defmodule MjolnirJsonDatasource do
  @moduledoc """
  Provides data from json files.
  """
  use Mjolnir.Datasource

  alias Mjolnir.Entity

  @impl true
  def find(spec) when is_binary(spec) do
    spec |> parse_pathspec() |> find(spec)
  end

  def find(pathspec) when is_struct(pathspec, Entity.Pathspec) do
    find(pathspec, pathspec |> to_path())
  end

  def find(pathspec, spec) do
    case all(pathspec) do
      {:ok, entities} ->
        entity =
          Enum.filter(entities, fn entity ->
            entity.id == spec
          end)
          |> List.first()

        if is_nil(entity) do
          {:error, :notfound}
        else
          {:ok, entity}
        end

      {:error, error} ->
        {:error, error}
    end
  end

  @impl true
  def all(spec) when is_binary(spec) do
    spec |> parse_pathspec |> all()
  end

  def all(pathspec) when is_struct(pathspec, Entity.Pathspec) do
    marshall_from_pathspec(pathspec)
  end

  @impl true
  def create(entity) do
    pathspec = entity.id |> parse_pathspec()

    case find(pathspec) do
      {:ok, entity} ->
        {:error, "an entity with id of' '#{entity.id}' already exists"}

      {:error, :notfound} ->
        unmarshall_to_pathspec(entity, pathspec)
    end
  end

  @impl true
  def update(entity) do
    pathspec = entity.id |> parse_pathspec()

    case find(pathspec) do
      {:ok, _entity} ->
        unmarshall_and_replace_in_pathspec(entity, pathspec)

      {:error, :notfound} ->
        {:error, :notfound}
    end
  end

  @impl true
  def destroy(entity) do
    pathspec = entity.id |> parse_pathspec()

    case find(pathspec) do
      {:ok, _entity} ->
        unmarshall_and_remove_from_pathspec(pathspec)

      {:error, :notfound} ->
        {:error, :notfound}
    end
  end

  defp unmarshall_and_replace_in_pathspec(entity, pathspec) do
    case all(pathspec) do
      {:ok, entities} ->
        entities =
          Enum.filter(entities, fn entity ->
            entity.pathspec.id != pathspec.id
          end)

        unmarshall_and_encode(entities ++ [entity], pathspec)

      {:error, message} ->
        {:error, message}
    end
  end

  defp unmarshall_and_remove_from_pathspec(pathspec) do
    case all(pathspec) do
      {:ok, entities} ->
        entities =
          Enum.filter(entities, fn entity ->
            entity.pathspec.id != pathspec.id
          end)

        unmarshall_and_encode(entities, pathspec)

      {:error, message} ->
        {:error, message}
    end
  end

  defp unmarshall_to_pathspec(entity, pathspec) do
    case all(pathspec) do
      {:ok, entities} ->
        unmarshall_and_encode(entities ++ [entity], pathspec)

      {:error, error} ->
        {:error, error}
    end
  end

  defp unmarshall_and_encode(entities, pathspec) do
    entities =
      Enum.reduce(entities, [], fn entity, acc ->
        acc ++ [Entity.unmarshall(entity)]
      end)

    case Jason.encode(entities) do
      {:ok, encoded} ->
        File.write("#{pathspec.path}.json", encoded)

      {:error, message} ->
        {:error, message}
    end
  end

  defp marshall_from_pathspec(pathspec) do
    case File.read("#{pathspec.path}.json") do
      {:ok, content} ->
        decode_and_marshall(content, pathspec)

      {:error, _message} ->
        {:error, "The file at '#{pathspec.path}' could not be read"}
    end
  end

  defp decode_and_marshall(content, pathspec) do
    case Jason.decode(content) do
      {:ok, maps} ->
        {:ok,
         Enum.reduce(maps, [], fn map, acc ->
           acc ++ [Entity.marshall(map, pathspec)]
         end)}

      {:error, message} ->
        {:error, message}
    end
  end

  defp parse_pathspec(spec) do
    Entity.parse_pathspec(spec)
  end

  defp to_path(pathspec) do
    Entity.Pathspec.to_path(pathspec)
  end
end