defmodule LoggerBinary do
  @moduledoc """
  A custom `Logger` formatter for handling binary data.

  This formatter is designed to correctly format binary messages. It converts
  non-printable binary data into a hexadecimal string representation and can
  optionally add a directional indicator (`"< "` or `"> "`) when needed.

  ## Features

    * Formats binary messages as uppercase hexadecimal strings.
    * Prepends formatted binary messages with a directional indicator if the `:direction`
      metadata is present. Supported values for the `:direction` metadata are `:in` and `:out`.
    * Uses the default Logger format for all other types of messages (e.g., printable strings and charlists).

  ## Direction Indicator

  You can add the `:direction` metadata to your log messages to indicate if the
  binary data is an incoming or outgoing message:

      Logger.debug(<<0x01, 0x02, 0x03>>, direction: :in)
      # Logs: "[debug] < 01 02 03"

      Logger.debug(<<0x01, 0x02, 0x03>>, direction: :out)
      # Logs: "[debug] > 01 02 03"

  Without directional metadata, it simply logs the formatted binary:

      Logger.debug(<<0x01, 0x02, 0x03>>)
      # Logs: "[debug] 01 02 03"
  """

  @doc """
  Formats binary data as space-separated uppercase hex pairs.

  Returns the original value inspected if not a binary.

  ## Examples

      iex> LoggerBinary.format(<<0xDE, 0xAD, 0xBE, 0xEF>>)
      "DE AD BE EF"

      iex> LoggerBinary.format(:timeout)
      ":timeout"

  """
  def format(data) when is_binary(data) do
    data
    |> Base.encode16(case: :upper)
    |> String.graphemes()
    |> Enum.chunk_every(2)
    |> Enum.map_join(" ", &Enum.join(&1))
  end

  def format(data), do: inspect(data)
end