Interview - Positioning and resetting a ball in the center - ios

Question

I was faced with this question in one of my interviews and was completely stumped. The only solution I could think of was storing the currentAngle in a NSArray to calculate the next angle.
Question:
Move a 35px ball across the screen utilizing the iPhone's compass. Once the ball is in the center of the screen, let the user tap it to 'reset' the position. Once reset, the ball will go back to the Min position. Remember that the compass may start somewhere between 0-359, the task is to find the nearest capture angle and focus on that angle until the ball is aligned. Once the ball is aligned & reset, the iPhone will move to the next angle and so forth until the ball has been reset 18 times. 18 resets * 20 degree angles = 360.
Assigned Variables:
int currentAngle = (Ranging between 0-359) (Constant updates as the user twirls around)
int captureAngle = 20
int centerX = view.center.x (160) - 35 (size of ball)
int ballSize = 35 (ball.width/2)
The paper looked something like this:
Function so far:
-(void)testMotion{
motionQueue = [[NSOperationQueue alloc] init];
motionManager = [[CMMotionManager alloc] init];
motionManager.deviceMotionUpdateInterval = 1.0f / 60.0f;
if (([CMMotionManager availableAttitudeReferenceFrames] & CMAttitudeReferenceFrameXTrueNorthZVertical) != 0) {
[motionManager startDeviceMotionUpdatesUsingReferenceFrame:CMAttitudeReferenceFrameXTrueNorthZVertical
toQueue:motionQueue
withHandler:^(CMDeviceMotion *motion, NSError *error)
{
if (!error) {
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
CMAttitude *attitude = motion.attitude;
CMRotationMatrix rm = attitude.rotationMatrix;
// Get the heading.
double heading = M_PI + atan2(rm.m22, rm.m12);
heading = heading*180/M_PI;
int currentAngle = (int)heading;
NSLog(#"Current Angle: %d",currentAngle);
int captureAngle = 20; // 20 Degress Capture Angle
}];
}
}];
}
}

Answer 1

If I understood you, then it's something like this:
calculate the x and y movement from the angle
(see
https://en.wikipedia.org/wiki/Rotation_of_axes
http://keisan.casio.com/has10/SpecExec.cgi?id=system/2006/1223522781
)
Then move the ball according to those values, and if it moved by an angle of 20 - allow reset it or get out of the loop (for your choice)
while(1) {
x = r \cos(currentAngle)
y = r \sin(currentAngle)
//change the ball position,
ball.position.x += x*speed
ball.position.y += y*speed
//check if angel is +20 or -20
if (((currentAngle + 20) % 360) != captureAngle && (abs(currentAngle - 20) % 360) != captureAngle)) {
allow_reset_ball = true
break;
}
}