structure Grid =
struct
  type square = {r: int, g: int, b: int, a: int}

  type t = square vector vector

  val emptyPixel = {r = 0, g = 0, b = 0, a = 0}

  fun isBlank ({a, ...}: square) = a = 0

  fun changeGridSize maxSide grid =
    Vector.tabulate (maxSide, fn i =>
      if i < Vector.length grid then
        let
          val yAxis = Vector.sub (grid, i)
        in
          Vector.tabulate (maxSide, fn ii =>
            if ii < Vector.length yAxis then Vector.sub (yAxis, ii)
            else emptyPixel)
        end
      else
        Vector.tabulate (maxSide, fn _ => emptyPixel))
end