defmodule Para do
  @moduledoc """
  Structured and declarative way to parse and validate parameters.

  Para uses Ecto under the hood and therefore inherits most of
  its utilities such as changeset and built-in validators.

  ## Usage

  Let's imagine that you have a controller named `Web.UserController` and
  wanted to validate the parameters for its `:create` and `:update` actions.

  First, let's define your parameters schema.

      defmodule Web.UserParams do
        use Para

        validator :create do
          required :name, :string
          required :age, :integer
          required :email, :string
          optional :phone, :string
        end

        validator :update do
          required :name, :string
          required :age, :integer
          required :email, :string
          optional :phone, :string
        end
      end

  This will generate two `validate/2` functions for your module
  with action `name` and `params` as arguments.

      defmodule Web.UserController do
        use Web, :controller
        alias Web.UserParams, as: Params

        def create(conn, params) do
          with {:ok, data} <- Params.validate(:create, params) do
            # ...
          end
        end

        def update(conn, params) do
          with {:ok, data} <- Params.validate(:update, params) do
            # ...
          end
        end
      end

  The `validate/2` function will return either an `{:ok, map}` or `{:error, changeset}`
  tuple.

  ## Inline validators

  Inline validator is a convenient way to validate your fields. This is
  especially useful when you need to perform some basic validation
  using `Ecto.Changeset`'s built-in validators.

      defmodule UserParams do
        use Para

        validator :update do
          required :name, :string, validator: {:validate_length, [min: 3, max: 100]}
        end
      end

  You can also use custom inline validators by supplying the function name
  as an atom. Similar to most of Ecto's built-in validators, the function will
  receive `changeset`, `key`, and `opts` as the arguments.

      defmodule UserParams do
        use Para

        validator :update do
          required :age, :string, validator: :validate_age
          required :gender, :string, validator: {:validate_gender, [allow: :non_binary]}
        end

        def validate_age(changeset, key, opts) do
          # ...
        end
      end

  ## Callback validator

  Sometimes, you might want to use custom validators or need to perform
  additional data manipulations. For this, you can use the `callback/1` macro.

  The `callback/1` macro will always be the last function to be called
  after the parametes have been parsed and validated.

      defmodule Web.UserParams do
        use Para

        validator :create do
          required :name, :string
          required :age, :integer
          required :email, :string
          optional :phone, :string
          callback :create_validators
        end

        def create_validators(changeset, params) do
          changeset
          |> format_email(params)
          |> format_phone(params)
          |> validate_age()
        end

        def format_email(changeset, params) do
          # ...
        end

        def format_phone(changeset, params) do
          # ...
        end

        def validate_age(changeset) do
          # ...
        end
      end

  """

  @type t :: {:ok, map()} | {:error, Ecto.Changeset.t()}
  @type data :: %{atom => term}
  @type spec :: %{
          data: map,
          types: map,
          embeds: map,
          permitted: list,
          required: list,
          validators: map
        }

  @doc """
  Parse and validate parameters for a given action.

  The function will cast all the returned map keys into atoms except
  for embedded map or list.

  ## Examples

      defmodule OrderParams do
        use Para

        validator :create do
          required :title
          required :data, {:array, :map}
        end
      end

      # Validate action with parameters
      OrderParams.validate(:create, %{
        "title" => "test"
        "data" => [%{"color" => "black", "material" => "cotton"}]
      })
      #=> {:ok, %{
        title: "test"
        data: [%{"color" => "black", "material" => "cotton"}]
      }}
  """
  @callback validate(atom, map) :: {:ok, data} | {:error, Ecto.Changeset.t()}

  @doc """
  Returns a basic spec.

  This is useful when you need to build a custom changeset,
  or when you just need the basic structure of your schema.

  Also see: `Ecto.Changeset.change/2`

  ## Examples

      defmodule OrderParams do
        use Para

        validator :create do
          required :title
          required :data, {:array, :map}
        end
      end

      def changeset(:new, params) do
        spec = spec(:create, params)
        Ecto.Changeset.change(spec.data, spec.types)
      end
  """
  @callback spec(atom, map) :: spec()

  @doc false
  defmacro __using__(_) do
    quote do
      @behaviour Para

      import Para,
        only: [
          validator: 2,
          required: 1,
          required: 2,
          required: 3,
          optional: 1,
          optional: 2,
          optional: 3,
          callback: 1,
          embeds_one: 2,
          embeds_many: 2
        ]
    end
  end

  @doc """
  Define a validator schema with an action name and field definitions.

