Collate a list of lists in sequence until the longest list is exhausted.

collate.ex

defmodule Util.Enum do
  @doc ~S"""
  Collates a list of lists in sequence until the longest
  list is exhausted.

  ## Examples

      iex> Util.Enum.collate([[1, 2, 3], [1, 2], [1, 2, 3, 4]])
      [1, 1, 1, 2, 2, 2, 3, 3, 4]

      iex> Util.Enum.collate([[], [1, 2], [1, 2, 3, 4]])
      [1, 1, 2, 2, 3, 4]

      iex> Util.Enum.collate([[1]])
      [1]

      iex> Util.Enum.collate([[]])
      []

      iex> Util.Enum.collate([])
      []

  """
  def collate(lists) do
    lists
    |> collate([])
    |> Enum.reverse()
  end

  defp collate(lists, heads) when lists == [], do: heads

  defp collate(lists, heads) do
    {heads, tails} =
      Enum.reduce(lists, {heads, []}, fn
        [head | tail], {heads, tails} ->
          {[head | heads], [tail | tails]}

        [], {heads, tails} ->
          {heads, tails}
      end)

    collate(tails, heads)
  end
end