Calculate binomial coefficient of one diagonal of pascal's triangle

pascal-triangle.ex

defmodule Easydiagonal do

  def diagonal(n, p) do
    {sum, _acc} = build_triangle(n, p)
    sum
  end

  defp build_triangle(n, p) do
    # [[1], [1, 1], [1, 2, 1], [1, 3, 3, 1], [1, 4, 6, 4, 1], ...]  
    Stream.iterate({0, [1]}, fn {sum, last} -> 
      new_sum =
        case Enum.at(last, p) do
          nil -> sum
          num -> sum + num
        end
        
      {new_sum, pascalit(last)}
    end)
    |> Enum.take(n + 2)
    |> Enum.at(-1)
  end

  defp pascalit([1]), do: [1, 1]
  defp pascalit(acc) do
    result = 
      Stream.chunk(acc, 2, 1)
      |> Enum.reduce([], fn [h, t], acc -> 
        [h + t | acc]
      end) 

    [1] ++ result ++ [1]
  end
end