feat: add server cache

lib/todo/cache.ex

@@ -0,0 +1,31 @@
+defmodule Todo.Cache do
+  use GenServer
+
+  @impl GenServer
+  def init(_init_args) do
+    {:ok, %{}}
+  end
+
+  @impl GenServer
+  def handle_call({:server_process, name}, _from, state) do
+    case Map.fetch(state, name) do
+      {:ok, process} ->
+        {:reply, process, state}
+
+      :error ->
+        new_process = Todo.Server.start()
+
+        new_state = Map.put(state, name, new_process)
+
+        {:reply, new_process, new_state}
+    end
+  end
+
+  def start() do
+    GenServer.start(__MODULE__, nil)
+  end
+
+  def server_process(pid, name) do
+    GenServer.call(pid, {:server_process, name})
+  end
+end

test/todo/cache_test.exs

@@ -0,0 +1,36 @@
+defmodule Todo.TestCache do
+  use ExUnit.Case
+  alias Todo.Cache
+  alias Todo.Server
+
+  test "different names get different servers" do
+    {:ok, pid} = Cache.start()
+
+    {:ok, alice_server} = Cache.server_process(pid, "Alice")
+
+    {:ok, bob_server} = Cache.server_process(pid, "Bob")
+
+    assert alice_server != bob_server
+  end
+
+  test "same name gets same server" do
+    {:ok, pid} = Cache.start()
+
+    {:ok, alice_server} = Cache.server_process(pid, "Alice")
+
+    {:ok, alice_server2} = Cache.server_process(pid, "Alice")
+
+    assert alice_server == alice_server2
+  end
+
+  test "simple usage" do
+    {:ok, pid} = Cache.start()
+
+    {:ok, alice_server} = Cache.server_process(pid, "Alice")
+
+    Server.add(alice_server, %{date: ~D[2024-08-04], title: "Learn Elixir"})
+
+    assert [%{date: ~D[2024-08-04], title: "Learn Elixir"}] =
+             Server.entries(alice_server, ~D[2024-08-04])
+  end
+end