defmodule Sorcery.Domain.Server do
  @moduledoc """
  The server component that manages domain instances and processes commands.

  The DomainServer is responsible for:
  1. Managing domain process lifecycles
  2. Processing commands and generating events
  3. Maintaining domain state by applying events
  4. Handling event subscriptions

  ## Types of Domains

  Sorcery supports two types of domains:

  1. **ID-Based Domains** - Multiple instances of the same domain type, each with a unique ID
  2. **Singleton Domains** - Single instance domains that manage global state

  ## Auto-Starting

  Domains can be configured to auto-start when your application boots. This ensures that:
  1. All domains are ready to handle commands immediately
  2. Domains listening for events from other domains don't miss any events
  3. Event handlers are always active

  Configure auto-starting in your config:

      config :sorcery, :auto_start,
        domains: [
          MyApp.UserDomain,
          MyApp.SettingsDomain
        ]

  Domains not configured for auto-start will be started on-demand when they receive their first command.

  ## ID-Based Domains

  Use these when you need multiple instances of the same domain type:

  ```elixir
  # Start and execute a command on a user domain
  Sorcery.DomainServer.execute(
    MyApp.UserDomain,
    "user-123",
    %MyApp.UserDomain.Commands.RegisterUser{
      name: "John Doe",
      email: "john@example.com"
    }
  )
  ```

  ## Singleton Domains

  Use these for global state or configuration:

  ```elixir
  # Start and execute a command on a settings domain
  Sorcery.DomainServer.execute(
    MyApp.SettingsDomain,
    %MyApp.SettingsDomain.Commands.UpdateTheme{
      theme: "dark"
    }
  )
  ```

  ## Process Management

  Each domain instance is a separate process that:
  - Subscribes to the event store
  - Maintains its own state
  - Processes commands
  - Generates and applies events

  ## Event Handling

  When events occur:
  1. The event is stored in the event store
  2. All domain processes receive the event
  3. Each domain filters events relevant to it
  4. Relevant events are applied to update state

  ## Process Registry

  Domain processes are registered with unique names:
  - ID-based domains: `{DomainModule, "id"}`
  - Singleton domains: `DomainModule`

  This ensures:
  - Only one process per domain instance
  - Easy lookup of existing processes
  - Automatic process cleanup on termination
  """

  use GenServer

  @doc """
  Starts a singleton instance of a domain.
  This is used for auto-starting domains at application boot.
  """
  def start_singleton(domain_module) do
    name = via_tuple(domain_module, nil)
    case GenServer.start_link(__MODULE__, {domain_module, "singleton"}, name: name) do
      {:ok, pid} -> {:ok, pid}
      {:error, {:already_started, pid}} -> {:ok, pid}
      error -> error
    end
  end

  @doc """
  Starts a new DomainServer process.

  ## Arguments

  Can be called in two ways:

  1. With module and ID:
     `start_link(domain_module, id_or_command, maybe_command)`

  2. With keyword options:
     `start_link(domain: domain_module, id: id)`
  """
  def start_link(opts) when is_list(opts) do
    domain_module = Keyword.fetch!(opts, :domain)
    id = Keyword.get(opts, :id)
    start_link(domain_module, id)
  end

  def start_link(domain_module, id_or_command, maybe_command \\ nil) do
    case {id_or_command, maybe_command} do
      {id, _command} when is_binary(id) and not is_nil(maybe_command) ->
        # Case 1: ID-based domain
        name = via_tuple(domain_module, id)
        GenServer.start_link(__MODULE__, {domain_module, id}, name: name)

      {_command, nil} ->
        # Case 2: Singleton domain
        start_singleton(domain_module)
    end
  end

  @doc """
  Gets the current state of a domain instance.

  ## Arguments

    * `domain_module` - The domain module
    * `id` - The instance ID (or nil for singleton domains)

  ## Returns

    * `{:ok, state}` - The current state of the domain
    * `{:error, :not_found}` - If the domain instance doesn't exist
  """
  def get_state(domain_module, id) do
    name = via_tuple(domain_module, id)
    try do
      state = GenServer.call(name, :get_state)
      {:ok, state}
    catch
      :exit, {:noproc, _} -> {:error, :not_found}
    end
  end

  @doc """
  Initializes a domain process with its initial state.

