--- | Takes a 'Grid', and returns all 'Cube's belonging to it that
--- contain the given 'Point'.
-find_containing_cubes :: Grid -> Point -> [Cube]
-find_containing_cubes g p =
- filter contains_our_point all_cubes
+
+-- The first cube along any axis covers (-h/2, h/2). The second
+-- covers (h/2, 3h/2). The third, (3h/2, 5h/2), and so on.
+--
+-- We translate the (x,y,z) coordinates forward by 'h' so that the
+-- first covers (0, h), the second covers (h, 2h), etc. This makes
+-- it easy to figure out which cube contains the given point.
+calculate_containing_cube_coordinate :: Grid -> Double -> Int
+calculate_containing_cube_coordinate g coord
+ -- Don't use a cube on the boundary if we can help it.
+ | coord == delta && (xsize > 0 && ysize > 0 && zsize > 0) = 1
+ | otherwise = (ceiling ( (coord + delta) / cube_width )) - 1
+ where
+ (xsize, ysize, zsize) = dims (function_values g)
+ delta = (h g)
+ cube_width = 2 * delta
+
+
+-- | Takes a 'Grid', and returns a 'Cube' containing the given 'Point'.
+-- Since our grid is rectangular, we can figure this out without having
+-- to check every cube.
+find_containing_cube :: Grid -> Point -> Cube
+find_containing_cube g p =
+ case cube_at g i j k of
+ Just c -> c
+ Nothing -> error "No cube contains the given point."