case <-pollCh:
			d.pollCheck(ctx)
		}
	}
}

// applyInitialState determines the current hardware state and applies the
// matching layout. This is the failsafe: if the daemon crashed while docked,
// a restart will detect that externals are gone and re-enable the built-in.
func (d *Daemon) applyInitialState(ctx context.Context) {
	ctx, cancel := context.WithTimeout(ctx, 10*time.Second)
	defer cancel()

	monitors, err := d.getMonitorsWithRetry(ctx)
	if err != nil {
		d.logger.Error("cannot get monitors on startup", "error", err)
		return
	}

	state := d.determineState(monitors)
	d.logger.Info("initial state determined", "state", state)

	if err := d.applyState(ctx, state); err != nil {
		d.logger.Error("failed to apply initial state", "state", state, "error", err)
	}
}

// checkAndApply is called after the debounce timer fires. It re-queries
// monitors and applies a new layout if the state has changed.
func (d *Daemon) checkAndApply(ctx context.Context) {
	ctx, cancel := context.WithTimeout(ctx, 10*time.Second)
	defer cancel()

	monitors, err := d.getMonitorsWithRetry(ctx)
	if err != nil {
		d.logger.Error("cannot get monitors", "error", err)
		return
	}

	newState := d.determineState(monitors)
	if newState == d.state {
		return
	}

	if err := d.applyState(ctx, newState); err != nil {
		d.logger.Error("failed to apply state", "state", newState, "error", err)
	}
}

// pollCheck is the polling fallback. It hashes the current monitor state
// and applies a new layout if the hash has changed.
func (d *Daemon) pollCheck(ctx context.Context) {
	ctx, cancel := context.WithTimeout(ctx, 10*time.Second)
	defer cancel()

	monitors, err := d.backend.GetMonitors(ctx)
	if err != nil {
		return // silent; socket events are primary
	}

	current := d.determineState(monitors)
	if current == d.state {
		return
	}

	d.logger.Debug("polling detected state change", "state", current)
	if err := d.applyState(ctx, current); err != nil {
		d.logger.Error("polling apply failed", "state", current, "error", err)
	}
}

// determineState checks whether any external monitor is physically connected.
func (d *Daemon) determineState(monitors []backend.MonitorInfo) backend.State {
	for _, m := range monitors {
		// Skip phantom or disconnected monitors.
		if m.Name == "" {
			continue
		}
		// A monitor is physically present if it reports non-zero dimensions.
		if m.Width > 0 && m.Height > 0 {
			if d.config.MatchesExternal(m.Name, m.Description) {
				return backend.StateDocked
			}
		}
	}
	return backend.StatePortable
}

// applyState applies a monitor layout and runs post-switch hooks.
// Safety invariants are enforced before any layout change.
func (d *Daemon) applyState(ctx context.Context, state backend.State) error {
	d.logger.Debug("applying state", "state", state)

	mode, ok := d.config.Modes[state.String()]
	if !ok {
		return fmt.Errorf("no mode config for %s", state)
	}

	enabled := countEnabled(mode.Monitors)
	if enabled == 0 {
		return fmt.Errorf("refusing %s mode: 0 enabled monitors", state)
	}

	// Safety: if entering docked mode, verify at least one external is
	// physically connected before we disable the built-in display.
	if state == backend.StateDocked {
		monitors, err := d.backend.GetMonitors(ctx)
		if err != nil {
			return fmt.Errorf("verify externals before dock: %w", err)
		}
		if !d.anyExternalConnected(monitors) {
			d.logger.Warn("docked mode skipped: no external monitors connected")
			// Stay in the current state instead of risking a black screen.
			return fmt.Errorf("docked mode: no external monitors connected")
		}
	}

	// Convert config entries to backend format.
	monitors := make([]backend.MonitorConfig, len(mode.Monitors))
	for i, e := range mode.Monitors {
		monitors[i] = backend.MonitorConfig{
			Name:     e.Name,
			Enabled:  e.Enabled,
			Mode:     e.Mode,
			Position: e.Position,
			Scale:    e.Scale,
		}
	}

	if err := d.backend.ApplyLayout(ctx, monitors); err != nil {
		return fmt.Errorf("apply layout: %w", err)
	}

	d.state = state
	d.logger.Info("layout applied", "state", state)

	// Run hooks asynchronously; failures are logged but never returned.
	hookKey := "on_dock"
	if state == backend.StatePortable {
		hookKey = "on_undock"
	}
	if cmds, ok := d.config.Hooks[hookKey]; ok && len(cmds) > 0 {
		// Hooks run with their own context so they outlive the request.
		go d.hookRunner.Run(context.Background(), cmds)
	}

	return nil
}

// anyExternalConnected returns true if at least one configured external
// monitor is physically present.
func (d *Daemon) anyExternalConnected(monitors []backend.MonitorInfo) bool {
	for _, m := range monitors {
		if m.Width > 0 && m.Height > 0 && m.Name != "" {
			if d.config.MatchesExternal(m.Name, m.Description) {
				return true
			}
		}
	}
	return false
}

// getMonitorsWithRetry calls GetMonitors up to 3 times with 500ms delays.
func (d *Daemon) getMonitorsWithRetry(ctx context.Context) ([]backend.MonitorInfo, error) {
	const maxRetries = 3
	const retryDelay = 500 * time.Millisecond

	var lastErr error
	for i := 0; i < maxRetries; i++ {
		monitors, err := d.backend.GetMonitors(ctx)
		if err == nil {
			return monitors, nil
		}
		lastErr = err
		d.logger.Debug("get monitors retry", "attempt", i+1, "error", err)

		select {
		case <-ctx.Done():
			return nil, ctx.Err()
		case <-time.After(retryDelay):
		}
	}
	return nil, fmt.Errorf("after %d retries: %w", maxRetries, lastErr)
}

// onShutdown applies the portable layout before exit if configured.
func (d *Daemon) onShutdown(ctx context.Context) {
	if d.config.RestoreOnExit == nil || !*d.config.RestoreOnExit {
		return
	}
	if d.state == backend.StatePortable {
		return // already portable
	}

	d.logger.Info("shutdown: restoring portable layout")
	ctx, cancel := context.WithTimeout(ctx, 10*time.Second)
	defer cancel()

	if err := d.applyState(ctx, backend.StatePortable); err != nil {
		d.logger.Error("shutdown restore failed", "error", err)
	}
}

// countEnabled returns how many MonitorEntries are enabled.
func countEnabled(entries []config.MonitorEntry) int {
	n := 0
	for _, e := range entries {
		if e.Enabled {
			n++
		}
	}
	return n
}