ex_bencode

defmodule ExBencode do
  @moduledoc """

  Documentation for ExBencode.

  """

  def encode!(t) do
    case encode(t) do
      {:ok, b} -> b
      {:error, reason} -> raise reason
    end
  end

  def decode!(s) do
    case decode(s) do
      {:ok, t} -> t
      {:error, reason} -> raise reason
    end
  end

  @doc """

  Decode the bencoded binary value.



  ## Examples



  Decoding integers



      iex> ExBencode.decode("i10e")

      {:ok, 10}



      iex> ExBencode.decode("i-10e")

      {:ok, -10}



    Doubles and scientific notation is **not** supported



      iex> ExBencode.decode("i4.2e")

      {:error, :invalid_integer}



      iex> ExBencode.decode("i1.5e7e")

      {:error, :invalid_integer}



  Decoding strings



      iex> ExBencode.decode("4:spam")

      {:ok, "spam"}



      iex> ExBencode.decode("4:too much spam")

      {:error, :unexpected_content, %{index: 6, unexpected: "much spam"}}



  Bytes are handled using the string type, with the preceding number

  representing the byte size, not the string length.



      iex> ExBencode.decode(<<?3, ?:, 1, 2, 3>>)

      {:ok, <<1, 2, 3>>}



      iex> ExBencode.decode("7:hełło")

      {:ok, "hełło"}



      iex> ExBencode.decode("5:hełło")

      {:error, :unexpected_content, %{index: 7, unexpected: <<130, 111>>}}



  Decoding lists



      iex> ExBencode.decode("le")

      {:ok, []}



      iex> ExBencode.decode("l4:spam4:eggse")

      {:ok, ["spam", "eggs"]}



  Decoding Dictionaries



      iex> ExBencode.decode("de")

      {:ok, %{}}



      iex> ExBencode.decode("d3:cow3:mooe")

      {:ok, %{"cow" => "moo"}}



      iex> ExBencode.decode("d8:shoppingl4:eggs4:milkee")

      {:ok, %{"shopping" => ["eggs", "milk"]}}

  """
  def decode(s) when is_binary(s) do
    case extract_next(s) do
      {:ok, body, ""} -> {:ok, body}
      # Fail if there's anything leftover after we parse

      {:ok, _, unexpected} ->
        {
          :error,
          :unexpected_content,
          %{
            index: byte_size(s) - byte_size(unexpected),
            unexpected: unexpected
          }
        }
      {:error, msg} -> {:error, msg}
      {:error, msg, details} -> {:error, msg, details}
    end
  end

  defp extract_next(<<"i", _rest :: bits>> = s), do: extract_int(s)


  defp extract_next(<<i, _rest :: bits>> = s) when i >= ?0 and i <= ?9 do
    with [len_bin | _] <- :binary.split(s, ":"),
         header_size <- byte_size(len_bin) + 1,
         str_and_rest <- after_n(s, header_size),
         {length, ""} <- Integer.parse(len_bin)
    do
      if byte_size(str_and_rest) < length do
        {:error, :invalid_string, %{expected_size: length, actual_size: byte_size(str_and_rest)}}
      else
        str = first_n(str_and_rest, length)
        rest = after_n(str_and_rest, length)

        if byte_size(str) != length do
          {:error, :invalid_string, %{expected_size: length, actual_size: byte_size(str)}}
        else
          {:ok, str, rest}
        end
      end
    else
      _ -> {:error, :invalid_string}
    end
  end


  defp extract_next(<<"l", rest :: bits>> = s) do
    extract_list_contents(rest)
  end

  defp extract_next(<<"d", tail :: bits>>) do
    with  {:ok, contents, rest} <- extract_list_contents(tail)
    do
      mapcontents =
        contents
        |> Enum.chunk(2)
        |> Enum.map(fn [a, b] -> {a, b} end)
        |> Map.new
      {:ok, mapcontents, rest}
    else
      err -> err
    end
  end

