Planets in Elixir

planet.exs

defmodule Planet do
  defstruct [:x, :y]

  def gen_planet(x, y) do
    xr = rem x, 7
    yr = rem y, 11

    if x > 4 and y > 8 and xr > 3 and xr < 6 and rem(yr, xr) === 1 do
      %Planet{x: x, y: y}
    else
      nil
    end
  end
end

defmodule Octree do
  defmodule Cube do
    defstruct [:size, :cx, :cy, :children]
  end

  defmodule Children do
    defstruct [:tr, :tl, :br, :bl]
  end

  def empty?(%Octree.Cube{children: children}) do
    empty? children
  end
  def empty?(%Octree.Children{tr: tr, tl: tl, br: br, bl: bl}) do
    tr === nil and tl === nil and br === nil and bl === nil
  end
end

defmodule Generate do
  def gen(cube = %Octree.Cube{size: size, cx: cx, cy: cy}) when size === 1 do
    children = %Octree.Children{
      tr: Planet.gen_planet(cx, cy),
      tl: Planet.gen_planet(cx - 1, cy),
      br: Planet.gen_planet(cx, cy - 1),
      bl: Planet.gen_planet(cx - 1, cy - 1)
    }

    if Octree.empty? children do
      nil
    else
      Map.put(
        cube,
        :children,
        children
      )
    end
  end
  def gen(%Octree.Cube{size: size, cx: cx, cy: cy}) when size > 1 do
    half_size = div size, 2

    cube = %Octree.Cube{
      size: size,
      cx: cx,
      cy: cy,
      children: %Octree.Children{
        tr: Generate.gen(%Octree.Cube{size: half_size, cx: cx + half_size, cy: cy + half_size}),
        tl: Generate.gen(%Octree.Cube{size: half_size, cx: cx - half_size, cy: cy + half_size}),
        br: Generate.gen(%Octree.Cube{size: half_size, cx: cx + half_size, cy: cy - half_size}),
        bl: Generate.gen(%Octree.Cube{size: half_size, cx: cx - half_size, cy: cy - half_size})
      }
    }

    if Octree.empty? cube do
      nil
    else
      cube
    end
  end
  def gen(_) do
    raise ArgumentError, "Invalid cube!"
  end
end

defmodule Main do
  def main do
    planets = Generate.gen(%Octree.Cube{size: 32, cx: 0, cy: 0})

    IO.inspect planets
  end
end

Main.main()