defmodule ShortUUID do
  @moduledoc """
  ShortUUID is a simple UUID shortener library.

  ## Installation

  Add ShortUUID to your list of dependencies in `mix.exs`:

      def deps do
        [{:shortuuid, "~> #{ShortUUID.Mixfile.project()[:version]}"}]
      end


  ShortUUID.encode/1 will translate UUIDs to base57 using lowercase and uppercase letters
  and digits while avoiding similar-looking characters such as l, 1, I, O and 0.

  ## Typical usage

  Use ShortUUID to shorten UUID strings generated by other libraries such as
  [Ecto](https://hexdocs.pm/ecto/Ecto.UUID.html),
  [Elixir UUID](https://github.com/zyro/elixir-uuid) and [Erlang UUID](https://github.com/okeuday/uuid).

  ## Notes

  The output is padded to a length of 22 characters, the padding character is "2", the first character of the alphabet.

      iex> ShortUUID.encode!("00000000-0000-0000-0000-000000000000")
      "2222222222222222222222"


      iex> ShortUUID.encode!("00000001-0001-0001-0001-000000000001")
      "UD6ibhr3V4YXvriP822222"


      iex> ShortUUID.decode!("UD6ibhr3V4YXvriP822222")
      "00000001-0001-0001-0001-000000000001"

  If we didn't do the padding the nil UUID for example would simply encode into an empty string
  since it's int value is 0.

  Some supported UUID input formats are:

  * `2a162ee5-02f4-4701-9e87-72762cbce5e2`
  * `2a162ee502f447019e8772762cbce5e2`
  * `{2a162ee5-02f4-4701-9e87-72762cbce5e2}`
  * `{2a162ee502f447019e8772762cbce5e2}`

  Letter case is not relevant.

  ## Acknowledgments
    This project was inspired by [skorokithakis/shortuuid](https://github.com/skorokithakis/shortuuid).
  """

  @alphabet "23456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz"
  @alphabet_tuple @alphabet
                  |> String.split("", trim: true)
                  |> List.to_tuple()
  @char_to_int_map @alphabet
                   |> String.split("", trim: true)
                   |> Enum.with_index()
                   |> Enum.reduce(%{}, fn {char, idx}, acc -> Map.put_new(acc, char, idx) end)

  @alphabet_length 57
  @padding_length 22
  @padding_char "2"
  @only_alphabet ~r/^[#{@alphabet}]*$/

  @doc """
  Encode a UUID to ShortUUID.

  ## Examples

      iex> ShortUUID.encode("2a162ee5-02f4-4701-9e87-72762cbce5e2")
      {:ok, "keATfB8JP2ggT7U9JZrpV9"}

  """
  @spec encode(String.t()) :: {:ok, String.t()} | {:error, String.t()}
  def encode(uuid) do
    stripped_uuid = strip_uuid(uuid)

    case stripped_uuid |> String.length() == 32 do
      true ->
        encoded_uuid =
          stripped_uuid
          |> String.to_integer(16)
          |> int_to_string
          |> pad_shortuuid

        {:ok, encoded_uuid}

      _anything_else ->
        {:error, "Invalid UUID"}
    end
  end

  @doc """
  Encode a UUID to ShortUUID.

  Similar to `encode/1` but raises an ArgumentError if it cannot process the UUID.

  ## Examples

      iex> ShortUUID.encode!("2a162ee5-02f4-4701-9e87-72762cbce5e2")
      "keATfB8JP2ggT7U9JZrpV9"

  """
  @spec encode!(String.t()) :: String.t()
  def encode!(uuid) do
    case encode(uuid) do
      {:ok, encoded_uuid} ->
        encoded_uuid

      {:error, msg} ->
        raise ArgumentError, message: msg
    end
  end

  @doc """
  Decode a ShortUUID.

  ## Examples

      iex> ShortUUID.decode("keATfB8JP2ggT7U9JZrpV9")
      {:ok, "2a162ee5-02f4-4701-9e87-72762cbce5e2"}

  """
  @spec decode(String.t()) :: {:ok, String.t()} | {:error, String.t()}
  def decode(string) when is_binary(string) do
    case string |> matches_alphabet? do
      true ->
        string_to_int(string)
        |> int_to_hex_string
        |> pad_uuid
        |> format_uuid_string

      false ->
        {:error, "Invalid input"}
    end
  end

  @doc """
  Decode a ShortUUID.

  Similar to `decode/1` but raises an ArgumentError if the encoded UUID is invalid.

  ## Examples

      iex> ShortUUID.decode!("keATfB8JP2ggT7U9JZrpV9")
      "2a162ee5-02f4-4701-9e87-72762cbce5e2"
  """
  @spec decode!(String.t()) :: String.t()
  def decode!(string) do
    case decode(string) do
      {:ok, uuid} -> uuid
      {:error, message} -> raise ArgumentError, message: message
    end
  end

  defp int_to_hex_string(number) do
    number
    |> to_hex_string
    |> String.downcase()
  end

  @spec divmod(Integer.t(), Integer.t()) :: [Integer.t()]
  defp divmod(dividend, divisor) do
    [div(dividend, divisor), rem(dividend, divisor)]
  end

  @spec int_to_string(Integer.t(), list()) :: [String.t()]
  defp int_to_string(number, acc \\ [])

  defp int_to_string(number, acc) when number > 0 do
    [result, remainder] = divmod(number, @alphabet_length)
    int_to_string(result, [acc | elem(@alphabet_tuple, remainder)])
  end

  defp int_to_string(0, acc), do: acc |> to_string

  defp strip_uuid(uuid) do
    uuid
    |> String.downcase()
    |> String.replace(~r/[^0-9a-f]/, "")
  end

  defp matches_alphabet?(string) do
    Regex.match?(@only_alphabet, string)
  end

  defp string_to_int(string) do
    string
    |> String.split("", trim: true)
    |> Enum.reverse()
    |> Enum.reduce(0, fn char, acc ->
      %{^char => index} = @char_to_int_map
      acc * @alphabet_length + index
    end)
  end

  defp to_hex_string(number) do
    number |> Integer.to_string(16)
  end

  defp pad_shortuuid(encoded_uuid) do
    encoded_uuid
    |> String.pad_trailing(@padding_length, @padding_char)
  end

  defp pad_uuid(string) do
    string |> String.pad_leading(32, "0")
  end

  defp format_uuid_string(<<u0::64, u1::32, u2::32, u3::32, u4::96>>) do
    {:ok, <<u0::64, ?-, u1::32, ?-, u2::32, ?-, u3::32, ?-, u4::96>>}
  end

  defp format_uuid_string(_invalid), do: {:error, "Invalid UUID"}
end