  defp extract_next(_), do: {:error, :not_bencoded_form}

  defp extract_int(<<"i", rest :: bits>>) when byte_size(rest) > 1 do
    [intbin, afterint] = :binary.split(rest, "e", [])
    case Integer.parse(intbin) do
       {int, ""} -> {:ok, int, afterint}
       {_, _} -> {:error, :invalid_integer}
    end
  end

  defp extract_int(_) do
    {:error, :invalid_integer}
  end

  defp first_n(subject, n) when byte_size(subject) < n  do
    :error
  end

  defp first_n(subject, n) do
    :binary.part(subject, 0, n)
  end

  defp after_n(subject, n) when byte_size(subject) < n do
    :error
  end

  defp after_n(subject, n) do
    :binary.part(subject, n, byte_size(subject)-n)
  end

  defp extract_list_contents(<<b::bits>>) do
    extract_list_contents({:ok, [], b})
  end

  defp extract_list_contents({:ok, list, <<?e, rest::bits>>}) do
    {:ok, Enum.reverse(list), rest}
  end

  defp extract_list_contents({:ok, list, rest}) do
    with {:ok, next, rest} <- extract_next(rest)
    do extract_list_contents({:ok, [next|list], rest})
    else err -> err
    end
  end

  defprotocol Bencode do
    @fallback_to_any true
    @doc "Encode an erlang term."
    def encode(term)
  end

  defimpl Bencode, for: Integer do
    def encode(term) do
      ["i", Integer.to_string(term), "e"]
    end
  end

  defimpl Bencode, for: BitString do
    def encode(term) do
      len = Integer.to_string byte_size(term)
      [len, ":", term]
    end
  end

  defimpl Bencode, for: List do
    def encode(term) do
      ["l", encode_contents(term), "e"]
    end

    defp encode_contents(term) when is_list(term) do
      Enum.map(term, &Bencode.encode/1)
    end
  end

  defimpl Bencode, for: Map do
    def encode(term) do
      ["d", encode_contents(term), "e"]
    end

    defp encode_contents(term) when is_map(term) do
      term
      |> Map.to_list
      |> List.keysort(0)
      |> Enum.map(&Tuple.to_list/1)
      |> Enum.map(&encode_contents/1)
    end

    defp encode_contents(term) when is_list(term) do
      Enum.map(term, &Bencode.encode/1)
    end
  end

  defimpl Bencode, for: Tuple do
    def encode(term) do
      term |> Tuple.to_list() |> Bencode.encode()
    end
  end

  defimpl Bencode, for: Any do
    def encode(term) do
      term |> to_string() |> Bencode.encode()
    end
  end

  @doc """

  Encode an erlang term.



  ## Examples



      iex> ExBencode.encode(1)

      {:ok, "i1e"}



      iex> ExBencode.encode("hi!")

      {:ok, "3:hi!"}



      iex> ExBencode.encode([])

      {:ok, "le"}



      iex> ExBencode.encode([1])

      {:ok, "li1ee"}



      iex> ExBencode.encode(%{})

      {:ok, "de"}



      iex> ExBencode.encode(%{"cow" => "moo"})

      {:ok, "d3:cow3:mooe"}



    Note that a keyword list counts as a list of lists, so convert keyword

    lists to maps before encoding. Otherwise, an empty keyword list

    could either be encoded as an empty list or an empty dict, and the

    library avoids making that kind of arbitrary decision.



      iex> ExBencode.encode([cow: "moo"])

      {:ok, "ll3:cow3:mooee"}



    Use `Enum.into/2` to convert a keyword list into a map



      iex> Enum.into [cow: "moo"], %{}

      %{cow: "moo"}



      iex> ExBencode.encode(%{cow: "moo"})

      {:ok, "d3:cow3:mooe"}





  """
  def encode(term) do
    {:ok, Bencode.encode(term) |> :erlang.iolist_to_binary()}
  end
end