Infinite loop by creating arrays to swap node positions - arrays

Question

There are 20 red balls sitting in 20 positions on 4 shelves. I'd like to move each ball to a new position on its own shelf by generating a new array that contains the new positions for all 20 balls. I am using a function (below) that allows me to do this. However, I find that this function keeps crashing; when I print out values it is generating, it seems to hang at the last coordinate of available position but then doesn't exit the "while" loop.
func generateNewLocationArray(previous: [String: CGPoint]) -> [String : CGPoint] {
var refreshedLocations = [String : CGPoint]()
for (location, _) in previous {
let node = fgNode.childNode(withName: location) as? LocationNode
var newLocation = generateRandomLocation(check: refreshedLocations)
let previousLocation = previous[(node?.name!)!]
while (newLocation == previousLocation) || (newLocation.y != previousLocation?.y) {
newLocation = generateRandomLocation(check: refreshedLocations)
}
node?.position = newLocation
refreshedLocations[location] = newLocation
}
return refreshedLocations
}
What I'm trying to achieve:
The function takes in an array of CGPoints, this is the "previous"
array, of positions where all the balls were.
It then creates a brand-new array, called "refreshedLocations".
It loads an existing node, which is located at a position contained in the previous array.
It then generates a new position for that node. This position is obtained from a pre-set array of 20 positions. This new location cannot be the same position the node is currently in; nor can it have a different y-coordinate (so the balls stay on the shelf).
Until this criteria is met, it keeps generating a new position.
It then loads this position into the refreshedLocations array, and the next node is also checked against this array to ensure there are no repeated positions.
The problem again: this code works with 10, or 15 balls. But when the number is pushed to 20, it seems more likely to hang. It will identify how to move all the balls around to new positions, but it gets stuck in the "while" loop more often than not. What's going wrong here?
EDIT: This is the function which returns a random location, but first checks whether the array that is being generated already contains the point being added.
func generateRandomLocation(check: [String : CGPoint]) -> CGPoint {
let randomIndex = GKRandomDistribution(lowestValue: 0, highestValue: allPositions.count - 1)
var position = allPositions[randomIndex.nextInt()]
while check.values.contains(position) {
position = allPositions[randomIndex.nextInt()]
}
return position
}

Answer 1

As far as I can see,
while (newLocation == previousLocation) || (newLocation.y != previousLocation?.y)
is always going to be true if you get to the last ball on a row and the only position left is the one it is already in. You can fix it by detecting how many positions on the shelf have been filled and if it is n - 1 out of n and the only position left is the one the ball is in, just swap it with a randomly selected other ball on the same shelf.
However, there must be a better way of doing this. I'd try a modified Fisher Yates shuffle.
Firstly, organise the balls by shelf and position. Then, for each shelf of n balls (numbered 0 to n - 1)
select a random ball from the first n - 2 balls and swap it with ball n - 1
select a random ball from the first n - 3 balls and swap it with ball n - 2
select a random ball from the first n - 4 balls and swap it with ball n - 3
and so on. Stop when the random selection would be from 0 balls. At the end of this process, all the balls will have changed position.
Repeat for each shelf.