  This will generate a new function called `validate/2` with the action `name`
  and `params` as the arguments.

      iex> defmodule UserParams do
      ...>   use Para
      ...>
      ...>   validator :create do
      ...>     required :name
      ...>   end
      ...> end
      ...>
      ...> UserParams.validate(:create, %{"name" => "Syamil MJ"})
      {:ok, %{name: "Syamil MJ"}}

  """
  defmacro validator(name, do: block) do
    fields =
      case block do
        {:__block__, _, fields} -> fields
        block -> [block]
      end

    quote do
      def validate(unquote(name), params) do
        Para.validate(__MODULE__, unquote(fields), params)
      end

      def spec(unquote(name), params) do
        Para.build_spec(unquote(fields), params)
      end
    end
  end

  @doc """
  Define a custom callback function that will be called to perform any
  additional manipulation to the changeset or parameters.

  The callback function must accept two arguments namely `changeset` and
  `params` and return an `Ecto.Changeset` struct.

  ## Examples

      # Define callback function to be called
      validator :create do
        callback :validate_price
      end

      def validate_price(changeset, params) do
        #...
      end
  """
  defmacro callback(name) do
    quote do
      {:callback, unquote(name)}
    end
  end

  @doc """
  Define a required field.

  ## Options

    * `:default` - Assign a default value if the not set by input parameters

    * `:validator` - Define either one of the built-in Ecto.Changeset's validators
      or use your own custom inline validator. Refer: [Custom inline validator](#required/3-custom-inline-validator)

    * `:droppable` - Drop the field when the key doesn't exist in parameters. This
      is useful when you need to perform partial update by leaving out certain fields.

  ## Custom inline validator

  You can define your own validator as such:

      def validate_country(changeset, field) do
        # ...
      end

  Then use it as an inline validator for your field

      validator :create do
        required :country, :string, [validator: :validate_country]
      end

  You can also supply options with your custom inline validator

      validator :create do
        required :country, :string, [validator: {:validate_country, region: :asia}]
      end

  """
  defmacro required(name, type \\ :string, opts \\ []) do
    quote do
      {:required, unquote(name), unquote(type), unquote(opts)}
    end
  end

  @doc """
  Define an optional field.

  Please refer to `required/3` for the list of available options.
  """
  defmacro optional(name, type \\ :string, opts \\ []) do
    quote do
      {:optional, unquote(name), unquote(type), unquote(opts)}
    end
  end

  @doc """
  Define an embedded map field

  It accepts similar schema definition like `validator/2`.

  ## Examples

      defmodule ParentParams do
        use Para

        validator :create do
          embeds_one :child do
            optional :name, :string
            optional :age,  :integer
          end
        end
      end
  """
  defmacro embeds_one(name, do: block) do
    fields =
      case block do
        {:__block__, _, fields} -> fields
        block -> [block]
      end

    quote do
      {:embed_one, unquote(name), unquote(fields)}
    end
  end

  @doc """
  Define an embedded array of maps field.

  It accepts similar schema definition like `validator/2`.

  ## Examples

      defmodule OrderParams do
        use Para

        validator :create do
          embeds_many :items do
            required :title
            required :price, :float
          end
        end
      end
  """
  defmacro embeds_many(name, do: block) do
    fields =
      case block do
        {:__block__, _, fields} -> fields
        block -> [block]
      end

    quote do
      {:embed_many, unquote(name), unquote(fields)}
    end
  end

  @doc false
  def validate(module, fields, params) do
    case changeset = do_validate(module, fields, params) do
      %{valid?: true} -> {:ok, apply_changes(changeset)}
      _ -> {:error, changeset}
    end
  end

  @doc false
  def do_validate(module, fields, params) do
    spec = build_spec(fields, params)

    callback =
      Enum.find_value(fields, fn
        {:callback, name} -> name
        _ -> nil
      end)

    {spec.data, spec.types}
    |> Ecto.Changeset.cast(params, spec.permitted)
    |> Ecto.Changeset.validate_required(spec.required)
    |> validate_embeds(module, spec)
    |> apply_inline_validators(module, spec.validators)
    |> apply_callback(module, callback, params)
  end

  @doc false
  def build_spec(fields, params) do
    default = %{data: %{}, types: %{}, embeds: %{}, permitted: [], required: [], validators: %{}}

    fields
    |> discard_droppable_fields(params)
    |> Enum.reduce(default, fn
      {:embed_one, name, block}, acc ->
        acc
        |> put_in([:data, name], nil)
        |> put_in([:embeds, name], {:embed_one, block})
        |> put_in([:types, name], {:map, :string})

      {:embed_many, name, block}, acc ->
        acc
        |> put_in([:data, name], nil)
        |> put_in([:embeds, name], {:embed_many, block})
        |> put_in([:types, name], {:map, :string})

