defmodule Diamond do
  def build(n) do
    half = Enum.map(1..n, &build_line(&1, n))
    [_ | t] = Enum.reverse(half)
    half ++ t
  end

  defp build_line(i, n) do
    spaces(n - i) <> middle(i) <> spaces(n - i)
  end

  defp middle(1), do: letter(0)

  defp middle(n) do
    char = letter(n - 1)
    char <> spaces(2 * n - 3) <> char
  end

  defp letter(n), do: <<n + 65>>
  defp spaces(n), do: String.duplicate(" ", n)
end