structure ParseGrid =
struct
  fun make (canvasWidth, canvasHeight) =
    let
      val maxPoints = Int.max (canvasWidth, canvasHeight)
      val emptyYAxis = Vector.tabulate (maxPoints, fn _ =>
        {r = 0, g = 0, b = 0, a = 0})
    in
      Vector.tabulate (maxPoints, fn _ => emptyYAxis)
    end

  local
    fun loopY (yAxis, x, ex, y, ey, colour) =
      if y > ey orelse y >= Vector.length yAxis then
        yAxis
      else
        let val yAxis = Vector.update (yAxis, y, colour)
        in loopY (yAxis, x, ex, y + 1, ey, colour)
        end

    fun loopX (grid, x, ex, y, ey, colour) =
      if x > ex orelse x >= Vector.length grid then
        grid
      else
        let
          val yAxis = Vector.sub (grid, x)
          val yAxis = loopY (yAxis, x, ex, y, ey, colour)
          val grid = Vector.update (grid, x, yAxis)
        in
          loopX (grid, x + 1, ex, y, ey, colour)
        end
  in
    fun applyItem (grid, x, y, ex, ey, colour) =
      loopX (grid, x, ex, y, ey, colour)
  end
end