find best magnetometer offset fit to a log file

def magfit(logfile):
    '''find best magnetometer offset fit to a log file'''

    print("Processing log %s" % filename)
    mlog = mavutil.mavlink_connection(filename, notimestamps=args.notimestamps)

    data = []

    last_t = 0
    offsets = Vector3(0,0,0)
    motor_ofs = Vector3(0,0,0)
    motor = 0.0

    # now gather all the data
    while True:
        m = mlog.recv_match(condition=args.condition)
        if m is None:
            break
        if m.get_type() == "PARAM_VALUE" and m.param_id == "RC3_MIN":
            rc3_min = float(m.param_value)
        if m.get_type() == "SENSOR_OFFSETS":
            # update current offsets
            offsets = Vector3(m.mag_ofs_x, m.mag_ofs_y, m.mag_ofs_z)
        if m.get_type() == "SERVO_OUTPUT_RAW":
            motor_pwm = m.servo1_raw + m.servo2_raw + m.servo3_raw + m.servo4_raw
            motor_pwm *= 0.25
            rc3_min = mlog.param('RC3_MIN', 1100)
            rc3_max = mlog.param('RC3_MAX', 1900)
            motor = (motor_pwm - rc3_min) / (rc3_max - rc3_min)
            if motor > 1.0:
                motor = 1.0
            if motor < 0.0:
                motor = 0.0
        if m.get_type() == "RAW_IMU":
            mag = Vector3(m.xmag, m.ymag, m.zmag)
            # add data point after subtracting the current offsets
            data.append((mag - offsets + noise(), motor))

    print("Extracted %u data points" % len(data))
    print("Current offsets: %s" % offsets)

    data = select_data(data)

    # do an initial fit with all data
    (offsets, motor_ofs, field_strength) = fit_data(data)

    for count in range(3):
        # sort the data by the radius
        data.sort(lambda a,b : radius_cmp(a,b,offsets,motor_ofs))

        print("Fit %u    : %s  %s field_strength=%6.1f to %6.1f" % (
            count, offsets, motor_ofs,
            radius(data[0], offsets, motor_ofs), radius(data[-1], offsets, motor_ofs)))

        # discard outliers, keep the middle 3/4
        data = data[len(data)/8:-len(data)/8]

        # fit again
        (offsets, motor_ofs, field_strength) = fit_data(data)

    print("Final    : %s  %s field_strength=%6.1f to %6.1f" % (
        offsets, motor_ofs,
        radius(data[0], offsets, motor_ofs), radius(data[-1], offsets, motor_ofs)))
    print("mavgraph.py '%s' 'mag_field(RAW_IMU)' 'mag_field_motors(RAW_IMU,SENSOR_OFFSETS,(%f,%f,%f),SERVO_OUTPUT_RAW,(%f,%f,%f))'" % (
        filename,
        offsets.x,offsets.y,offsets.z,
        motor_ofs.x, motor_ofs.y, motor_ofs.z))