Resolve "Vérification de l’existence dans la piscine/sandbox/mempool"

Closes #183 (closed)

lib/doc/mempool.ex

+defmodule Doc.Mempool do
+  @moduledoc """
+  Module for the mempool, i.e. documents that are yet to be added to a block
+
+  Used this [pattern](https://elixirforum.com/t/ets-or-genserver-for-caching-messages/42065) :
+  - GenServer owning an ETS table
+  - GenServer handling write access
+  - direct ETS calls for read access and concurency
+  """
+  use GenServer
+
+  def init(_init_arg) do
+    :ets.new(:mempool, [:set, :protected, :named_table])
+    {:ok, 0}
+  end
+
+  def start_link(_initial_value) do
+    GenServer.start_link(__MODULE__, nil, name: __MODULE__)
+  end
+
+  @doc """
+  Check whether a document is already in the mempool or not
+  """
+  def can_add?(document) do
+    case :ets.match(:mempool, {:"$1", document}) do
+      [] -> true
+      _ -> false
+    end
+  end
+
+  @doc """
+  Add a document in the mempool
+  """
+  def add(document) do
+    GenServer.call(__MODULE__, {:add, document})
+  end
+
+  def handle_call({:add, document}, _from, state) do
+    :ets.insert(:mempool, {state + 1, document})
+    {:reply, :ok, state + 1}
+  end
+end