monitor-lets-go/internal/backend/sway.go

package backend

import (
	"bytes"
	"context"
	"encoding/binary"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"log/slog"
	"net"
	"os"
	"os/exec"
	"strings"
	"sync"
	"time"
)

// i3 IPC protocol constants.
// Sway uses the same protocol as i3 with the socket at $SWAYSOCK.
const (
	i3Magic            = "i3-ipc"
	i3Command          = 0  // run a command
	i3Subscribe        = 2  // subscribe to events
	i3GetOutputs       = 3  // get output (monitor) list
	i3EventOutput      = 1  // event: output added, removed, or changed
	i3EventMask uint32 = 0x80000000 // flag on event message types
)

// swayBackend implements Backend for the Sway Wayland compositor via the
// i3-compatible IPC protocol.
//
// Monitor queries:   swaymsg -t get_outputs
// Layout application: swaymsg 'output <name> enable; output <name> resolution ... position ... scale ...; output <name> disable'
// Hotplug events:    raw IPC socket ($SWAYSOCK), subscribing to ["output"]
type swayBackend struct {
	logger *slog.Logger

	// socketPath is the sway IPC socket ($SWAYSOCK or discovered).
	socketPath string

	// conn is the current IPC connection for the event stream; guarded by mu.
	conn net.Conn
	mu   sync.Mutex
}

// NewSway creates a Sway backend. It discovers the IPC socket from the
// SWAYSOCK environment variable or by scanning XDG_RUNTIME_DIR.
//
// opts carries backend-specific configuration from the config file's
// backend_config section. Sway currently supports no backend-specific
// options — layout is applied via swaymsg commands, not config files.
func NewSway(logger *slog.Logger, _ map[string]any) (Backend, error) {
	socketPath, err := findSwaySocket()
	if err != nil {
		return nil, err
	}
	return &swayBackend{logger: logger, socketPath: socketPath}, nil
}

func (s *swayBackend) Name() string { return "sway" }

// ---------------------------------------------------------------------------
// Monitor querying
// ---------------------------------------------------------------------------

// swayOutput mirrors the JSON structure returned by swaymsg -t get_outputs
// and the output event payload. Pointer fields handle null values for
// disabled outputs.
type swayOutput struct {
	Name        string     `json:"name"`
	Make        string     `json:"make"`
	Model       string     `json:"model"`
	Serial      string     `json:"serial"`
	Active      bool       `json:"active"`
	Scale       float64    `json:"scale"`
	CurrentMode *swayMode  `json:"current_mode"`
	Rect        swayRect   `json:"rect"`
	Modes       []swayMode `json:"modes"`
}

type swayMode struct {
	Width   int     `json:"width"`
	Height  int     `json:"height"`
	Refresh float64 `json:"refresh"` // millihertz, e.g. 60002 = 60.002 Hz
}

type swayRect struct {
	X      int `json:"x"`
	Y      int `json:"y"`
	Width  int `json:"width"`
	Height int `json:"height"`
}

// nativeResolution returns the highest-pixel-count mode from the modes list
// as a stable hardware identifier, regardless of the display's current power
// or enabled state.
func nativeResolution(modes []swayMode) (int, int) {
	var bestW, bestH, bestPixels int
	for _, m := range modes {
		if p := m.Width * m.Height; p > bestPixels {
			bestW, bestH, bestPixels = m.Width, m.Height, p
		}
	}
	return bestW, bestH
}

// toMonitorInfo converts a raw swayOutput into the backend-agnostic
// MonitorInfo. Disabled outputs have a nil CurrentMode and report
// Width=0, Height=0 so determineState correctly skips them.
func (so *swayOutput) toMonitorInfo() MonitorInfo {
	// Build a description from make + model + serial.
	// Serial helps differentiate monitors with identical make+model
	// (e.g. two Xiaomi Mi Monitors). We also append the native resolution
	// (largest pixel count from the modes list) as a stable hardware
	// identifier that never changes.
	desc := strings.TrimSpace(so.Make + " " + so.Model)
	if so.Serial != "" && so.Serial != "Unknown" {
		desc += " " + so.Serial
	}

	// Include the native resolution from the modes list. This gives a
	// stable unique identifier that works even when the monitor is disabled
	// (CurrentMode is nil but modes list is always populated).
	if maxW, maxH := nativeResolution(so.Modes); maxW > 0 && maxH > 0 {
		desc += fmt.Sprintf(" %dx%d", maxW, maxH)
	}

	width := 0
	height := 0
	refresh := 0.0

	if so.CurrentMode != nil {
		width = so.CurrentMode.Width
		height = so.CurrentMode.Height
		refresh = so.CurrentMode.Refresh / 1000.0 // mHz → Hz
	}

