tla_connect

defmodule TlaConnect.Emitter do
  @moduledoc """
  Approach 3, part 1: NDJSON state emitter.

  Writes state transitions as newline-delimited JSON to a file.
  Each line is `{"action":"...", ...state}`.
  """

  alias TlaConnect.ValidationError

  defstruct [:path, :io_device, count: 0, finished: false]

  @type t :: %__MODULE__{
          path: Path.t(),
          io_device: IO.device() | nil,
          count: non_neg_integer(),
          finished: boolean()
        }

  @doc "Create a new emitter writing to the given file path."
  @spec new(Path.t()) :: t()
  def new(file_path) do
    {:ok, io} = File.open(file_path, [:write, :utf8])
    %__MODULE__{path: file_path, io_device: io}
  end

  @doc "Emit a state transition as an NDJSON line."
  @spec emit(t(), String.t(), map()) :: t()
  def emit(%{finished: true}, _action, _state) do
    raise ValidationError, message: "Cannot emit after finish()"
  end

  def emit(_emitter, _action, state) when not is_map(state) or is_struct(state) do
    raise ValidationError, message: "State must be a plain map"
  end

  def emit(emitter, action, state) do
    line = Map.put(state, "action", action) |> Jason.encode!()
    IO.write(emitter.io_device, line <> "\n")
    %{emitter | count: emitter.count + 1}
  end

  @doc """
  Finish emitting. Returns `{count, emitter}` where count is lines written.
  No further calls to `emit/3` are allowed.
  """
  @spec finish(t()) :: {non_neg_integer(), t()}
  def finish(%{finished: true} = emitter), do: {emitter.count, emitter}

  def finish(emitter) do
    if emitter.io_device, do: File.close(emitter.io_device)
    emitter = %{emitter | finished: true, io_device: nil}
    {emitter.count, emitter}
  end
end