  The process will:
  1. Subscribe to the event store
  2. Load historical events
  3. Build initial state by applying events
  """
  def init({domain_module, id}) do
    Sorcery.EventStore.subscribe()

    # Load history and build initial state
    {:ok, events} = Sorcery.EventStore.get_events()
    state =
      events
      |> Enum.filter(fn event ->
        if id do
          event.metadata["instance_id"] == id
        else
          event.metadata["domain"] == domain_module
        end
      end)
      |> Enum.reduce(domain_module.new(), fn event, state ->
        # Event Store -> Domain Event
        try do
          # Convert the event data to the proper struct
          event_module = Module.concat([domain_module, Events, Macro.camelize(event.type)])

          # Convert string keys to atoms in event data
          event_data = for {key, val} <- event.data, into: %{} do
            {String.to_existing_atom(key), val}
          end

          event_struct = struct(event_module, event_data)

          # Apply the event using the domain module
          domain_module.apply_event(state, event_struct)
        rescue
          e ->
            IO.inspect(e, label: "Error applying event")
            state
        end
      end)

    {:ok, {domain_module, id, state}}
  end

  def handle_call(:get_state, _from, {_domain_module, _id, state} = full_state) do
    {:reply, state, full_state}
  end

  def handle_call({:command, command}, _from, {domain_module, id, state}) do
    case domain_module.handle_command(state, command) do
      {:ok, events} ->
        # Domain Event -> Event Store
        # Only append events, don't apply them directly
        events
        |> Enum.map(&Sorcery.EventConverter.domain_to_event_store(&1, domain_module, id))
        |> Sorcery.EventStore.append()
        {:reply, :ok, {domain_module, id, state}}
      {:error, reason} ->
        {:reply, {:error, reason}, {domain_module, id, state}}
    end
  end

  def handle_info({:event, event}, {domain_module, id, state} = current_state) do
    # For ID-based domains, only apply events that belong to this instance
    # For singleton domains, apply all events for this domain
    should_apply = if id do
      event.metadata.instance_id == id
    else
      event.metadata.domain == domain_module
    end

    if should_apply do
      # Event Store -> Domain Event
      event_struct = Sorcery.EventConverter.event_store_to_domain(event)
      new_state = domain_module.apply_event(state, event_struct)
      {:noreply, {domain_module, id, new_state}}
    else
      {:noreply, current_state}
    end
  end

  defp via_tuple(domain_module, id) do
    registry_key = if id, do: {domain_module, id}, else: domain_module
    {:via, Registry, {Sorcery.Domain.Registry, registry_key}}
  end

  @doc """
  Executes a command on a domain.

  This is the main entry point for changing domain state. It will:
  1. Start the domain process if it doesn't exist
  2. Send the command to the process
  3. Handle command validation
  4. Generate and store events
  5. Apply events to update state

  ## Examples

  For ID-based domains:

      Sorcery.DomainServer.execute(
        MyApp.UserDomain,
        "user-123",
        %MyApp.UserDomain.Commands.RegisterUser{
          name: "John Doe",
          email: "john@example.com"
        }
      )

  For singleton domains:

      Sorcery.DomainServer.execute(
        MyApp.SettingsDomain,
        %MyApp.SettingsDomain.Commands.UpdateTheme{
          theme: "dark"
        }
      )

  ## Returns

    * `:ok` - Command succeeded
    * `{:error, reason}` - Command failed with reason
  """
  def execute(domain_module, id_or_command, maybe_command \\ nil) do
    case {id_or_command, maybe_command} do
      {id, command} when is_binary(id) and not is_nil(command) ->
        # Case 1: ID-based domain
        case start_link(domain_module, id, command) do
          {:ok, pid} ->
            GenServer.call(pid, {:command, command})
          {:error, {:already_started, pid}} ->
            GenServer.call(pid, {:command, command})
          error ->
            error
        end

      {command, nil} ->
        # Case 2: Singleton domain
        case start_link(domain_module, command) do
          {:ok, pid} ->
            GenServer.call(pid, {:command, command})
          {:error, {:already_started, pid}} ->
            GenServer.call(pid, {:command, command})
          error ->
            error
        end
    end
  end
end