defmodule ProtobufLite do
  import Bitwise

  # 0   Varint  int32, int64, uint32, uint64, sint32, sint64, bool, enum
  def dvarint(bin), do: dvarint(bin, 0, 0)

  def dvarint(<<1::size(1), i::size(7), r::binary>>, count, acc),
    do: dvarint(r, count + 1, acc + (i <<< (count * 7)))

  def dvarint(<<0::size(1), i::size(7), r::binary>>, count, acc) do
    num = acc + (i <<< (count * 7))
    bin = <<num::64>>
    <<signed::64-signed>> = bin
    {signed, r}
  end

  # 1   64-bit  fixed64, sfixed64, double
  def dbit64(<<int64::little-signed-size(64), r::binary>>), do: {int64, r}

  # 2   Length-delimited    string, bytes, embedded messages, packed repeated fields
  def dlen_delimited(bin) do
    {len, r} = dvarint(bin)
    <<content::binary-size(len), r::binary>> = r
    {content, r}
  end

  # 3   Start group groups (deprecated)
  # def group(r), do: {0, r}
  # 4   End group groups (deprecated)
  # def group_end(r), do: {0, r}

  # 5   32-bit  fixed32, sfixed32, float
  def dbit32(<<int32::little-signed-size(32), r::binary>>), do: {int32, r}

  def dtag_type(<<tag::size(5), type::size(3), r::binary>>) do
    case tag &&& 16 do
      16 ->
        {value, r} = dvarint(r)
        {(value <<< 4) + (tag &&& 0xF), type, r}

      _ ->
        {tag, type, r}
    end
  end

  def decode(bin), do: decode(bin, [])
  def decode(<<>>, acc), do: acc

  def decode(bin, acc) do
    {tag, type, r} = dtag_type(bin)
    # IO.puts "#{type} #{tag} #{inspect r}"
    case type do
      # varint
      0 ->
        {value, r} = dvarint(r)
        decode(r, acc ++ [{:varint, tag, value}])

      # int64
      1 ->
        {value, r} = dbit64(r)
        decode(r, acc ++ [{:int64, tag, value}])

      # binary
      2 ->
        {value, r} = dlen_delimited(r)

        decode(r, acc ++ [{:binary, tag, value}])

      # group
      3 ->
        {nest, r} = decode(r, [])
        decode(r, acc ++ [{:group, tag, nest}])

      # group_end
      4 ->
        {acc, r}

      # int32
      5 ->
        {value, r} = dbit32(r)
        decode(r, acc ++ [{:int32, tag, value}])
    end
  end

  def evarint(num), do: evarint(num, <<>>)
  def evarint(num, acc) when num <= 127, do: acc <> <<0::integer-size(1), num::integer-size(7)>>

  def evarint(num, acc) do
    <<r::integer-little-size(1), n::integer-little-size(7)>> = <<num::integer-size(8)-little>>
    acc = acc <> <<1::integer-size(1), n::integer-size(7)>>
    evarint((num >>> 8 <<< 1) + r, acc)
  end

  def etag_type(tag, type) do
    cond do
      tag < 16 ->
        <<tag::size(5), type::size(3)>>

      true ->
        btag = (tag &&& 0xF) ||| 0x10
        <<btag::size(5), type::size(3), evarint(tag >>> 4)::binary>>
    end
  end

  def encode(proplists), do: encode(proplists, <<>>)
  def encode([], acc), do: acc

  def encode([data | rest], acc) do
    acc =
      case data do
        {:varint, tag, n} ->
          acc <> etag_type(tag, 0) <> evarint(n)

        {:int64, tag, n} ->
          acc <> etag_type(tag, 1) <> <<n::little-size(64)>>

        {:binary, tag, n} ->
          acc <> etag_type(tag, 2) <> evarint(byte_size(n)) <> n

        {:group, tag, n} ->
          acc <> etag_type(tag, 3) <> encode(n) <> etag_type(tag, 4)

        {:int32, tag, n} ->
          # IO.inspect {acc, etag_type(tag, 5), n}
          acc <> etag_type(tag, 5) <> <<n::little-size(32)>>
      end

    encode(rest, acc)
  end

  # Dangerous! dont use often
  def to_map(proplist) do
    Enum.reduce(proplist, %{}, fn
      {type, tag, value}, a ->
        v =
          case type do
            :group -> to_map(value)
            _ -> value
          end

        case Map.has_key?(a, tag) do
          false ->
            Map.merge(a, %{tag => v})

          true ->
            oldV = Map.get(a, tag)

            v =
              if is_list(oldV) do
                oldV ++ [v]
              else
                [oldV, v]
              end

            Map.merge(a, %{tag => v})
        end
    end)
  end
end