ex_bencode

defmodule ExBencode do
  @moduledoc """
  Documentation for ExBencode.
  """

  @doc """
  Decode the bencoded binary value.

  ## Examples

  Decoding integers

      iex> ExBencode.decode("i10e")
      {:ok, 10}

      iex> ExBencode.decode("i-10e")
      {:ok, -10}

    Scientific notation is **not** supported

      iex> ExBencode.decode("i1.5e7e")
      {:error, :unexpected_content, %{index: 5, unexpected: "7e"}}

  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(b)

  def decode(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(s) do
    case s do
      _ when is_nil(s) -> {:error, :not_binary}
      _ when not is_binary(s) -> {:error, :not_binary}
      <<"i", _rest :: binary>> -> extract_int(s)
      <<i, _rest :: binary>> when i >= ?0 and i <= ?9 -> extract_string(s)
      <<"l", _rest :: binary>> -> extract_list(s)
      <<"d", _rest :: binary>> -> extract_dict(s)
      _ -> {:error, :not_bencoded_form}
    end
  end

  defp extract_int(s) do
    with [substr | rest] <- String.split(s, ~r/i|e/, parts: 2, trim: true),
         {int, _} <- Integer.parse(substr)
    do
      case rest do
        [] -> {:ok, int, ""}
        _ -> {:ok, int, hd(rest)}
      end
    else
      _err -> {:error, :invalid_integer}
    end
  end

  defp extract_string(s) do
    with [lenstr, rest] <- String.split(s, ":", parts: 2),
         {length, _} <- Integer.parse(lenstr),
         {str, rest} <- binary_split(rest, length)
    do
      if byte_size(str) != length do
        {:error, :invalid_string, %{expected_size: length, actual_size: byte_size(str)}}
      else
        {:ok, str, rest}
      end
    else
      _ -> {:error, :invalid_string}
    end
  end

  defp binary_split(binary, position) do
    if byte_size(binary) <= position do
      {binary, ""}
    else
      tailsize = byte_size(binary) - position
      head = binary_part(binary, 0, position)
      tail = binary_part(binary, position, tailsize)
      {head, tail}
    end
  end

  defp extract_list(s) do
    {"l", tail} = String.split_at(s, 1)
    extract_list_contents({:ok, [], tail})
  end

  defp extract_dict(s) do
    with  {"d", tail} <- String.split_at(s, 1),
          {:ok, contents, rest} <- extract_list_contents({:ok, [], 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_list_contents({:ok, list, rest}) do
    # Build the list in reverse, then reverse it at the very end.
    # This lets us prepend entries to the list, which is much faster.
    if String.starts_with?(rest, "e") do
      {"e", afterlist} = String.split_at(rest, 1)
      {:ok, Enum.reverse(list), afterlist}
    else
      with {:ok, next, rest} <- extract_next(rest)
      do extract_list_contents({:ok, [next|list], rest})
      else err -> err
      end
    end
  end
end