//go:build unix

package ui

import (
	"bytes"
	"errors"
	"os"
	"time"

	"golang.org/x/sys/unix"
	"golang.org/x/term"
)

// probeKittyGraphics asks the terminal whether it understands the Kitty
// graphics protocol: a tiny query image (which a capable terminal answers with
// "OK" and does not show) followed by a Primary Device Attributes request,
// which every terminal answers. If the DA answer arrives without a graphics
// answer before it, the protocol is not supported. Runs before the screen is
// set up, on the controlling terminal.
func probeKittyGraphics(timeout time.Duration) bool {
	tty, err := os.OpenFile("/dev/tty", os.O_RDWR, 0)
	if err != nil {
		return false
	}
	defer tty.Close()
	fd := int(tty.Fd())
	state, err := term.MakeRaw(fd)
	if err != nil {
		return false
	}
	defer term.Restore(fd, state)

	const query = "\x1b_Gi=31,s=1,v=1,a=q,t=d,f=24;AAAA\x1b\\" + "\x1b[c"
	if _, err := tty.WriteString(query); err != nil {
		return false
	}
	// A terminal that does not know APC sequences may print the query; erase it.
	defer tty.WriteString("\r\x1b[2K")

	// The wait uses select(2): a read deadline does not work on a terminal
	// device on macOS (kqueue does not support them), which made the probe
	// give up at once there.
	deadline := time.Now().Add(timeout)
	var got []byte
	buf := make([]byte, 256)
	for {
		left := time.Until(deadline)
		if left <= 0 {
			return false // no answer in time: assume no support
		}
		var fds unix.FdSet
		fds.Set(fd)
		tv := unix.NsecToTimeval(left.Nanoseconds())
		n, err := unix.Select(fd+1, &fds, nil, nil, &tv)
		if errors.Is(err, unix.EINTR) {
			continue
		}
		if err != nil {
			return false
		}
		if n == 0 {
			continue // timed out; the loop ends above
		}
		n, err = unix.Read(fd, buf)
		if errors.Is(err, unix.EINTR) || errors.Is(err, unix.EAGAIN) {
			continue
		}
		if err != nil || n <= 0 {
			return false
		}
		got = append(got, buf[:n]...)
		if bytes.Contains(got, []byte("_Gi=31;OK")) {
			return true
		}
		if da := bytes.Index(got, []byte("\x1b[?")); da >= 0 && bytes.IndexByte(got[da:], 'c') >= 0 {
			return false // device attributes answered, graphics query not
		}
	}
}