	// Disabled outputs carry rect={0,0,0,0}; use sensible defaults.
	x := so.Rect.X
	y := so.Rect.Y

	if so.Scale <= 0 {
		so.Scale = 1
	}

	return MonitorInfo{
		Name:        so.Name,
		Description: desc,
		Make:        so.Make,
		Model:       so.Model,
		Serial:      so.Serial,
		Width:       width,
		Height:      height,
		RefreshRate: refresh,
		X:           x,
		Y:           y,
		Scale:       so.Scale,
		Enabled:     so.Active,
	}
}

// GetMonitors calls swaymsg -t get_outputs and parses the JSON response.
// It returns both active and inactive monitors so the daemon can detect
// physically-connected but disabled externals.
func (s *swayBackend) GetMonitors(ctx context.Context) ([]MonitorInfo, error) {
	cmd := exec.CommandContext(ctx, "swaymsg", "-t", "get_outputs")
	cmd.Env = s.swayEnv()
	out, err := cmd.Output()
	if err != nil {
		return nil, fmt.Errorf("swaymsg get_outputs: %w", err)
	}

	var raw []swayOutput
	if err := json.Unmarshal(out, &raw); err != nil {
		return nil, fmt.Errorf("parse swaymsg output: %w\nraw: %s", err, string(out))
	}

	monitors := make([]MonitorInfo, len(raw))
	for i, so := range raw {
		monitors[i] = so.toMonitorInfo()
	}
	return monitors, nil
}

// ---------------------------------------------------------------------------
// Layout application
// ---------------------------------------------------------------------------

// ApplyLayout applies monitor configurations via swaymsg commands.
// Enabled monitors are configured first (enable → resolution → position → scale),
// then disabled monitors are turned off. All commands are sent in a single
// swaymsg call separated by ';' so Sway executes them as one IPC message.
//
// Monitor names prefixed with "desc:" are resolved to actual connector names
// by querying the current monitor state. This makes configs portable across
// dock ports and reboots.
//
// Safety: the daemon guarantees at least one enabled monitor exists before
// calling ApplyLayout. Sway refuses to disable the last active output, so
// enable commands always precede disables.
func (s *swayBackend) ApplyLayout(ctx context.Context, monitors []MonitorConfig) error {
	var err error
	monitors, err = resolveMonitorNames(ctx, monitors, s.GetMonitors, s.logger)
	if err != nil {
		return fmt.Errorf("resolve monitor names: %w", err)
	}

	cmd := s.buildCommands(monitors)
	if cmd == "" {
		return errors.New("no commands to apply")
	}

	c := exec.CommandContext(ctx, "swaymsg", cmd)
	c.Env = s.swayEnv()
	if out, err := c.CombinedOutput(); err != nil {
		return fmt.Errorf("swaymsg: %w\noutput: %s", err, string(out))
	}

	s.logger.Info("layout applied", "monitors", len(monitors))
	return nil
}

// buildCommands assembles the swaymsg command string.
// Enabled monitors go first (enable + configure), then disabled ones.
// Commands are joined with ';' for batch execution.
func (s *swayBackend) buildCommands(monitors []MonitorConfig) string {
	var cmds []string

	for _, m := range monitors {
		if !m.Enabled {
			continue
		}

		cmds = append(cmds, fmt.Sprintf("output %s enable", m.Name))
		// Explicitly wake the monitor from DPMS sleep. Sway sometimes
		// keeps dpms=false after docking cycles, leaving the screen black.
		cmds = append(cmds, fmt.Sprintf("output %s dpms on", m.Name))

		if mode := s.formatMode(m.Mode); mode != "" {
			cmds = append(cmds, fmt.Sprintf("output %s resolution %s", m.Name, mode))
		}

		if x, y, ok := parseSwayPosition(m.Position); ok {
			cmds = append(cmds, fmt.Sprintf("output %s position %d %d", m.Name, x, y))
		}

		if m.Scale > 0 {
			cmds = append(cmds, fmt.Sprintf("output %s scale %s", m.Name, formatScale(m.Scale)))
		}
	}

	// Disable monitors only after all enabled ones are configured.
	for _, m := range monitors {
		if m.Enabled {
			continue
		}
		cmds = append(cmds, fmt.Sprintf("output %s disable", m.Name))
	}

	return strings.Join(cmds, "; ")
}

// formatMode converts a user-configured mode (e.g. "2560x1440@144")
// into Sway's syntax ("2560x1440@144Hz"). Returns "" for "preferred"
// or empty, meaning Sway should use its default mode.
func (s *swayBackend) formatMode(mode string) string {
	if mode == "" || mode == "preferred" {
		return ""
	}
	if !strings.HasSuffix(mode, "Hz") {
		mode += "Hz"
	}
	return mode
}

