defmodule AshDispatch.Channel do
  @moduledoc """
  Represents a delivery channel for an event.

  A channel specifies:
  - **Transport** - How to deliver (email, in_app, discord, etc.)
  - **Audience** - Who receives (:user, :admin, custom)
  - **Time** - When to deliver (immediate, delayed, scheduled)
  - **Policy** - Delivery policy (:always, :skip_if_read)
  - **Variant** - Template variant (e.g., :admin for admin-specific templates)

  ## Examples

      %Channel{
        transport: :email,
        audience: :user,
        time: {:in, 300},  # Delay 5 minutes
        policy: :skip_if_read
      }

      %Channel{
        transport: :discord,
        audience: :admin,
        time: :immediate,
        opts: %{webhook_url: "https://..."}
      }
  """

  @type transport :: :email | :in_app | :discord | :sms | :slack | :webhook | atom()
  @type audience :: :user | :admin | atom()
  @type time ::
          :immediate
          | {:in, non_neg_integer()}
          | {:at, DateTime.t()}
          | {:window, map()}
  @type policy :: :always | :skip_if_read | {:gate, function()}
  @type variant :: atom() | nil

  @type t :: %__MODULE__{
          transport: transport(),
          audience: audience(),
          time: time(),
          policy: policy(),
          variant: variant(),
          webhook_url: String.t() | nil,
          content: map(),
          metadata: map(),
          opts: map(),
          load: [atom() | {atom(), any()}]
        }

  @enforce_keys [:transport, :audience]
  defstruct [
    :transport,
    :audience,
    :variant,
    :webhook_url,
    time: {:in, 0},
    policy: :always,
    content: %{},
    metadata: %{},
    opts: %{},
    load: []
  ]

  @doc """
  Creates a new channel.

  ## Examples

      iex> Channel.new(:email, :user)
      %Channel{transport: :email, audience: :user, time: {:in, 0}}

      iex> Channel.new(:email, :user, time: 5.minutes(), skip_if_read: true)
      %Channel{
        transport: :email,
        audience: :user,
        time: {:in, 300},
        policy: :skip_if_read
      }
  """
  def new(transport, audience, opts \\ []) do
    time = Keyword.get(opts, :time, {:in, 0})
    policy = if Keyword.get(opts, :skip_if_read), do: :skip_if_read, else: :always

    %__MODULE__{
      transport: transport,
      audience: audience,
      time: normalize_time(time),
      policy: Keyword.get(opts, :policy, policy),
      variant: Keyword.get(opts, :variant),
      webhook_url: Keyword.get(opts, :webhook_url),
      opts: Keyword.get(opts, :opts, %{})
    }
  end

  @doc """
  Helper function to create a channel struct.
  Used in DSL and event modules for cleaner syntax.

  ## Examples

      def channels(_ctx) do
        [
          channel(:in_app, :user),
          channel(:email, :user, time: 5.minutes(), skip_if_read: true)
        ]
      end
  """
  def channel(transport, audience, opts \\ []) do
    new(transport, audience, opts)
  end

  @doc """
  Normalizes time specification to internal format.

  ## Examples

      iex> Channel.normalize_time(:immediate)
      {:in, 0}

      iex> Channel.normalize_time(300)
      {:in, 300}

      iex> Channel.normalize_time({:in, 300})
      {:in, 300}
  """
  def normalize_time(:immediate), do: {:in, 0}
  def normalize_time(seconds) when is_integer(seconds), do: {:in, seconds}
  def normalize_time({:in, _} = time), do: time
  def normalize_time({:at, %DateTime{}} = time), do: time
  def normalize_time({:window, _} = time), do: time
  def normalize_time(time), do: time

  @doc """
  Calculates delay in seconds from time specification.

  ## Examples

      iex> Channel.calculate_delay(%Channel{time: {:in, 300}})
      300

      iex> Channel.calculate_delay(%Channel{time: :immediate})
      0
  """
  def calculate_delay(%__MODULE__{time: {:in, seconds}}), do: seconds
  def calculate_delay(%__MODULE__{time: :immediate}), do: 0

  def calculate_delay(%__MODULE__{time: {:at, datetime}}) do
    DateTime.diff(datetime, DateTime.utc_now(), :second)
  end

  def calculate_delay(%__MODULE__{time: {:window, _window}}) do
    # TODO: Implement business hours calculation
    # For now, send immediately if in window, or delay to window start
    0
  end

  def calculate_delay(_), do: 0

  @doc """
  Checks if a channel matches given transport and/or audience filters.

  ## Examples

      iex> channel = %Channel{transport: :in_app, audience: :user}
      iex> Channel.matches?(channel, transport: :in_app)
      true

      iex> Channel.matches?(channel, transport: :email)
      false

      iex> Channel.matches?(channel, transport: :in_app, audience: :user)
      true
  """
  def matches?(%__MODULE__{} = channel, filters) do
    Enum.all?(filters, fn
      {:transport, transport} -> channel.transport == transport
      {:audience, audience} -> channel.audience == audience
      {:variant, variant} -> channel.variant == variant
      _ -> true
    end)
  end
end