pascal

defmodule Pascal do
  @moduledoc """
  Provides a set of functions that calculate Pascal's triangles
  """

  @typedoc """
  Line of Pascal's triangle
  """
  @type line :: nonempty_list(pos_integer)

  @typedoc """
  Pascal's triangle
  """
  @type triangle :: nonempty_list(line)

  @doc """
  Generate Pascal's `t:triangle/0` of given `height`.

  Returns `t:triangle/0`.

  ## Examples

      iex> Pascal.triangle(1)
      [[1]]

      iex> Pascal.triangle(2)
      [[1], [1, 1]]

      iex> Pascal.triangle(3)
      [[1], [1, 1], [1, 2, 1]]

  """
  @spec triangle(pos_integer) :: triangle
  def triangle(height) do
    Enum.reverse triangle(height, [[1]])
  end

  @doc """
  Generate next `t:line/0` of Pascal's `t:triangle/0` for given previous `t:line/0`.

  Returns `t:line/0`.

  ## Examples

      iex> Pascal.next_triangle_line([1])
      [1, 1]

      iex> Pascal.next_triangle_line([1, 1])
      [1, 2, 1]

      iex> Pascal.next_triangle_line([1, 2, 1])
      [1, 3, 3, 1]

  """
  @spec next_triangle_line(line) :: line
  def next_triangle_line(line) do
    Enum.zip([0 | line], line ++ [0])
    |> Enum.map(fn {a, b} -> a + b end)
  end

  @spec triangle(pos_integer, triangle) :: triangle
  defp triangle(1, triangle), do: triangle
  defp triangle(n, [line | _rest] = triangle) do
    next_line = next_triangle_line(line)
    triangle(n - 1, [next_line | triangle])
  end
end