// parseSwayPosition splits a position string "1920x0" into (1920, 0).
// Returns false for "auto", empty, or invalid formats.
func parseSwayPosition(pos string) (int, int, bool) {
	if pos == "" || pos == "auto" {
		return 0, 0, false
	}
	parts := strings.SplitN(pos, "x", 2)
	if len(parts) != 2 {
		return 0, 0, false
	}
	// Simple atoi; errors are rare at this point since the config
	// is validated, but we guard anyway.
	x, errX := parseInt(parts[0])
	y, errY := parseInt(parts[1])
	if errX != nil || errY != nil {
		return 0, 0, false
	}
	return x, y, true
}

// parseInt is a small helper for parseSwayPosition.
func parseInt(s string) (int, error) {
	var n int
	neg := false
	if len(s) > 0 && s[0] == '-' {
		neg = true
		s = s[1:]
	}
	for _, ch := range s {
		if ch < '0' || ch > '9' {
			return 0, fmt.Errorf("invalid digit %c", ch)
		}
		n = n*10 + int(ch-'0')
	}
	if neg {
		n = -n
	}
	return n, nil
}

// ---------------------------------------------------------------------------
// Hotplug events via raw IPC socket
// ---------------------------------------------------------------------------

// Events opens a connection to the Sway IPC socket, subscribes to output
// events, and emits parsed hotplug notifications. On connection loss it
// attempts reconnection with exponential backoff.
//
// The caller MUST read from both channels. The event channel is buffered
// (depth 8) to absorb bursts during reconnection.
func (s *swayBackend) Events(ctx context.Context) (<-chan Event, <-chan error) {
	eventCh := make(chan Event, 8)
	errCh := make(chan error, 1)

	go func() {
		defer close(eventCh)
		defer close(errCh)

		backoff := 1 * time.Second
		for {
			if err := s.readEvents(ctx, eventCh); err != nil {
				if ctx.Err() != nil {
					return // graceful shutdown
				}
				s.logger.Error("sway IPC disconnected, reconnecting",
					"error", err, "backoff", backoff)

				select {
				case <-ctx.Done():
					return
				case <-time.After(backoff):
				}

				backoff = min(backoff*2, 30*time.Second)
			} else {
				backoff = 1 * time.Second
			}
		}
	}()

	return eventCh, errCh
}

// readEvents connects to the Sway IPC socket, sends a SUBSCRIBE message
// for ["output"], then reads events until the connection breaks or ctx
// is cancelled.
func (s *swayBackend) readEvents(ctx context.Context, eventCh chan<- Event) error {
	var d net.Dialer
	conn, err := d.DialContext(ctx, "unix", s.socketPath)
	if err != nil {
		return fmt.Errorf("connect to %s: %w", s.socketPath, err)
	}
	defer conn.Close()

	s.mu.Lock()
	s.conn = conn
	s.mu.Unlock()
	defer func() {
		s.mu.Lock()
		s.conn = nil
		s.mu.Unlock()
	}()

	s.logger.Info("connected to sway IPC", "socket", s.socketPath)

	// Subscribe to output events.
	subscribePayload := `["output"]`
	if err := writeIPCMessage(conn, i3Subscribe, []byte(subscribePayload)); err != nil {
		return fmt.Errorf("subscribe: %w", err)
	}

	// Read the subscribe response. Sway/i3 may reply with type
	// SUBSCRIBE (2) or COMMAND (0); both carry {"success": true}.
	msgType, payload, err := readIPCMessage(conn)
	if err != nil {
		return fmt.Errorf("read subscribe response: %w", err)
	}
	if msgType != i3Command && msgType != i3Subscribe {
		s.logger.Warn("unexpected subscribe response type",
			"type", msgType, "payload", string(payload))
	}

	// Read events indefinitely. Each event message carries the type
	// i3EventOutput | i3EventMask.
	for {
		msgType, payload, err := readIPCMessage(conn)
		if err != nil {
			return fmt.Errorf("read event: %w", err)
		}

		if msgType != (i3EventOutput | i3EventMask) {
			s.logger.Debug("ignoring non-output event", "type", msgType&^i3EventMask)
			continue
		}

		evt, ok := parseOutputEvent(payload)
		if !ok {
			continue
		}

		select {
		case eventCh <- evt:
		case <-ctx.Done():
			return ctx.Err()
		}
	}
}

