gistfile1.ex

def add_river(chunk, rng) do
  {x, rng} = random(0..chunk_size - 1, rng)
  y = 0
  {cells, rng} = add_river_parts({x, y}, chunk[:cells], rng)
  {%__MODULE__{ chunk | cells: cells }, rng}
end

defp add_river_parts({x, y}, cells, rng) do
  if y == chunk_size do
    {cells, rng}
  else
    {dx, rng} = random(-1..1, rng)
    x = x + dx
    y = if(dx == 0, do: y + 1, else: y)
    cells = Dict.put(cells, {x, y}, :water)
    add_river_parts({x, y}, cells, rng)
  end
end