      {requirement, name, type, opts}, acc ->
        acc
        |> put_in([:data, name], opts[:default])
        |> put_in([:types, name], type)
        |> assign_permitted_fields(name)
        |> assign_required_fields(requirement, name)
        |> assign_inline_validators(name, opts)

      _, acc ->
        acc
    end)
  end

  @doc false
  def discard_droppable_fields(fields, params) do
    Enum.filter(fields, fn
      # optional/required fields
      {_, name, _, opts} ->
        with true <- opts[:droppable],
             false <- Map.has_key?(params, Atom.to_string(name)) do
          false
        else
          _ -> true
        end

      # embed fields
      {_, name, opts} ->
        with true <- opts[:droppable],
             false <- Map.has_key?(params, Atom.to_string(name)) do
          false
        else
          _ -> true
        end

      any ->
        any
    end)
  end

  @doc false
  def assign_permitted_fields(spec, name) do
    put_in(spec, [:permitted], spec.permitted ++ [name])
  end

  @doc false
  def assign_required_fields(spec, :required, name) do
    put_in(spec, [:required], spec.required ++ [name])
  end

  def assign_required_fields(spec, _, _), do: spec

  @doc false
  def assign_inline_validators(spec, name, opts) do
    if validator = opts[:validator] do
      put_in(spec, [:validators, name], validator)
    else
      spec
    end
  end

  @doc false
  def validate_embeds(changeset, module, %{embeds: embeds}) do
    Enum.reduce(embeds, changeset, fn {name, embed}, acc ->
      validate_embed(acc, module, name, embed, changeset.params)
    end)
  end

  def validate_embeds(changeset, _, _), do: changeset

  @doc false
  def validate_embed(changeset, module, name, {:embed_one, block}, params) do
    params = Map.get(params, Atom.to_string(name))

    case do_validate(module, block, params) do
      %{valid?: true} = valid_changeset ->
        Ecto.Changeset.put_change(changeset, name, valid_changeset)

      invalid_changeset ->
        Ecto.Changeset.put_change(%{changeset | valid?: false}, name, invalid_changeset)
    end
  end

  def validate_embed(changeset, module, name, {:embed_many, block}, params) do
    params = Map.get(params, Atom.to_string(name))

    if is_list(params) do
      Enum.reduce(params, changeset, fn embedded_params, acc ->
        embedded_changesets = Ecto.Changeset.get_change(acc, name, [])

        case do_validate(module, block, embedded_params) do
          %{valid?: true} = valid_changeset ->
            Ecto.Changeset.put_change(
              acc,
              name,
              embedded_changesets ++ [valid_changeset]
            )

          invalid_changeset ->
            Ecto.Changeset.put_change(
              %{acc | valid?: false},
              name,
              embedded_changesets ++ [invalid_changeset]
            )
        end
      end)
    else
      changeset
    end
  end

  @doc false
  def apply_inline_validators(changeset, module, validators) do
    Enum.reduce(validators, changeset, fn {key, validator}, acc ->
      apply_inline_validator(acc, module, key, validator)
    end)
  end

  @doc false
  def apply_inline_validator(changeset, module, key, validator) do
    case validator do
      {function, data_or_opts} ->
        do_apply_inline_validator(module, function, [changeset, key] ++ [data_or_opts])

      {function, data, opts} ->
        do_apply_inline_validator(module, function, [changeset, key] ++ [data, opts])

      function when is_atom(function) ->
        do_apply_inline_validator(module, function, [changeset, key, []])

      _ ->
        changeset
    end
  end

  @doc false
  def do_apply_inline_validator(module, function, params) do
    arity = length(params)

    if function_exported?(Ecto.Changeset, function, arity) do
      apply(Ecto.Changeset, function, params)
    else
      apply(module, function, params)
    end
  end

  @doc false
  def apply_callback(changeset, _, nil, _), do: changeset

  def apply_callback(changeset, module, callback, params) do
    apply(module, callback, [changeset, params])
  end

  @doc false
  def apply_changes(%{changes: changes, data: data}) do
    Enum.reduce(changes, data, fn
      {key, list}, acc when is_list(list) ->
        Map.put(acc, key, apply_changes(list))

      {key, %Ecto.Changeset{} = changeset}, acc ->
        Map.put(acc, key, apply_changes(changeset))

      {key, value}, acc ->
        Map.put(acc, key, value)
    end)
  end

  def apply_changes(list) when is_list(list) do
    Enum.map(list, &apply_changes/1)
  end

  def apply_changes(any), do: any
end