defmodule Sorcery.Domain do
  @moduledoc """
  A powerful macro for defining event-sourced domains in a declarative way.

  Domains in Sorcery represent your business entities and their rules. Each domain:
  - Defines its state shape
  - Declares what events can occur
  - Specifies how events change state
  - Handles commands that generate events

  ## Domain Structure

  A domain consists of:

  1. **State** - The current data structure of your domain
  2. **Events** - Things that have happened (facts)
  3. **Commands** - User intentions that may generate events
  4. **Event Handlers** - Functions that update state based on events
  5. **Command Handlers** - Functions that validate commands and generate events

  ## Auto-Starting

  Domains are automatically started 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

  ## Example

  ```elixir
  defmodule MyApp.UserDomain do
    use Sorcery.Domain

    domain do
      # Define the state shape
      state :name, default: nil
      state :email, default: nil
      state :status, default: :inactive

      # Define an event
      event :user_registered do
        field :name, type: :string
        field :email, type: :string

        # How this event changes state
        apply fn state, event ->
          %{state |
            name: event.name,
            email: event.email,
            status: :active
          }
        end
      end

      # Define a command
      command :register_user do
        field :name, type: :string
        field :email, type: :string

        # Command validation and event generation
        handle fn state, command ->
          cond do
            state.status == :active ->
              {:error, :already_registered}
            !valid_email?(command.email) ->
              {:error, :invalid_email}
            true ->
              event = %__MODULE__.Events.UserRegistered{
                name: command.name,
                email: command.email
              }
              {:ok, [event]}
          end
        end
      end
    end
  end
  ```

  ## State Definition

  State fields are defined using the `state/2` macro:

  ```elixir
  state :name, default: nil
  state :email, default: nil
  state :status, default: :inactive
  ```

  ## Event Definition

  Events are defined using the `event/2` macro and must include:
  - Fields that the event carries
  - An apply function that updates state

  ```elixir
  event :user_registered do
    field :name, type: :string
    field :email, type: :string

    apply fn state, event ->
      %{state | name: event.name, email: event.email}
    end
  end
  ```

  ## Command Definition

  Commands are defined using the `command/2` macro and must include:
  - Fields that the command requires
  - A handle function that validates and generates events

  ```elixir
  command :register_user do
    field :name, type: :string
    field :email, type: :string

    handle fn state, command ->
      # Return {:ok, [events]} on success
      # Return {:error, reason} on failure
      {:ok, [%Events.UserRegistered{...}]}
    end
  end
  ```

  ## External Events

  You can handle events from other domains using `external_event/2`:

  ```elixir
  external_event OtherDomain.Events.SomethingHappened do
    apply fn state, event ->
      # Update state based on external event
      state
    end
  end
  ```

  ## Best Practices

  1. **Keep Domains Focused**: Each domain should represent a single business concept
  2. **Events are Facts**: Name events in past tense, they represent things that happened
  3. **Commands May Fail**: Always handle validation in command handlers
  4. **State is Private**: Only expose state changes through commands
  5. **Event Handlers are Pure**: They should only transform state, no side effects
  """

  defmacro __using__(_opts) do
    quote do
      import Sorcery.Domain

      # Mark this module as a Sorcery domain for auto-discovery
      @sorcery_domain true

      # Event tracking
      Module.register_attribute(__MODULE__, :current_event, accumulate: false)
      Module.register_attribute(__MODULE__, :events, accumulate: true)
      Module.register_attribute(__MODULE__, :event_fields, accumulate: true)
      Module.register_attribute(__MODULE__, :event_applies, accumulate: true)
      Module.register_attribute(__MODULE__, :event_handlers, accumulate: true)

      # Command tracking
      Module.register_attribute(__MODULE__, :current_command, accumulate: false)
      Module.register_attribute(__MODULE__, :commands, accumulate: true)
      Module.register_attribute(__MODULE__, :command_fields, accumulate: true)
      Module.register_attribute(__MODULE__, :command_handlers, accumulate: true)

      # State tracking
      Module.register_attribute(__MODULE__, :states, accumulate: true)

      @before_compile Sorcery.Domain
    end
  end

  @doc """
  Defines a domain block where state, events, and commands are declared.

  ## Example

      domain do
        state :name, default: nil
        event :user_registered do
          # ...
        end
        command :register_user do
          # ...
        end
      end
  """
  defmacro domain(do: block) do
    quote do
      import Sorcery.Domain
      unquote(block)
    end
  end

  @doc """
  Declares a state field with an optional default value.

  ## Options

    * `:default` - The default value for this field

  ## Example

      state :name, default: nil
      state :status, default: :pending
  """
  defmacro state(name, opts \\ []) do
    quote do
      @states {unquote(name), unquote(opts)}
    end
  end

  @doc """
  Defines an event type and its structure.

