def write_seatalk (xx, yy):
        # thanks to http://www.thomasknauf.de/seatalk.htm
        with serial.Serial() as ser:
                ser.baudrate = 4800
                ser.port = '/dev/serial0'
                ser.stopbits=serial.STOPBITS_ONE
                ser.bytesize=serial.EIGHTBITS

                ser.open()
                ser.parity = serial.PARITY_MARK
                ser.write(b'\x86')
                ser.parity = serial.PARITY_SPACE
                ser.write(b'\x11' + chr(int(xx, 16)) + chr(int(yy, 16)))
                ser.close()

angle = 0
previous_angle = 0

def send_command(command):
        global angle
        print "command="+str(command)

        if command == -10:
                write_seatalk("06", "F9")
        if command == -1:
                write_seatalk("05", "FA")
        if command == +1:
                write_seatalk("07", "F8")
        if command == +10:
                write_seatalk("08", "F7")
        angle = angle - command


def steer_to_angle(angle_to_steer_to):
        global angle
        angle = angle_to_steer_to

        while angle != 0:
                if angle <= -10:
                        send_command(-10)
                if angle > -10 and angle < 0:
                        send_command(-1)
                if angle > 0 and angle < 10:
                        send_command(+1)
                if angle >= 10:
                        send_command(+10)

def steer_into_wind():
        global previous_angle

        try:
                with open('/tmp/AWA', 'r') as myfile:
                        line = myfile.read().replace("\n", "")
                        awa = int(line)
        except:
                awa = 0

        angle=(awa+180) % 360-180
        previous_angle = angle
        print "awa=" + str(awa) + "; angle=" + str(angle)

        steer_to_angle(angle)


def steer_previous_angle():
        global previous_angle

        steer_to_angle(-previous_angle)