//go:build linux

package evidence

import (
	"context"

	"github.com/jezek/xgb/xproto"
	"github.com/jezek/xgbutil"
	"github.com/jezek/xgbutil/ewmh"
	"github.com/jezek/xgbutil/icccm"
	"github.com/jezek/xgbutil/xevent"
	"github.com/jezek/xgbutil/xprop"
	"github.com/jezek/xgbutil/xwindow"
)

// NewSource returns the real X11 active-window sensor.
func NewSource() Source { return &x11Source{} }

type x11Source struct{}

// Watch opens one long-lived X connection, subscribes to _NET_ACTIVE_WINDOW
// changes on the root window, and emits a snapshot immediately plus on every
// change. Any failure degrades to an Unavailable snapshot; it never panics the
// daemon.
func (s *x11Source) Watch(ctx context.Context, onChange func(WindowSnapshot)) {
	X, err := xgbutil.NewConn()
	if err != nil {
		onChange(unavailable("cannot connect to X server: " + err.Error()))
		<-ctx.Done()
		return
	}
	defer X.Conn().Close()

	root := X.RootWin()
	activeAtom, err := xprop.Atm(X, "_NET_ACTIVE_WINDOW")
	if err != nil {
		onChange(unavailable("no _NET_ACTIVE_WINDOW atom: " + err.Error()))
		<-ctx.Done()
		return
	}

	// Listen for property changes on the root window.
	if err := xwindow.New(X, root).Listen(xproto.EventMaskPropertyChange); err != nil {
		onChange(unavailable("cannot listen on root window: " + err.Error()))
		<-ctx.Done()
		return
	}

	emit := func() { onChange(snapshot(X)) }
	emit() // immediate current window

	xevent.PropertyNotifyFun(func(_ *xgbutil.XUtil, ev xevent.PropertyNotifyEvent) {
		if ev.Atom == activeAtom {
			emit()
		}
	}).Connect(X, root)

	// Run the event loop until ctx is cancelled.
	go func() {
		<-ctx.Done()
		xevent.Quit(X)
	}()
	xevent.Main(X)
}

func snapshot(X *xgbutil.XUtil) WindowSnapshot {
	active, err := ewmh.ActiveWindowGet(X)
	if err != nil || active == 0 {
		return unavailable("no active window")
	}
	title, err := ewmh.WmNameGet(X, active)
	if err != nil || title == "" {
		// Fall back to ICCCM WM_NAME.
		if t, e := icccm.WmNameGet(X, active); e == nil {
			title = t
		}
	}
	var class string
	if wmClass, err := icccm.WmClassGet(X, active); err == nil && wmClass != nil {
		class = wmClass.Class
	}
	return WindowSnapshot{
		Title:  title,
		Class:  class,
		Health: EvidenceHealth{Available: true},
	}
}