defmodule Ash.Resource.Validation do
  @moduledoc """
  Represents a validation in Ash.

  See `Ash.Resource.Validation.Builtins` for a list of builtin validations.

  To write your own validation, define a module that implements the `c:init/1` callback
  to validate options at compile time, and `c:validate/2` callback to do the validation.

  Then, in a resource, you can say:

  ```
  validations do
    validate {MyValidation, [foo: :bar]}
  end
  ```
  """
  defstruct [
    :validation,
    :module,
    :opts,
    :only_when_valid?,
    :description,
    :message,
    :before_action?,
    :where,
    on: []
  ]

  @type t :: %__MODULE__{
          validation: {atom(), list(atom())},
          module: atom(),
          opts: list(atom()),
          only_when_valid?: boolean(),
          description: String.t() | nil,
          where: list({atom(), list(atom())}),
          on: list(atom())
        }

  @type path :: [atom | integer]
  @type ref :: {module(), Keyword.t()} | module()

  @callback init(Keyword.t()) :: {:ok, Keyword.t()} | {:error, String.t()}
  @callback validate(Ash.Changeset.t(), Keyword.t()) :: :ok | {:error, term}
  @callback describe(Keyword.t()) :: String.t() | [{:message, String.t()} | {:vars, Keyword.t()}]
  @callback atomic?(Keyword.t()) :: boolean

  @optional_callbacks describe: 1

  @validation_type {:spark_function_behaviour, Ash.Resource.Validation,
                    Ash.Resource.Validation.Builtins, {Ash.Resource.Validation.Function, 1}}

  @schema [
    validation: [
      type: @validation_type,
      required: true,
      doc:
        "The module (or module and opts) that implements the `Ash.Resource.Validation` behaviour. Also accepts a one argument function that takes the changeset."
    ],
    where: [
      type: {:or, [@validation_type, {:list, @validation_type}]},
      required: false,
      default: [],
      doc: """
      Validations that should pass in order for this validation to apply. Any of these validations failing will result in this validation being ignored.
      """
    ],
    on: [
      type: {:custom, __MODULE__, :on, []},
      default: [:create, :update],
      doc: """
      The action types the validation should run on. Many validations don't make sense in the context of deletion, so by default it is not included.
      """
    ],
    only_when_valid?: [
      type: :boolean,
      default: false,
      doc:
        "If the validation should only run on valid changes. Useful for expensive validations or validations that depend on valid data."
    ],
    message: [
      type: :string,
      doc: "If provided, overrides any message set by the validation error"
    ],
    description: [
      type: :string,
      doc: "An optional description for the validation"
    ],
    before_action?: [
      type: :boolean,
      default: false,
      doc: "If set to `true`, the validation will be run in a before_action hook"
    ]
  ]

  @action_schema Keyword.delete(@schema, :on)

  defmacro __using__(_) do
    quote do
      @behaviour Ash.Resource.Validation

      def init(opts), do: {:ok, opts}
      def atomic?(_), do: false

      defp with_description(keyword, opts) do
        if Kernel.function_exported?(__MODULE__, :describe, 1) do
          keyword ++ apply(__MODULE__, :describe, [opts])
        else
          keyword
        end
      end

      defoverridable init: 1, atomic?: 1
    end
  end

  @doc false
  def transform(%{validation: {module, opts}, where: where} = validation) do
    {:ok,
     %{
       validation
       | validation: {module, opts},
         module: module,
         opts: opts,
         where: List.wrap(where)
     }}
  end

  def opt_schema, do: @schema
  def action_schema, do: @action_schema

  def on(list) do
    list
    |> List.wrap()
    |> Enum.all?(&(&1 in [:create, :update, :destroy]))
    |> case do
      true ->
        {:ok, List.wrap(list)}

      false ->
        {:error, "Expected items of [:create, :update, :destroy], got: #{inspect(list)}"}
    end
  end

  def validation_type, do: @validation_type
end