package serialport

import (
	"fmt"
	"os"

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

func Open115200(path string) (*os.File, error) {
	file, err := os.OpenFile(path, os.O_RDWR|unix.O_NOCTTY, 0)
	if err != nil {
		return nil, err
	}

	termios, err := unix.IoctlGetTermios(int(file.Fd()), unix.TCGETS)
	if err != nil {
		file.Close()
		return nil, err
	}

	configureRaw115200(termios)

	if err := unix.IoctlSetTermios(int(file.Fd()), unix.TCSETS, termios); err != nil {
		file.Close()
		return nil, fmt.Errorf("configure %s: %w", path, err)
	}

	effective, err := unix.IoctlGetTermios(int(file.Fd()), unix.TCGETS)
	if err != nil {
		file.Close()
		return nil, fmt.Errorf("read back %s: %w", path, err)
	}
	if err := verifyBaud(*effective, unix.B115200); err != nil {
		file.Close()
		return nil, fmt.Errorf("verify %s: %w", path, err)
	}
	return file, nil
}

// configureRaw115200 keeps the raw 115200 8N1 blocking-read configuration.
// On Linux TCSETS takes the line speed from the CBAUD bits in Cflag, so
// CBAUD must be cleared and B115200 selected there; Ispeed/Ospeed alone
// leave an inherited baud (for example 9600 from boot defaults) in effect.
func configureRaw115200(termios *unix.Termios) {
	termios.Iflag = 0
	termios.Oflag = 0
	termios.Lflag = 0
	termios.Cflag &^= unix.CSIZE | unix.PARENB | unix.CSTOPB | unix.CRTSCTS | unix.CBAUD
	termios.Cflag |= unix.B115200
	termios.Cflag |= unix.CS8 | unix.CREAD | unix.CLOCAL
	termios.Ispeed = unix.B115200
	termios.Ospeed = unix.B115200
	termios.Cc[unix.VMIN] = 1
	termios.Cc[unix.VTIME] = 0
}

func verifyBaud(termios unix.Termios, baud uint32) error {
	if got := termios.Cflag & unix.CBAUD; got != baud {
		return fmt.Errorf("effective baud bits %#o, want %#o", got, baud)
	}
	return nil
}