defmodule Mob.ScreenState do
  @moduledoc """
  Persistent store for screen assigns.

  Backed by the app's configured Ecto Repo (via `config :mob, :repo, MyApp.Repo`).
  All functions are no-ops when no Repo is configured, so screens compiled with
  `vsn:` enabled don't crash in environments without a database.

  ## Setup

  Generated projects include the required table migration and config automatically.
  For existing projects, add to `config/config.exs`:

      config :mob, :repo, MyApp.Repo

  And run the migration (see `priv/repo/migrations/*_create_mob_screen_states.exs`
  generated by `mix mob.new`).

  ## Storage

  State is keyed by screen module name by default. Screens with per-user or
  parameterised state should implement `screen_key/1`:

      def screen_key(assigns), do: "\#{__MODULE__}:\#{assigns.user_id}"

  ## Serialisation

  Values are encoded with `:erlang.term_to_binary/1` after stripping
  non-serialisable terms (PIDs, references, ports, functions). The `:safe`
  flag is used on decode to prevent atom-table pollution from untrusted data.
  """

  @select_sql "SELECT vsn, data FROM mob_screen_states WHERE key = ?"
  @upsert_sql """
  INSERT OR REPLACE INTO mob_screen_states (key, vsn, data, updated_at)
  VALUES (?, ?, ?, ?)
  """
  @delete_sql "DELETE FROM mob_screen_states WHERE key = ?"

  @doc """
  Persist the current assigns of `socket` for `module`.

  Calls `module.dump_state/1`, strips non-serialisable values, and writes to
  the configured Repo. Returns `:ok` regardless — persistence failures are
  silent so a missing or misconfigured Repo never crashes a screen.
  """
  @spec dump(module(), Mob.Socket.t()) :: :ok
  def dump(module, socket) do
    with repo when not is_nil(repo) <- repo(),
         key <- screen_key(module, socket),
         vsn <- module.__mob_vsn__(),
         raw <- module.dump_state(socket.assigns),
         {:ok, data} <- safe_encode(raw) do
      ts = System.system_time(:second)
      apply(repo, :query!, [@upsert_sql, [key, vsn, data, ts]])
    end

    :ok
  end

  @doc """
  Load previously persisted assigns for `module`.

  Returns `{:ok, stored_vsn, raw_map}` when a record exists, `:not_found`
  otherwise (including when no Repo is configured or the data cannot be decoded).
  """
  @spec load(module(), Mob.Socket.t()) :: {:ok, non_neg_integer(), map()} | :not_found
  def load(module, socket) do
    with repo when not is_nil(repo) <- repo(),
         key <- screen_key(module, socket) do
      case apply(repo, :query!, [@select_sql, [key]]) do
        %{rows: [[stored_vsn, blob]]} when is_binary(blob) ->
          case safe_decode(blob) do
            {:ok, raw} -> {:ok, stored_vsn, raw}
            :error -> :not_found
          end

        _ ->
          :not_found
      end
    else
      _ -> :not_found
    end
  end

  @doc """
  Delete the persisted state for `module`. Used when resetting or logging out.
  """
  @spec delete(module(), Mob.Socket.t()) :: :ok
  def delete(module, socket) do
    with repo when not is_nil(repo) <- repo(),
         key <- screen_key(module, socket) do
      apply(repo, :query!, [@delete_sql, [key]])
    end

    :ok
  end

  # ── Private ────────────────────────────────────────────────────────────────

  defp repo, do: Application.get_env(:mob, :repo)

  defp screen_key(module, socket) do
    if function_exported?(module, :screen_key, 1),
      do: module.screen_key(socket.assigns),
      else: to_string(module)
  end

  defp safe_encode(term) do
    {:ok, :erlang.term_to_binary(strip(term))}
  rescue
    _ -> :error
  end

  defp safe_decode(blob) do
    {:ok, :erlang.binary_to_term(blob, [:safe])}
  rescue
    _ -> :error
  end

  defp strip(map) when is_map(map) and not is_struct(map) do
    map
    |> Enum.reject(fn {_k, v} -> ephemeral?(v) end)
    |> Map.new(fn {k, v} -> {k, strip(v)} end)
  end

  defp strip(list) when is_list(list) do
    list |> Enum.reject(&ephemeral?/1) |> Enum.map(&strip/1)
  end

  defp strip(v), do: v

  defp ephemeral?(v),
    do: is_pid(v) or is_reference(v) or is_port(v) or is_function(v)
end