defmodule Codex.Base64 do
  import Codex

  @capitals 65..90 |> Enum.map(&:binary.encode_unsigned/1)
  @lowers 97..122 |> Enum.map(&:binary.encode_unsigned/1)
  @numbers 48..57 |> Enum.map(&:binary.encode_unsigned/1)

  @char_list (@capitals ++ @lowers ++ @numbers ++ ["+", "/"]) |> Enum.with_index()

  @char_to_num @char_list |> Enum.into(%{})
  @num_to_char @char_list |> Enum.map(fn {char, num} -> {num, char} end) |> Enum.into(%{})

  @spec codec() :: Codex.codec(String.t())
  def codec() do
    padding = value("=")
    char_6 = bits(6) |> index_to_char()
    char_2 = bits(2) |> done() |> pad(4) |> index_to_char()
    char_4 = bits(4) |> done() |> pad(2) |> index_to_char()

    list(
      choice([
        list_of(4, char_6) |> join(),
        [char_6, char_2, padding, padding] |> sequence() |> join(),
        [char_6, char_6, char_4, padding] |> sequence() |> join()
      ])
    )
    |> join(4)
  end

  defp index_to_char(index),
    do: convert(index, &Map.fetch!(@num_to_char, &1), &Map.fetch!(@char_to_num, &1))
end