defmodule TrackIndices do
  @moduledoc """
  Provides index documentation for schema modules.

  ## Usage

  Add `use TrackIndices` to your schema module and define @indices:

      defmodule MyApp.MySchema do
        use Ecto.Schema
        use TrackIndices

        @indices [
          %{columns: [:user_id], unique: false},
          %{columns: [:email], unique: true}
        ]

        schema "my_table" do
          # ...
        end
      end

  This will generate a `__indices__/0` function that returns the documented indices.

  ## Index Format

  Each index is a map with the following keys:

    * `:columns` - List of column names (atoms)
    * `:unique` - Boolean indicating if the index is unique
    * `:where` - (Optional) Partial index condition as a string

  ## Example

      @indices [
        %{columns: [:email], unique: true},
        %{columns: [:event_id, :email], unique: true, where: "email IS NOT NULL"}
      ]
  """

  defmacro __using__(_opts) do
    quote do
      Module.register_attribute(__MODULE__, :track_indices_table, persist: true)
      @before_compile TrackIndices
    end
  end

  defmacro __before_compile__(env) do
    indices = Module.get_attribute(env.module, :indices, [])

    quote do
      @doc """
      Returns the documented indices for this schema.
      """
      def __indices__ do
        unquote(Macro.escape(indices))
      end

      @doc """
      Returns the table name for this schema.
      """
      def __table_name__ do
        __schema__(:source)
      end
    end
  end

  @doc """
  Returns all modules that use TrackIndices along with their table names.
  """
  def all_schemas do
    # Get all loaded applications and their modules
    :application.loaded_applications()
    |> Enum.flat_map(fn {app, _, _} ->
      case :application.get_key(app, :modules) do
        {:ok, modules} -> modules
        _ -> []
      end
    end)
    |> Enum.filter(fn module ->
      Code.ensure_loaded?(module) and function_exported?(module, :__table_name__, 0)
    end)
    |> Enum.map(fn module ->
      {module, module.__table_name__()}
    end)
    |> Enum.reject(fn {_module, table} -> is_nil(table) end)
  end

  @doc """
  Gets the Ecto repo from application config.
  """
  def get_repo do
    case Application.get_env(:track_indices, :repo) do
      nil -> raise_no_config_error()
      repo when is_atom(repo) -> repo
      other -> raise_invalid_repo_error(other)
    end
  end

  defp raise_no_config_error do
    raise """
    No repo configured for TrackIndices.

    Please add to your config/config.exs:

        config :track_indices, repo: YourApp.Repo
    """
  end

  defp raise_invalid_repo_error(other) do
    raise "Expected :repo to be a module, got: #{inspect(other)}"
  end

  @doc """
  Retrieves documented indices from all schema modules.
  Returns a map of table_name => [documented_index]
  """
  def get_documented_indices do
    all_schemas()
    |> Enum.reduce(%{}, fn {module, table_name}, acc ->
      indices = module.__indices__()
      Map.put(acc, table_name, indices)
    end)
  end
end