combinations_benchmark.exs

defmodule CombinationsBenchmark do
  def combinations(collection, k) do
    List.last(do_combinations(collection, k))
  end

  defp do_combinations(list, k) do
    combinations_by_length = [[[]]|List.duplicate([], k)]

    List.foldr list, combinations_by_length, fn x, next ->
      sub = :lists.droplast(next)
      step = [[]|(for l <- sub, do: (for s <- l, do: [x|s]))]
      :lists.zipwith(&:lists.append/2, step, next)
    end
  end

  def combinations2(_, 0), do: [[]]
  def combinations2([], _), do: []
  def combinations2([h|t], k) do
    (for l <- combinations2(t, k - 1), do: [h|l]) ++ combinations2(t, k)
  end
end


list = Enum.to_list(1..50)

{ fast, _ } = :timer.tc(fn ->
  for k <- 1..6 do
    CombinationsBenchmark.combinations(list, k)
  end
end, [])

IO.puts "CombinationsBenchmark.combinations completed in #{fast}"

{ slow, _ } = :timer.tc(fn ->
  for k <- 1..6 do
    CombinationsBenchmark.combinations2(list, k)
  end
end, [])

IO.puts "CombinationsBenchmark.combinations2 completed in #{slow}"