// swayEvent is a minimal view of an output event payload.
// We only need the change type and monitor name; additional fields
// (make, model, etc.) are ignored because the daemon calls
// GetMonitors after the debounce period to get the full state.
type swayEvent struct {
	Change string `json:"change"`
	Name   string `json:"name"`
}

// parseOutputEvent extracts a monitor hotplug event from an IPC payload.
// Sway emits "added" and "removed" change values. Other change types
// (e.g. mode/scale changes fired by our own layout application) are
// ignored to avoid noise.
func parseOutputEvent(payload []byte) (Event, bool) {
	var se swayEvent
	if err := json.Unmarshal(payload, &se); err != nil {
		return Event{}, false
	}

	switch se.Change {
	case "added":
		return Event{Type: EventMonitorAdded, MonitorName: se.Name}, true
	case "removed":
		return Event{Type: EventMonitorRemoved, MonitorName: se.Name}, true
	default:
		// "unknown", "unspecified", or our own configuration changes.
		// Ignore to avoid reacting to our own ApplyLayout calls.
		return Event{}, false
	}
}

// ---------------------------------------------------------------------------
// IPC protocol helpers
// ---------------------------------------------------------------------------

// readIPCMessage reads one framed message from a Sway IPC connection.
// The i3 protocol header is:
//
//	"i3-ipc"  (6 bytes magic)
//	uint32 LE (4 bytes payload length)
//	uint32 LE (4 bytes message type)
//	payload    (variable bytes)
func readIPCMessage(r io.Reader) (msgType uint32, payload []byte, err error) {
	var magic [6]byte
	if _, err := io.ReadFull(r, magic[:]); err != nil {
		return 0, nil, fmt.Errorf("read magic: %w", err)
	}
	if string(magic[:]) != i3Magic {
		return 0, nil, fmt.Errorf("bad magic: %q", string(magic[:]))
	}

	var length uint32
	if err := binary.Read(r, binary.LittleEndian, &length); err != nil {
		return 0, nil, fmt.Errorf("read length: %w", err)
	}

	if err := binary.Read(r, binary.LittleEndian, &msgType); err != nil {
		return 0, nil, fmt.Errorf("read type: %w", err)
	}

	payload = make([]byte, length)
	if _, err := io.ReadFull(r, payload); err != nil {
		return 0, nil, fmt.Errorf("read payload: %w", err)
	}

	return msgType, payload, nil
}

// writeIPCMessage sends one framed message to a Sway IPC connection.
func writeIPCMessage(w io.Writer, msgType uint32, payload []byte) error {
	var buf bytes.Buffer
	buf.WriteString(i3Magic)
	_ = binary.Write(&buf, binary.LittleEndian, uint32(len(payload)))
	_ = binary.Write(&buf, binary.LittleEndian, msgType)
	buf.Write(payload)

	_, err := w.Write(buf.Bytes())
	return err
}

// ---------------------------------------------------------------------------
// Socket discovery
// ---------------------------------------------------------------------------

// findSwaySocket locates the Sway IPC socket. It checks the SWAYSOCK
// environment variable first, then falls back to scanning XDG_RUNTIME_DIR
// for sway-ipc.*.sock files.
func findSwaySocket() (string, error) {
	if sock := os.Getenv("SWAYSOCK"); sock != "" {
		return sock, nil
	}

	runtimeDir := os.Getenv("XDG_RUNTIME_DIR")
	if runtimeDir == "" {
		runtimeDir = fmt.Sprintf("/run/user/%d", os.Getuid())
	}

	entries, err := os.ReadDir(runtimeDir)
	if err != nil {
		return "", fmt.Errorf("cannot read runtime dir %s: %w", runtimeDir, err)
	}

	for _, e := range entries {
		if strings.HasPrefix(e.Name(), "sway-ipc.") && strings.HasSuffix(e.Name(), ".sock") {
			return runtimeDir + "/" + e.Name(), nil
		}
	}

	return "", errors.New("cannot find Sway IPC socket; set SWAYSOCK or ensure Sway is running")
}

// swayEnv returns the environment for swaymsg subprocesses, injecting
// SWAYSOCK so the commands work even when launched from systemd or a
// context where the variable is not inherited.
func (s *swayBackend) swayEnv() []string {
	return append(os.Environ(), "SWAYSOCK="+s.socketPath)
}

// ---------------------------------------------------------------------------
// Close
// ---------------------------------------------------------------------------

func (s *swayBackend) Close() error {
	s.mu.Lock()
	defer s.mu.Unlock()
	if s.conn != nil {
		return s.conn.Close()
	}
	return nil
}