  Events must include:
  - One or more fields using `field/2`
  - An apply function using `apply/1`

  ## Example

      event :user_registered do
        field :name, type: :string
        field :email, type: :string

        apply fn state, event ->
          %{state | name: event.name, email: event.email}
        end
      end
  """
  defmacro event(name, do: block) do
    event_name = Macro.escape(name)

    quote do
      Module.put_attribute(__MODULE__, :current_event, unquote(event_name))
      Module.put_attribute(__MODULE__, :events, unquote(event_name))

      unquote(block)

      Module.put_attribute(__MODULE__, :current_event, nil)
    end
  end

  @doc """
  Declares a field for an event or command.

  ## Options

    * `:type` - The type of the field (e.g., `:string`, `:integer`)

  ## Example

      field :name, type: :string
      field :age, type: :integer
  """
  defmacro field(name, opts) do
    quote do
      current_command = Module.get_attribute(__MODULE__, :current_command)

      if current_command do
        @command_fields {current_command, {unquote(name), unquote(opts)}}
      else
        current_event = Module.get_attribute(__MODULE__, :current_event)
        if current_event do
          @event_fields {current_event, {unquote(name), unquote(opts)}}
        else
          raise "field/2 must be called within an event or command block"
        end
      end
    end
  end

  @doc """
  Defines how an event changes the domain state.

  The function receives the current state and event as arguments and should return the new state.

  ## Example

      apply fn state, event ->
        %{state | name: event.name, email: event.email}
      end
  """
  defmacro apply(func) do
    quote do
      current_event = Module.get_attribute(__MODULE__, :current_event)

      Module.put_attribute(
        __MODULE__,
        :event_applies,
        {current_event, unquote(Macro.escape(func))}
      )
    end
  end

  @doc """
  Defines a command that can be executed on the domain.

  Commands must include:
  - One or more fields using `field/2`
  - A handle function using `handle/1`

  ## Example

      command :register_user do
        field :name, type: :string
        field :email, type: :string

        handle fn state, command ->
          {:ok, [%Events.UserRegistered{...}]}
        end
      end
  """
  defmacro command(name, do: block) do
    quote do
      Module.put_attribute(__MODULE__, :current_command, unquote(name))
      Module.put_attribute(__MODULE__, :commands, unquote(name))

      unquote(block)

      Module.put_attribute(__MODULE__, :current_command, nil)
    end
  end

  @doc """
  Defines how to handle a command or event.

  For commands, the function should return:
  - `{:ok, events}` on success where events is a list of events to apply
  - `{:error, reason}` on failure

  ## Example

      handle fn state, command ->
        if valid?(command) do
          {:ok, [%Events.SomethingHappened{...}]}
        else
          {:error, :invalid_command}
        end
      end
  """
  defmacro handle(func) do
    quote do
      cond do
        @current_command != nil ->
          @command_handlers {@current_command, unquote(Macro.escape(func))}
        @current_event != nil ->
          @event_handlers {@current_event, unquote(Macro.escape(func))}
        true ->
          raise "handle/1 must be called within a command or event block"
      end
    end
  end

  @doc """
  Defines handling of events from other domains.

  This is useful when your domain needs to react to events from other parts
  of your system.

  ## Example

      external_event OtherDomain.Events.SomethingHappened do
        apply fn state, event ->
          # Update state based on external event
          state
        end
      end
  """
  defmacro external_event(event_struct, do: block) do
    quote do
      # Get the module name from the struct
      event_module = unquote(event_struct).__struct__
      event_name = Module.split(event_module)
                  |> List.last()
                  |> Macro.underscore() # Convert from CamelCase to snake_case
                  |> String.to_atom()

      Module.put_attribute(__MODULE__, :current_event, event_name)
      Module.put_attribute(__MODULE__, :events, event_name)

      # Mark this event as external and store the full module path
      Module.put_attribute(__MODULE__, :event_fields, {event_name, {:external, true}})
      Module.put_attribute(__MODULE__, :event_fields, {event_name, {:module, event_module}})

      unquote(block)

      Module.put_attribute(__MODULE__, :current_event, nil)
    end
  end

  defmacro __before_compile__(env) do
    states = Module.get_attribute(env.module, :states)
    events = Module.get_attribute(env.module, :events)
    event_fields = Module.get_attribute(env.module, :event_fields)
    event_applies = Module.get_attribute(env.module, :event_applies)
    commands = Module.get_attribute(env.module, :commands)
    command_fields = Module.get_attribute(env.module, :command_fields)
    command_handlers = Module.get_attribute(env.module, :command_handlers)
    event_handlers = Module.get_attribute(env.module, :event_handlers)
    parent_module = env.module

    state_fields =
      for {name, opts} <- states do
        {name, Keyword.get(opts, :default)}
      end

    state_fields_map = Map.new(state_fields)

