The SpiralDifferenceIterator should have direction instead of default right direction.

 

Consider the layout of tiling data as given below.
 TL  T  TR
  L  *   R
 BL  B  BR
 
where -

R  = right
TR = top right
T  = top
TL = top left
L  = left
BL = bottom left
B  = bottom
BR = bottom right

The spiral order {R, TR, T, TL, L, BL, B, BR} does not give better coverage when direction of scrolling (or anchor of zoom is considered) and the layout of tiling data with current implementation is

   x 0 1 2
  y.------
  0| 4 3 2
  1| 5 * 1   --->
  2| 6 7 8


When user is scrolling in right direction (inclined to bottom right), then tiles at right are most important and tiles at left are least important. In the above example tile at 8 would be needed before tiles at 4, 5, 6. The relative importance of tiles at different directions in this case is {R, BR, TR, B, T, BL, TL, L}. (the main coverage direction can be deduced by x-velocity and secondary direction can be deduced from y-velocity). The layout of tiling data would be

   x 0 1 2
  y.------
  0| 7 5 3
  1| 8 * 1    --->
  2| 6 4 2
  
Implement precedence in coverage directions for tiling data based on scroll direction or anchor of zooming instead of default right direction.