Find closest unit to a position

closest.swift

func findClosestUnitToPosition(destination:CGPoint, var positionedUnits:Array<CharacterNode>) -> CharacterNode {
    var closestUnit : CharacterNode? = nil
    var closestDistance : CGFloat? = nil
        
    // Loop over all of our units.
    for(i, unit) in enumerate(self.units) {
        
        // If the unit hasn't been moved already.
        if(!contains(positionedUnits, unit)) {
            
            // Helper function to calculate distance using hypotf()
            let distance = Coordinates.calculateDistance(unit.position, end: destination)
                
            // If no unit has been marked closest yet, set our first to closest.
            if(closestUnit == nil) {
                closestUnit = unit
                closestDistance = distance
            }
                
            // Else, if this unit is actually closer than the closest, set it as the new closest.
            else {
                if(distance < closestDistance) {
                    closestDistance = distance
                    closestUnit = unit
                }
            }
        }
    }
        
    return closestUnit!
}