package main

import (
	"bytes"
	"os"
	"time"
	"unsafe"

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

type dsrResult struct {
	row        int
	column     int
	extraBytes []byte
}

func main() {
	st, err := term.MakeRaw(0)
	if err != nil {
		os.Exit(1)
	}

	var res dsrResult
	select {
	case <-time.After(100 * time.Millisecond):
	case res = <-awaitDSR():
	}

	term.Restore(0, st)
	if res.column > 1 {
		unix.Write(1, []byte("🚫\n")) // U+1F6AB
	}

	term, _ := unix.IoctlGetTermios(1, unix.TCGETS)
	if term != nil {
		term.Lflag = term.Lflag &^ unix.ECHO
		unix.IoctlSetTermios(1, unix.TCSETS, term)
	}

	for _, b := range res.extraBytes {
		unix.Syscall(unix.SYS_IOCTL, 0, unix.TIOCSTI, uintptr(unsafe.Pointer(&b)))
	}

	if term != nil {
		term.Lflag = term.Lflag | unix.ECHO
		unix.IoctlSetTermios(1, unix.TCSETS, term)
	}
}

func awaitDSR() <-chan dsrResult {
	ch := make(chan dsrResult)
	go func() {
		row, col, extra := readDSR()
		ch <- dsrResult{
			row:        row,
			column:     col,
			extraBytes: extra,
		}
	}()
	return ch
}

func readDSR() (row, col int, extra []byte) {
	unix.Write(1, []byte{
		'\033', '[', // ANSI escape code: CSI
		'6', 'n', // DSR / device status report
	})

	inputBuf := bytes.NewBuffer(make([]byte, 0, 128))
	termIn := make([]byte, 64)
	for {
		// read some bytes from stdin and append to our input buffer
		termIn = termIn[:64]
		n, err := unix.Read(0, termIn)
		inputBuf.Write(termIn[:n])
		if err != nil {
			return 0, 0, inputBuf.Bytes()
		}

		// does the input buffer contain a valid DSR result sequence?
		b := inputBuf.Bytes()
		row, col, pos, count, ok := parseDSR(b)
		if ok {
			// replay the non-DSR sequence bytes
			return row, col, append(b[:pos], b[pos+count:]...)
		}
	}
}

func parseDSR(input []byte) (row, col, pos, count int, ok bool) {
	// locate the start of the result
	pos = bytes.Index(input, []byte{'\033', '['})
	if pos == -1 {
		return
	}

	// parse the row and column numbers, separated by ';'
	p := pos + 2
rowLoop:
	for {
		if p == len(input) {
			return
		}
		switch input[p] {
		case ';':
			break rowLoop
		case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
			row *= 10
			row += int(input[p] - '0')
		default:
			return
		}
		p++
	}
	p++
	for {
		if p == len(input) {
			return
		}
		switch input[p] {
		case 'R':
			count = p - pos + 1
			ok = true
			return
		case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
			col *= 10
			col += int(input[p] - '0')
		default:
			return
		}
		p++
	}
}