-- | The Grid module just contains the Grid type and two constructors
--   for it. We hide the main Grid constructor because we don't want
--   to allow instantiation of a grid with h <= 0.
module Grid (empty_grid,
             function_values,
             Grid,
             h,
             make_grid)
where

-- | Our problem is defined on a Grid. The grid size is given by the
--   positive number h. The function values are the values of the
--   function at the grid points, which are distance h from one
--   another in each direction (x,y,z).
data Grid = Grid { h :: Double, -- MUST BE GREATER THAN ZERO!
                   function_values :: [[[Double]]] }
          deriving (Eq, Show)


-- | The constructor that we want people to use. If we're passed a
--   non-positive grid size, we throw an error.
make_grid :: Double -> [[[Double]]] -> Grid
make_grid grid_size values
    | grid_size <= 0 = error "grid size must be positive"
    | otherwise = Grid grid_size values


-- | Creates an empty grid with grid size 1.
empty_grid :: Grid
empty_grid = Grid 1 [[[]]]