defmodule Subaru do
  @moduledoc """
  The main facade and entry point for the Subaru graph database.
  """

  alias Subaru.Query

  @doc """
  Returns the configured store module.
  """
  def store do
    Application.get_env(:subaru, :store, Subaru.Store.Mnesia)
  end

  @doc """
  Runs a query plan and returns the results as a list.
  """
  def run(%Query{} = query) do
    Subaru.Executor.run(query, store())
  end

  @doc """
  Generate a sortable identifier.
  """
  defdelegate gen_id, to: Subaru.Id, as: :generate

  @doc """
  Adds an `:insert` operation for a vertex struct to a batch.
  """
  def insert(batch, %{id: id} = struct) when is_list(batch) and is_binary(id) do
    [{:insert, struct} | batch]
  end

  @doc """
  Adds a `:link` operation to a batch.
  """
  def link(batch, type, from_id, to_id, props \\ %{}) when is_list(batch) do
    [{:link, type, from_id, to_id, props} | batch]
  end

  @doc """
  Commits a batch of write operations to the store.
  """
  def commit(batch) when is_list(batch) do
    low_level_ops =
      batch
      |> Enum.reverse()
      |> Enum.map(&to_low_level_op/1)

    store().write_batch(low_level_ops)
  end

  # --- Passthrough functions to store ---

  @doc """
  Delegates to `c:Subaru.Store.Behaviour.backup/1`.
  """
  def backup(path), do: store().backup(path)

  @doc """
  Delegates to `c:Subaru.Store.Behaviour.restore/1`.
  """
  def restore(path), do: store().restore(path)

  # --- Private Helpers ---

  defp to_low_level_op({:insert, struct}) do
    {label, properties} = struct_to_vertex_parts(struct)
    value = %{label: label, properties: properties}
    {:put, {:node, struct.id}, value}
  end

  defp to_low_level_op({:link, type, from_id, to_id, props}) do
    id = gen_id()
    value = %{from: from_id, to: to_id, label: type, properties: props}
    {:put, {:edge, id}, value}
  end

  defp struct_to_vertex_parts(%{__struct__: schema} = struct) do
    label =
      schema
      |> Module.split()
      |> List.last()
      |> Macro.underscore()
      |> String.to_atom()

    properties =
      struct
      |> Map.from_struct()
      |> Map.drop([:id, :__struct__])

    {label, properties}
  end
end