defmodule Hugs.Gen do
  def parent_key, do: {__MODULE__, :_parent_key, []}
  def _parent_key(%{datapath: [_, parent | _]}), do: {:ok, parent}
  def _parent_key(_), do: {:error, "no parent key found in context"}

  def copy(other_norm_key), do: {__MODULE__, :_copy, [other_norm_key]}

  def _copy(%{parent_data: d}, key) when is_map_key(d, key) do
    {:ok, Map.fetch!(d, key)}
  end

  def _copy(%{parent_data: d}, key) when is_map(d) do
    reason = %KeyError{
      key: key,
      message: "cannot copy key #{key} from data, key not found",
      term: d
    }

    {:error, reason}
  end

  def _copy(%{parent_data: _}, _key) do
    {:error, "Hugs.Gen.copy/1 can only be used on field definitions"}
  end
end