Varint encoding and decoding utilities.

https://developers.google.com/protocol-buffers/docs/encoding#varints

For performance reasons, varint decoding must be built through a macro, so that binary match contexts are reused and no new large binaries get allocated. You can define your own varint decoders with the decoder macro, which generates function heads for up to 10-bytes long varint-encoded data.

defmodule VarintDecoders do
  import Protobuf.Wire.Varint

  decoder :def, :decode_and_sum, [:plus] do
    {:ok, value + plus, rest}
  end

  def decode_all(<<bin::bits>>), do: decode_all(bin, [])

  defp decode_all(<<>>, acc), do: acc

  defdecoderp decode_all(acc) do
    decode_all(rest, [value | acc])
  end
end

iex> VarintDecoders.decode_and_sum(<<35>>, 7)
{:ok, 42, ""}

iex> VarintDecoders.decode_all("abcd asdf")
[102, 100, 115, 97, 32, 100, 99, 98, 97]

Refer to efficiency guide for more on efficient binary handling.

Encoding on the other hand is simpler. It takes an integer and returns an iolist with its varint representation:

iex> Protobuf.Wire.Varint.encode(35)
[35]

iex> Protobuf.Wire.Varint.encode(1_234_567)
[<<135>>, <<173>>, 75]