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General

Remember to use the GenServer behaviour.
There is a built-in function that starts a linked GenServer process. The only thing that TakeANumberDeluxe.start_link/2 needs to do is call that function with the right arguments.
__MODULE__ is a special variable that holds the name of the current module.
Implement the GenServer callback used when starting the process.
The callback should return either {:ok, state} or {:stop, reason}.
Read the options from the init_arg keyword list.
There is a built-in function to get a value from a keyword list.
Use TakeANumberDeluxe.State.new/2 to get the initial state.
Use @impl above your callback implementation to mark which behaviour this callback comes from.

There is a built-in function that sends a message to a GenServer process and receives a reply. The only thing that TakeANumberDeluxe.report_state/1 needs to do is call that function with the right arguments.
The messages sent to a server can be anything, but atoms are best.
Implement the GenServer callback used when handling messages that need a reply.
The callback should return {:reply, reply, state}.
Pass the state as the reply.
Use @impl above your callback implementation to mark which behaviour this callback comes from.

There is a built-in function that sends a message to a GenServer process and receives a reply. The only thing that TakeANumberDeluxe.queue_new_number/1 needs to do is call that function with the right arguments.
The messages sent to a server can be anything, but atoms are best.
Implement the GenServer callback used when handling messages that need a reply.
The callback should return {:reply, reply, state}.
Get the reply and the new state by calling TakeANumberDeluxe.State.queue_new_number/1. Use a case expression to pattern match the return value.
The reply should be either {:ok, new_number} or {:error, error}.
Use @impl above your callback implementation to mark which behaviour this callback comes from.

There is a built-in function that sends a message to a GenServer process and receives a reply. The only thing that TakeANumberDeluxe.serve_next_queued_number/2 needs to do is call that function with the right arguments.
The messages sent to a server can be anything, but tuples are best if an argument needs to be sent in the message. Use a message like this: {:my_message_name, some_argument}.
Implement the GenServer callback used when handling messages that need a reply.
The callback should return {:reply, reply, state}.
Get the reply and the new state by calling TakeANumberDeluxe.State.serve_next_queued_number/2. Use a case expression to pattern match the return value.
The reply should be either {:ok, next_number} or {:error, error}.
Use @impl above your callback implementation to mark which behaviour this callback comes from.

There is a built-in function that sends a message to a GenServer process and does not wait for a reply. The only thing that TakeANumberDeluxe.reset_state/1 needs to do is call that function with the right arguments.
The messages sent to a server can be anything, but atoms are best.
Implement the GenServer callback used when handling messages that do not need a reply.
The callback should return {:noreply, state}.
Use TakeANumberDeluxe.State.new/2 to get the new state, just like in init/1.
Use @impl above your callback implementation to mark which behaviour this callback comes from.

Extend all init/1 and handle_* callbacks to return one extra element in their tuples. Its value should be state.auto_shutdown_timeout.
The return value {:stop, reason} of init/1 does not need a timeout.
Implement the GenServer callback used when handling messages that weren't sent in the usual GenServer way.
This callback needs to handle :timeout messages and exit the process, but also catch and ignore any other messages.
To exit a GenServer process, return {:stop, reason, state} from the callback.
The exit reason should be :normal.