bacstack

defmodule BACnet.Protocol.ResultFlags do
  @moduledoc """
  BACnet Result Flags conveys several flags that describe characteristics of the response data.

  * The FIRST_ITEM flag indicates whether this response includes the first list or array element
    (in the case of positional indexing), or the oldest timestamped item (in the case of time indexing).
  * The LAST_ITEM flag indicates whether this response includes the last list or array element
    (in the case of positional indexing), or the newest timestamped item (in the case of time indexing).
  * The MORE_ITEMS flag indicates whether more items matched the request but were not transmittable within the PDU.
  """

  alias BACnet.Protocol.ApplicationTags

  # TODO: Throw argument error in encode if not valid

  @typedoc """
  Represents BACnet result flags.
  """
  @type t :: %__MODULE__{
          first_item: boolean(),
          last_item: boolean(),
          more_items: boolean()
        }

  @fields [
    :first_item,
    :last_item,
    :more_items
  ]
  @enforce_keys @fields
  defstruct @fields

  @doc """
  Encodes a BACnet result flags into application tags encoding.
  """
  @spec encode(t(), Keyword.t()) :: {:ok, ApplicationTags.encoding_list()} | {:error, term()}
  def encode(%__MODULE__{} = flags, _opts \\ []) do
    {:ok, [to_bitstring(flags)]}
  end

  @doc """
  Creates from an application tag bitstring a result flags.
  """
  @spec from_bitstring(tuple()) :: t()
  def from_bitstring(bitstring) when is_tuple(bitstring) and tuple_size(bitstring) == 3 do
    %__MODULE__{
      first_item: elem(bitstring, 0),
      last_item: elem(bitstring, 1),
      more_items: elem(bitstring, 2)
    }
  end

  @doc """
  Parses a BACnet result flags from application tags encoding.
  """
  @spec parse(ApplicationTags.encoding_list()) ::
          {:ok, {t(), rest :: ApplicationTags.encoding_list()}}
          | {:error, term()}
  def parse(tags) when is_list(tags) do
    case tags do
      [{:bitstring, {_b1, _b2, _b3} = bs} | rest] -> {:ok, {from_bitstring(bs), rest}}
      _else -> {:error, :invalid_tags}
    end
  end

  @doc """
  Creates an application tag bitstring from a result flags.
  """
  @spec to_bitstring(t()) :: BACnet.Protocol.ApplicationTags.primitive_encoding()
  def to_bitstring(%__MODULE__{} = t) do
    {:bitstring, {t.first_item, t.last_item, t.more_items}}
  end

  @doc """
  Validates whether the given result flags is in form valid.

  It only validates the struct is valid as per type specification.
  """
  @spec valid?(t()) :: boolean()
  def valid?(
        %__MODULE__{
          first_item: first_item,
          last_item: last_item,
          more_items: more_items
        } = _t
      )
      when is_boolean(first_item) and is_boolean(last_item) and is_boolean(more_items),
      do: true

  def valid?(%__MODULE__{} = _t), do: false
end