    # Create event structs
    event_structs =
      for event_name <- events do
        is_external = Enum.any?(event_fields, fn
          {^event_name, {:external, true}} -> true
          _ -> false
        end)

        if is_external do
          # For external events, get the existing module
          {^event_name, {:module, existing_module}} =
            Enum.find(event_fields, fn
              {^event_name, {:module, _}} -> true
              _ -> false
            end)

          # Don't generate a new struct, use the existing one
          quote do
            alias unquote(existing_module)
          end
        else
          # Regular events remain the same
          fields =
            for {^event_name, {field_name, _opts}} <- event_fields,
                field_name not in [:external, :module] do
              {field_name, nil}
            end

          capitalized_name = event_name
                           |> to_string()
                           |> Macro.camelize() # Use Macro.camelize instead of String.capitalize
                           |> String.to_atom()

          quote do
            defmodule unquote(Module.concat([parent_module, Events, capitalized_name])) do
              @moduledoc false
              defstruct unquote(Macro.escape(fields))
            end
          end
        end
      end

    # Create command structs
    command_structs =
      for command_name <- commands do
        fields =
          command_fields
          |> Enum.filter(fn {cmd, _} ->
            cmd == command_name
          end)
          |> Enum.map(fn {_cmd, {field_name, _opts}} ->
            {field_name, nil}
          end)
          |> Enum.uniq()

        capitalized_name = command_name |> to_string() |> Macro.camelize()

        quote do
          defmodule unquote(Module.concat([parent_module, Commands, capitalized_name])) do
            @moduledoc false
            defstruct unquote(Macro.escape(fields))
          end
        end
      end

    # Generate apply_event functions
    apply_event_clauses =
      for event_name <- events do
        {^event_name, func_ast} =
          Enum.find(event_applies, fn
            {^event_name, _} -> true
            _ -> false
          end)

        # Check if it's an external event
        is_external = Enum.any?(event_fields, fn
          {^event_name, {:external, true}} -> true
          _ -> false
        end)

        if is_external do
          # For external events, use the stored module directly
          {^event_name, {:module, event_module}} =
            Enum.find(event_fields, fn
              {^event_name, {:module, _}} -> true
              _ -> false
            end)

          quote do
            def apply_event(state, %unquote(event_module){} = event) do
              unquote(func_ast).(state, event)
            end
          end
        else
          # For internal events, use the generated module path
          capitalized_name = event_name |> to_string() |> Macro.camelize()
          event_module = Module.concat([parent_module, Events, capitalized_name])

          quote do
            def apply_event(state, %unquote(event_module){} = event) do
              unquote(func_ast).(state, event)
            end
          end
        end
      end

    # Generate handle_command functions
    handle_command_clauses =
      for {command_name, handler} <- command_handlers do
        capitalized_name = command_name |> to_string() |> Macro.camelize()
        command_module = Module.concat([parent_module, Commands, capitalized_name])

        quote do
          def handle_command(state, %unquote(command_module){} = command) do
            unquote(handler).(state, command)
          end
        end
      end


    # Generate handle_event functions
    handle_event_clauses =
      for {event_name, handler} <- event_handlers do
        # Check if it's an external event
        is_external = Enum.any?(event_fields, fn
          {^event_name, {:external, true}} -> true
          _ -> false
        end)

        if is_external do
          # For external events, use the stored module directly
          {^event_name, {:module, event_module}} =
            Enum.find(event_fields, fn
              {^event_name, {:module, _}} -> true
              _ -> false
            end)

          quote do
            def handle_event(state, %unquote(event_module){} = event) do
              unquote(handler).(state, event)
            end
          end
        else
          # For internal events, use the generated module path
          capitalized_name = event_name |> to_string() |> Macro.camelize()
          event_module = Module.concat([parent_module, Events, capitalized_name])

          quote do
            def handle_event(state, %unquote(event_module){} = event) do
              unquote(handler).(state, event)
            end
          end
        end
      end

      default_handle_event = quote do
        def handle_event(_state, _event) do
          {:ok, []}
        end
      end

    quote do
      defstruct unquote(Macro.escape(state_fields))

      def new do
        struct(__MODULE__, unquote(Macro.escape(state_fields_map)))
      end

      def new(params) when is_map(params) do
        struct(__MODULE__, Map.merge(unquote(Macro.escape(state_fields_map)), params))
      end

      # Create the Events module structure
      defmodule Events do
        @moduledoc false
      end

      # Create the Commands module structure
      defmodule Commands do
        @moduledoc false
      end

      # Generate all the modules and functions
      unquote_splicing(event_structs)
      unquote_splicing(command_structs)
      unquote_splicing(apply_event_clauses)
      unquote_splicing(handle_command_clauses)
      unquote_splicing(handle_event_clauses)
      unquote(default_handle_event)
    end
  end
end