Diamond kata in Elixir

diamond.ex

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

diamond_test.exs

defmodule DiamondTest do
  use ExUnit.Case

  test "1 Line" do
    diamond = Diamond.build(1)
    assert List.first(diamond) == "A"
  end

  test "2 Lines (first line)" do
    diamond = Diamond.build(2)
    assert List.first(diamond) == " A "
  end

  test "2 Lines  (second line)" do
    diamond = Diamond.build(2)
    [_, s | _] = diamond
    assert s == "B B"
  end

  test "3 (first line)" do
    diamond = Diamond.build(3)
    assert List.first(diamond) == "  A  "
  end

  test "3 (second line)" do
    diamond = Diamond.build(3)
    [_, s | _] = diamond
    assert s == " B B "
  end

  test "3 (third line)" do
    diamond = Diamond.build(3)
    [_, _, s | _] = diamond
    assert s == "C   C"
  end

  test "2" do
    assert Diamond.build(2) == [" A ", "B B", " A "]
  end

  test "3" do
    assert Diamond.build(3) == ["  A  ", " B B ", "C   C", " B B ", "  A  "]
  end
end