defmodule Magnet do
  @moduledoc """
  `Magnet` struct which represents Magnet URI.

  See: https://en.wikipedia.org/wiki/Magnet_URI_scheme
  """

  defstruct name: nil,
            length: nil,
            info_hash: [],
            fallback: nil,
            source: [],
            keywords: [],
            manifest: nil,
            announce: [],
            experimental: %{}

  @type t :: %__MODULE__{
          name: String.t(),
          length: number,
          info_hash: [String.t()],
          fallback: String.t(),
          source: [String.t()],
          keywords: [String.t()],
          manifest: String.t(),
          announce: [String.t()],
          experimental: map
        }

  defdelegate decode(data), to: Magnet.Decoder
  defdelegate encode(data), to: Magnet.Encoder
end

defimpl Collectable, for: Magnet do
  @spec into(Magnet.t()) ::
          {Magnet.t(),
           (Magnet.t(), {:cont, {String.t(), String.t()}} | :done | :halt -> Magnet.t() | :ok)}
  def into(original) do
    {original,
     fn
       # ignore entries with empty values
       acc, {:cont, {_, ""}} ->
         acc

       # as (Acceptable Source) - Web link to the file online
       acc, {:cont, {"as", value}} ->
         uri = URI.decode(value)
         %Magnet{acc | fallback: uri}

       # dn (Display Name) - Suggested filename
       acc, {:cont, {"dn", value}} ->
         %Magnet{acc | name: value}

       # kt (Keyword Topic) - Key words for the given torrent
       acc, {:cont, {<<"kt", priority::binary>>, value}} ->
         entry = parse_suffix_number(priority, String.split(value, "+"))
         %Magnet{acc | keywords: [entry | acc.keywords]}

       # mt (Manifest Topic) - link to a file that contains a list of magneto (MAGMA - MAGnet MAnifest)
       acc, {:cont, {"mt", value}} ->
         %Magnet{acc | manifest: value}

       # tr (address TRacker) - Tracker/Announce URLs for BitTorrent downloads
       acc, {:cont, {<<"tr", priority::binary>>, value}} ->
         announce = URI.decode(value)
         entry = parse_suffix_number(priority, announce)
         %Magnet{acc | announce: [entry | acc.announce]}

       # xl (eXact Length) - File size in bytes
       acc, {:cont, {"xl", value}} ->
         length = String.to_integer(value)
         %Magnet{acc | length: length}

       # xs (eXact Source) - peer-to-peer links
       acc, {:cont, {<<"xs", priority::binary>>, value}} ->
         uri = URI.decode(value)
         entry = parse_suffix_number(priority, uri)
         %Magnet{acc | source: [entry | acc.source]}

       # xt (eXact Topic) - URN containing file hash
       acc, {:cont, {<<"xt", priority::binary>>, value}} ->
         entry = parse_suffix_number(priority, value)
         %Magnet{acc | info_hash: [entry | acc.info_hash]}

       acc, {:cont, {<<"x.", key::binary>>, value}} ->
         experimental = URI.decode(value)
         %Magnet{acc | experimental: Map.put(acc.experimental, key, experimental)}

       acc, :done ->
         keywords =
           acc.keywords
           |> sort_by_priority
           |> List.flatten()
           |> Enum.dedup()

         %Magnet{
           acc
           | info_hash: prepare_list(acc.info_hash),
             announce: prepare_list(acc.announce),
             source: prepare_list(acc.source),
             keywords: keywords
         }

       _, :halt ->
         :ok
     end}
  end

  @spec prepare_list([{number, String.t()}]) :: [String.t()]
  defp prepare_list(list) do
    list
    |> sort_by_priority
    |> Enum.dedup()
  end

  @spec sort_by_priority([{number, String.t()}]) :: [String.t()]
  defp sort_by_priority(priority_list) do
    priority_list
    |> Enum.sort_by(&elem(&1, 0))
    |> Enum.map(&elem(&1, 1))
  end

  @spec parse_suffix_number(String.t(), any) :: {number, any}
  defp parse_suffix_number("", value),
    do: {0, value}

  defp parse_suffix_number(<<".", number::binary>>, value) do
    with {num, _} <- Integer.parse(number), do: {num, value}
  end
end