defmodule Nebulex.Object.Version do
  @moduledoc """
  Version handler for cached objects.

  The purpose of this module is to allow users implement custom
  version generator and validator. This interface is used by the
  adapters to generate and/or validate object versions.

  ## Example

      defmodule MyApp.Versioner do
        @behaviour Nebulex.Object.Version

        def generate(nil), do: 0

        def generate(%Object{version: version}) do
          version + 1
        end
      end

  If you are going to rely on this feature, it is recommended to use a good
  version generation algorithm (e.g.:  **Vector Clocks**).
  """

  @doc """
  Generates a new version for the current cached object.
  """
  @callback generate(cached_object :: Nebulex.Object.t()) :: any

  alias Nebulex.Object

  @doc """
  Validates version conflicts for the given cached `object_or_key`.

  This function is used by cache's adapters which implement
  optimistic locking via object's version.

  For more information, check out adapters implementation.
  """
  @spec validate(
          object_or_key :: object | Nebulex.Cache.key(),
          cache :: Nebulex.Cache.t(),
          opts :: Nebulex.Cache.opts()
        ) :: {:override | :nothing, object | nil}
        when object: Nebulex.Object.t()
  def validate(nil, _cache, _opts), do: {:override, nil}

  def validate(object_or_key, cache, opts) do
    opts
    |> Keyword.get(:version)
    |> validate(object_or_key, cache, opts)
  end

  ## Helpers

  defp validate(nil, %Object{} = object, _cache, _opts) do
    {:override, object}
  end

  defp validate(nil, key, _cache, _opts) do
    {:override, %Object{key: key}}
  end

  defp validate(vsn, object_or_key, cache, opts) do
    cache
    |> maybe_get(object_or_key)
    |> on_conflict(vsn, Keyword.get(opts, :on_conflict, :raise))
  end

  defp maybe_get(_cache, %Object{} = object),
    do: object

  defp maybe_get(cache, key),
    do: cache.__adapter__.get(cache, key, [])

  defp on_conflict(%Object{version: version} = cached, version, _on_conflict),
    do: {:override, cached}

  defp on_conflict(cached, _version, on_conflict) when on_conflict in [:override, :nothing],
    do: {on_conflict, cached}

  defp on_conflict(cached, version, :raise),
    do: raise(Nebulex.VersionConflictError, cached: cached, version: version)

  defp on_conflict(_, _, other),
    do: raise(ArgumentError, "unknown value for :on_conflict, got: #{inspect(other)}")
end