defmodule SimpleRestView do
  alias User

  @moduledoc """
  Module takes care of creating a map which can be converted into json from a schema obtained in database query.\n
  Meant to make phoenix views shorter and more concise in REST projects
  """

  @adding_keyword :add
  @include_timestamps_keyword :include_timestamps
  @only_keyword :only
  @except_keyword :except
  @many_keyword :many
  @timestamp_fields  [:inserted_at, :updated_at]


  @doc """
  Function renders all fields of a given schema into a map that can be converted into json.
  It supports rendering fields of nested objects if specified with add optional parameter.\n
  Optional parameters are:\n
  many - true/false\n
  only - [:field1, :field2]\n
  except - [:field1, :field2]\n
  include_timestamps - true/false\n
  add - [field_name: {SchemaReference, :field_name}] OR [field_name: {SchemaReference, :field_name, opt}] OR [field_name: (fn schema -> ... end)] OR [field_name: %{custom_field: custom_val, ...}]

  ## Examples

      iex> render_schema(User, %User{id: 1, username: "joe", email: "joe@mail.com", ...})
      %{id: 1, username: "joe", email: "joe@mail.com", ...}



      iex> render_schema(User, %User{id: 1, username: "joe", email: "joe@mail.con", reviewed: [%Review{...}, %Review{...}]},
                            except: [:email, :password], add: [avg_rating: (fn user -> get_avg_rating(user.id)),
                                                               reviewed: {Review, :reviewed, many: true, only: [:comment]}])
      %{id: 1, username: "joe", avg_rating: 10, reviewed: [%{comment: "..."}, %{comment: "..."}]}


      iex> render_schema(User, %User{id: 1, username: "joe", email: "joe@mail.com", reviewed: [%Review{..., reviewer: %User{username: "mary", ...}}, %Review{..., reviewer: %User{username: "johnson", ...}}]},
                              except: [:email, :password], add: [avg_rating: (fn user -> get_avg_rating(user.id)),
                                                                 reviewed: {Review, :reviewed, many: true, only: [:comment],
                                                                    add: [reviewed_by: {User, :reviewer, only: [:username]}]}])
        %{id: 1, username: "joe", avg_rating: 10,
          reviewed: [%{comment: "...", reviewed_by: %{username: "mary"}},
                    %{comment: "...", reviewed_by: %{username: "johnson"}}]}


  """
  def render_schema(schema_ref, schema_map, opt \\ []) do
    cond do
      opt[@many_keyword] == true -> render_many_schema(schema_ref, schema_map, opt)
      true -> render_single_schema(schema_ref, schema_map, opt)
    end
  end


  @doc """
  Wraps a map inside of map with a data tag

  ## Examples

      iex> render_wrapper([%{id: 1, username: "joe"}, %{id: 2, username: "mary"}])
      %{data: [%{id: 1, username: "joe"}, %{id: 2, username: "mary"}]}

  """
  def render_wrapper(data) do
    %{data: data}
  end

  @doc """
  Wraps the result with pagination data obtained when using :scrivener library

  ## Examples

      iex> render_paginated_wrapper([%{id: 1, username: "joe"}, %{id: 2, username: "mary"}], %Scrivener.Page{page_number: 1, page_size: 2, total_pages: 5}, except: [:total_entries])
      %{data: [%{id: 1, username: "joe"}, %{id: 2, username: "mary"}],
        page_number: 1, page_size: 2, total_pages: 5}

  """
  def render_paginated_wrapper(data, query_result, opt \\ []) do
    %{data: data,
      page_number: value_or_nil(query_result, :page_number),
      page_size: value_or_nil(query_result, :page_size),
      total_entries: value_or_nil(query_result, :total_entries),
      total_pages: value_or_nil(query_result, :total_pages)}
    |> handle_base_fields(opt)
  end



  defp render_many_schema(schema_ref, schema_list_of_maps, opt) do
    schema_list_of_maps
    |> Enum.map(fn schema_map ->
      render_single_schema(schema_ref, schema_map, convert_options(schema_map, opt))
    end)
  end

  defp render_single_schema(schema_ref, schema_map, opt) do
    converted_opts = convert_options(schema_map, opt)

    schema_ref.__schema__(:fields)
    |> Enum.map(fn key -> {key, value_or_nil(schema_map, key)} end)
    |> Enum.reduce(%{}, fn {key, val}, acc -> Map.put(acc, key, val) end)
    |> handle_options(converted_opts)
  end


