secret_santa.ex

defmodule SecretSanta do
  @participants [:andrei, :dave, :john, :mary, :jane, :anna]

  def get_random, do: generate_pairs() |> Enum.random()

  defp generate_pairs(givers \\ @participants, receivers \\ @participants, existing_pairs \\ [])

  defp generate_pairs([giver | givers], receivers, existing_pairs) do
    for receiver <- receivers,
        valid_pair?(giver, receiver, existing_pairs),
        remaining_receivers = List.delete(receivers, receiver),
        pairs <- generate_pairs(givers, remaining_receivers, [{giver, receiver} | existing_pairs]) do
      [{giver, receiver} | pairs]
    end
  end

  defp generate_pairs([], [], _existing_pairs), do: [[]]

  defp valid_pair?(giver, receiver, existing_pairs) do
    cond do
      giver == receiver -> false
      {giver, receiver} == {:andrei, :dave} -> false
      {giver, receiver} == {:dave, :andrei} -> false
      {giver, receiver} == {:john, :mary} -> false
      {giver, receiver} == {:mary, :john} -> false
      Enum.member?(existing_pairs, {receiver, giver}) -> false
      true -> true
    end
  end
end