defmodule Unleash.Propagation.Serialization do
  @moduledoc """
  Module handling the (de)serialization of Unleash objects for propagation.
  See `Unleash.Propagation`.
  """
  defmodule ContextDeserializationError do
    @moduledoc false
    defexception [:message, :details]
    @type t :: %__MODULE__{message: String.t(), details: term()}
  end

  defmodule OverridesDeserializationError do
    @moduledoc false
    defexception [:message, :details]
    @type t :: %__MODULE__{message: String.t(), details: term()}
  end

  @propagated_context_schema %{
    "userId" => :string,
    "sessionId" => :string,
    "properties" => {:custom, &__MODULE__.validate_context_properties/1}
  }
  @spec deserialize_context(binary()) :: {:ok, Unleash.context()} | {:error, ContextDeserializationError.t()}
  def deserialize_context(serialized_context) do
    with {:ok, payload} <- Jason.decode(serialized_context),
         {:ok, validated} <- Peri.validate(@propagated_context_schema, payload) do
      context =
        Map.reject(
          %{
            user_id: validated["userId"],
            session_id: validated["sessionId"],
            properties: validated["properties"]
          },
          fn {_key, val} -> is_nil(val) end
        )

      {:ok, context}
    else
      {:error, %Jason.DecodeError{} = jason_error} ->
        {:error, %ContextDeserializationError{message: "invalid JSON", details: jason_error}}

      {:error, [%Peri.Error{} | _]} = peri_errors ->
        {:error, %ContextDeserializationError{message: "invalid context object", details: peri_errors}}
    end
  end

  @spec deserialize_context!(binary()) :: Unleash.context()
  def deserialize_context!(serialized_context) do
    case deserialize_context(serialized_context) do
      {:ok, context} -> context
      {:error, exception} -> raise exception
    end
  end

  @doc """
  Serializes context for further propagation to downstream services.

  Note that only propagation-friendly context fields are serialized.
  """
  @spec serialize_context!(Unleash.context()) :: String.t()
  def serialize_context!(context) do
    %{
      userId: context[:user_id],
      sessionId: context[:session_id],
      properties: context[:properties]
    }
    |> Map.update(:properties, nil, &stringify_properties/1)
    |> Map.reject(fn {_key, val} -> is_nil(val) end)
    |> Jason.encode!()
  end

  @propagated_overrides_schema %{
    "name" => {:required, :string},
    "isEnabled" => {:required, {:either, {:boolean, :string}}}
  }
  @spec deserialize_overrides(binary()) :: {:ok, Unleash.context()} | {:error, OverridesDeserializationError.t()}
  def deserialize_overrides(serialized_overrides) do
    with {:ok, payload} <- Jason.decode(serialized_overrides),
         {_tag, payload} when is_list(payload) <- {:check_list, payload},
         {:ok, overrides} <- parse_overrides(payload) do
      {:ok, overrides}
    else
      {:error, %Jason.DecodeError{} = jason_error} ->
        {:error, %OverridesDeserializationError{message: "invalid JSON", details: jason_error}}

      {:check_list, _not_a_list} ->
        {:error, %OverridesDeserializationError{message: "not a list of overrides objects"}}

      {:error, [%Peri.Error{} | _] = peri_errors} ->
        {:error, %OverridesDeserializationError{message: "invalid override object", details: peri_errors}}
    end
  end

  @spec deserialize_overrides!(binary()) :: Unleash.overrides()
  def deserialize_overrides!(serialized_overrides) do
    case deserialize_overrides(serialized_overrides) do
      {:ok, overrides} -> overrides
      {:error, exception} -> raise exception
    end
  end

  @doc """
  Serializes overrides for further propagation to downstream services.
  """
  @spec serialize_overrides!(Unleash.overrides()) :: String.t()
  def serialize_overrides!(overrides) do
    overrides
    |> Enum.map(&objectify_override_entry/1)
    |> Jason.encode!()
  end

  defp stringify_properties(nil), do: nil

  defp stringify_properties(%{} = props) do
    Map.new(
      props,
      fn
        {key, val} when is_atom(key) -> {Unleash.stringify_context_key(key), val}
        {key, val} when is_binary(key) -> {key, val}
      end
    )
  end

  defp objectify_override_entry({feature_name, state}) do
    state =
      case state do
        :off -> false
        :on -> true
        variant when is_binary(variant) -> variant
      end

    %{"name" => feature_name, "isEnabled" => state}
  end

  @spec parse_overrides(list(term())) :: {:ok, Unleash.overrides()} | {:error, Peri.Error.t()}
  defp parse_overrides(overrides_payload) do
    overrides_payload
    |> Enum.reduce_while(
      %{},
      fn override, acc ->
        case Peri.validate(@propagated_overrides_schema, override) do
          {:ok, %{"name" => name, "isEnabled" => enabled}} ->
            state =
              case enabled do
                true -> :on
                false -> :off
                variant when is_binary(variant) -> variant
              end

            {:cont, Map.put(acc, name, state)}

          {:error, error} ->
            {:halt, {:error, error}}
        end
      end
    )
    |> case do
      {:error, error} -> {:error, error}
      overrides -> {:ok, overrides}
    end
  end

  def validate_context_properties(properties) when is_map(properties) do
    properties
    |> Enum.all?(fn {key, val} -> is_binary(key) and is_binary(val) end)
    |> case do
      true -> :ok
      false -> {:error, "properties should be a flat object with string values", []}
    end
  end

  def validate_context_properties(_properties), do: {:error, "invalid properties", []}
end