#  Function takes care of converting data from current level and all nested levels
#  into maps that can be send from server.
#  This is where the magic happens!
  defp convert_options(schema_map, opt) do
    if opt[@adding_keyword] != nil do

      opt[@adding_keyword]
      |> Enum.reduce(opt, fn adding_options, accumulated_options ->
        case adding_options do

          # Rendering nested field
          {key, {schema_ref, field_atom, inner_options}} ->
            renewed_adding_options = handle_rendering_nested_schema(key, schema_ref, schema_map, field_atom, inner_options)
            concatenate_options(accumulated_options, renewed_adding_options)

          # Rendering nested field without options passed
          {key, {schema_ref, field_atom}} ->
            renewed_adding_options = handle_rendering_nested_schema(key, schema_ref, schema_map, field_atom)
            concatenate_options(accumulated_options, renewed_adding_options)

          # Adding field with a custom function
          {key, custom_func} when is_function(custom_func) ->
            concatenate_options(accumulated_options, [{@adding_keyword, [{key, custom_func.(schema_map)}]}])

          _ -> accumulated_options

        end
      end)

    else
      opt
    end
  end

  defp concatenate_options(full_options, options_to_concat) do
    current_add = full_options[@adding_keyword]
    if current_add != nil do
      to_add = Keyword.merge(current_add, options_to_concat[@adding_keyword])

      {_, new_keyword_list} = Keyword.pop(full_options, @adding_keyword)

      new_keyword_list
      |> Keyword.put(@adding_keyword, to_add)
    else
      full_options ++ options_to_concat
    end
  end

  defp handle_rendering_nested_schema(key, schema_ref, schema_map, field_name, options \\ []) do
    schema_to_render = value_or_nil(schema_map, field_name)

    inner_schema = render_schema(schema_ref, schema_to_render, options)

    [{@adding_keyword, [{key, inner_schema}]}]
  end

  defp value_or_nil(map, key) do
    with {:ok, val} <- Map.fetch(map, key) do
      val
    else _ ->
      nil
    end
  end


#  Function takes care of rendering a map based on options passed
#  Doesn't handle rendering on nested fields
  defp handle_options(map, opt) do
    map
    |> handle_base_fields(opt)
    |> handle_adding_fields(opt[@adding_keyword])
    |> handle_timestamps(opt[@include_timestamps_keyword])
  end

  defp handle_base_fields(map, opt) do
    cond do
      opt[@only_keyword] -> if is_atom(opt[@only_keyword]), do: map |> Map.take([opt[@only_keyword]]), else: map |> Map.take(opt[@only_keyword])
      opt[@except_keyword] -> if is_atom(opt[@except_keyword]), do: map |> Map.drop([opt[@except_keyword]]), else: map |> Map.drop(opt[@except_keyword])
      true -> map
    end
  end



#  Functions handle adding fields which get passed in opt parameter.
#  It doesn't convert values and doesn't handle adding for nested fields
  defp handle_adding_fields(map, {key, value}),
       do: map |> Map.put(key, value)

  defp handle_adding_fields(map, list_of_key_values) when is_list(list_of_key_values),
       do: list_of_key_values |> Enum.reduce(map, fn key_value_pair, acc -> handle_adding_fields(acc, key_value_pair) end)

  defp handle_adding_fields(map, _), do: map


  defp handle_timestamps(map, true), do: map

  defp handle_timestamps(map, false), do: map |> Map.drop(@timestamp_fields)

  defp handle_timestamps(map, _), do: handle_timestamps(map, false)

end