Extract tracker into supervision tree.

lib/tracker.ex

@@ -1,22 +1,27 @@
 defmodule Tracker do
-  def start(modules) do
-    total = modules
-            |> Enum.flat_map(&(&1.all_koans))
-            |> Enum.count
+  alias __MODULE__
 
-    Agent.start_link(fn -> %{total: total,
-                             koans: MapSet.new(),
-                             visited_modules: MapSet.new()} end, name: __MODULE__)
-    modules
+  defstruct total: 0, koans: MapSet.new(), visited_modules: MapSet.new()
+
+  def start_link do
+    Agent.start_link(fn -> %Tracker{} end, name: __MODULE__)
   end
 
-  defp get(),  do: Agent.get(__MODULE__, &(&1))
+  def set_total(modules) do
+    total = modules
+    |> Enum.flat_map(&(&1.all_koans))
+    |> Enum.count
+
+    Agent.update(__MODULE__, fn _ -> %Tracker{total: total} end)
+  end
 
   def completed(module, koan) do
-    Agent.update(__MODULE__, fn(%{koans: completed, visited_modules: modules} = all) ->
-      %{ all | koans: MapSet.put(completed, koan),
-               visited_modules: MapSet.put(modules, module)}
-    end)
+    Agent.update(__MODULE__, &mark_koan_completed(&1, module, koan))
+  end
+
+  defp mark_koan_completed(state, module, koan) do
+    %{state | koans: MapSet.put(state.koans, koan),
+       visited_modules: MapSet.put(state.visited_modules, module)}
   end
 
   def visited do
@@ -28,14 +33,13 @@ defmodule Tracker do
     total == completed
   end
 
-  def summarize, do: get() |> summarize()
-  defp summarize(%{total: total,
-                  koans: completed,
-                  visited_modules: modules}) do
+  def summarize do
+    state = Agent.get(__MODULE__, &(&1))
+
     %{
-      total: total,
-      current: MapSet.size(completed),
-      visited_modules: MapSet.to_list(modules)
+      total: state.total,
+      current: MapSet.size(state.koans),
+      visited_modules: MapSet.to_list(state.visited_modules)
     }
   end
 end