chained_stream_example.ex

defmodule ChainedStream do
  # [#Function<59.77324385/2 in Stream.unfold/2>, #Function<59.77324385/2 in Stream.unfold/2>]
  def test do
    states = count(%{}, ["a", "b", "c"]) |> Enum.to_list
    # [{%{"a" => 1}, ["b", "c"]}, {%{"a" => 1, "b" => 1},
    # ["c"]}, {%{"a" => 1, "b" => 1, "c" => 1}, []}]

    states |> Enum.at(-1) |> elem(0) |> count(["a", "b", "d"])
    # [{%{"a" => 2, "b" => 1, "c" => 1}, ["b", "d"]},
    # {%{"a" => 2, "b" => 2, "c" => 1}, ["d"]},
    # {%{"a" => 2, "b" => 2, "c" => 1, "d" => 1}, []}]
  end

  def count(%{} = stats, things) do
    Stream.unfold({stats, things}, &count_items/1)
  end

  def count_items({stats, []}), do: nil
  def count_items({stats, [h | t] = items}) do
    new_stats = Map.update(stats, h, 1, &(&1 + 1))
    {{new_stats, t}, {new_stats, t}}
  end
end