state machine with requests per guild

lib/metrics.ex

@@ -0,0 +1,96 @@
+defmodule PlantIdDiscordBot.Metrics do
+  use Agent
+  alias PlantIdDiscordBot.Metrics
+
+  @file_path "priv/metrics"
+
+  defstruct [
+    :guild_id,
+    :guild_name,
+    :total_requests
+  ]
+
+  def start_link(_opts) do
+    initial_state =
+      case File.exists?(Path.join(@file_path, "requests")) do
+        true -> read()
+        false -> %{}
+      end
+
+    Agent.start_link(fn -> initial_state end, name: via_tuple(:metrics))
+  end
+
+  defp via_tuple(key) do
+    PlantIdDiscordBot.ProcessRegistry.via_tuple({__MODULE__, key})
+  end
+
+  defp get_pid do
+    [{pid, _value}] = Registry.lookup(PlantIdDiscordBot.ProcessRegistry, {__MODULE__, :metrics})
+    pid
+  end
+
+  def get(key) do
+    get_pid()
+    |> Agent.get(&Map.get(&1, key))
+  end
+
+  def get_all() do
+    get_pid()
+    |> Agent.get(& &1)
+  end
+
+  def put(guild_id, guild_name) do
+    get_pid()
+    |> Agent.update(fn state ->
+      case Map.get(state, guild_id) do
+        nil ->
+          Map.put(state, guild_id, %Metrics{
+            guild_id: guild_id,
+            guild_name: guild_name,
+            total_requests: 1
+          })
+
+        existing ->
+          Map.put(state, guild_id, %Metrics{
+            existing
+            | total_requests: existing.total_requests + 1
+          })
+      end
+    end)
+  end
+
+  @doc """
+  Reset metrics state and remove the saved file from disk.
+  """
+  def reset() do
+    get_pid()
+    |> Agent.update(fn _ -> %{} end)
+
+    file = Path.join(@file_path, "requests")
+
+    if File.exists?(file) do
+      File.rm!(Path.join(@file_path, "requests"))
+    end
+  end
+
+  @doc """
+  Write the current state to disk. To be consumed by a CRON job.
+  """
+  def write() do
+    File.mkdir_p!("priv/metrics")
+
+    data =
+      get_all()
+      |> :erlang.term_to_binary()
+
+    File.write!(Path.join(@file_path, "requests"), data)
+  end
+
+  @doc """
+  Read the state from disk. To be loaded into state on process start.
+  """
+  def read() do
+    File.read!(Path.join(@file_path, "requests"))
+    |> :erlang.binary_to_term()
+  end
+end