g_counter.ex

defmodule GCounter.Node do
  use GenServer

  def start_link(node_name) do
    GenServer.start_link(__MODULE__, node_name, [name: node_name])
  end

  @impl true
  def init(node_name) do
    init_counter = %{node_name => 0}
    {:ok, {node_name, init_counter}}
  end

  @impl true
  def handle_cast(:increment, {node_name, counter}) do
    counter = Map.update!(counter, node_name, &(&1 + 1))

    {:noreply, {node_name, counter}}
  end

  @impl true
  def handle_cast({:merge, remote_counter}, {local_node_name, local_counter}) do
    local_counter =
      Enum.reduce(remote_counter, local_counter, fn {node_name, count}, local_counter ->
        Map.update(local_counter, node_name, 0, &max(&1, count))
      end)

    {:noreply, {local_node_name, local_counter}}
  end

  @impl true
  def handle_call(:count, _from, {_, counter} = state) do
    count = Enum.reduce(counter, 0, fn {_, count}, total -> total + count end)

    {:reply, count, state}
  end

  # TODO: Make these configurable.
  @remote_nodes [:node1, :node2, :node3]

  @impl true
  def handle_info(:gossip, {node_name, counter}) do
    Enum.each(@remote_nodes, &GenServer.cast(&1, {:merge, counter}))

    {:noreply, {node_name, counter}}
  end
end