basic admin endgame view and add (#3)

Reviewed-on: #3

lib/chesstrainer/cache/persist.ex

@@ -0,0 +1,29 @@
+defmodule Chesstrainer.Cache.Persist do
+  @moduledoc """
+  Persistance for cache. Backup and restore functions.
+  """
+
+  @backup_path "priv/static/backups"
+
+  def backup(table) do
+    File.mkdir_p!(@backup_path)
+    filename = Path.join(@backup_path, "#{table}.det") |> String.to_charlist()
+    {:ok, dets_table} = :dets.open_file(filename, type: :set)
+    :ets.to_dets(table, dets_table)
+    :dets.sync(dets_table)
+  end
+
+  def restore(table) do
+    File.mkdir_p!(@backup_path)
+    filename = Path.join(@backup_path, "#{table}.det") |> String.to_charlist()
+
+    case :dets.open_file(table, file: filename, type: :set) do
+      {:ok, ^table} ->
+        :dets.foldl(fn entry, _ -> :ets.insert(table, entry) end, :ok, table)
+        :dets.close(table)
+
+      {:error, _} ->
+        :ok
+    end
+  end
+end