#+TITLE: Practicing Elixir by Building Concurrent, Distributed, Multiplayer Games in the Terminal
#+AUTHOR: Lizzy Hunt (Simponic)

* Reminder: linux.usu.edu
This meeting should be being streamed live at [[https://linux.usu.edu/streams]].

* Introduction
#+BEGIN_SRC elixir
  defmodule Hello do
    def hello() do
      "Hello, Linux Club!"
      |> IO.puts
    end
  end

  Hello.hello()
#+END_SRC

** CheSSH
CheSSH is a multiplayer distributed game of chess over SSH - let's take a quick look before diving into Elixir!

[[https://chessh.linux.usu.edu]]

* Elixir - Functional Programming & Meta-Programming
Elixir is a self-proclaimed "dynamic, functional language for building scalable and maintainable applications". Obviously, one of Elixir's
main selling points is that of its functional paradigm - it's the second in the list. 

We'll take a quick look at some features of Elixir, and find that functional programming brings a lot to the table.

** Basic Data Types
1. ~int~s, ~bool~s, ~string~s are all here
   + ~1~, ~true~, ~"Hello"~
2. Atoms: prefixed with ":" are named constants whose name is their value, similar to symbols in LISP
   + ~:x~, ~:three~
4. Maps: regular key-value store; keys can be literally anything, including other maps
   + ~%{%{a: 1}: 2, %{a: 2}: :an_atom}~
5. Lists: lists are singly-linked elements of "stuff"
   + ~[1,2,3]~, ~[]~, ~[1, [2, :three, %{}]]~
6. Tuples: tuples are fixed-size collections of "stuff"
   + ~{1,2,3}~, ~{1, {2, 3}}~

** Pattern Matching
The match operator "=" does not mean its convential meaning of assignment, but instead an assertion of equivalence.

This gives way to one of Elixir's unique features of pattern matching similar to that found in Rust's ~match~ or Scala's ~case~.



* Elixir - Concurrency
Elixir is built on top of (and completely interoperable with) Erlang - a language developed to build massively fault-tolerant systems in the 80's
for large telephone exchanges with hundreds of thousands of users.

You can imagine (if you look past the many problems with this statement), Elixir and Erlang to be analogous to Python and C, respectively - but
without the massive performance penalty.

** The BEAM
The BEAM powers Elixir's concurrency magic; by running a VM executing Erlang bytecode that holds one OS thread per core,
and a separate process scheduler (and queue) on each. 

Imagine an army of little goblins, and you give each a todo list. The goblins then go complete the tasks in the order best
suited for them, and have the added benefit that they can talk to each other.

** Concurrency - Demo!
Here we will open up two terminals: one running an Elixir REPL on my machine, and another to SSH into my android:

#+BEGIN_SRC python
  import subprocess
  import string
  import random
  cookie = ''.join(random.choices(string.ascii_uppercase +
                                  string.digits, k=32))
  host = "host"
  android = "a02364151-23.bluezone.usu.edu"
  
  h = subprocess.Popen(f"alacritty -e rlwrap --always-readline iex --name lizzy@{host} --cookie {cookie}".split())
  a = subprocess.Popen(f"alacritty -e ssh u0_a308@{android} -p 2222 rlwrap --always-readline iex --name android@{android} --cookie {cookie}".split())
#+END_SRC

#+BEGIN_SRC elixir
  defmodule SpeakServer do
    @sleep_between_msg 2000

    def loop(queue \\ []) do
      case queue do
        [head | tail] ->
          speak(head)

          :timer.sleep(@sleep_between_msg)
          loop(tail)
        [] ->
          receive do
            msg ->
              loop(queue ++ [msg])
          end
      end
    end

    defp speak(msg) do
      System.cmd("espeak", [msg])
    end
  end
#+END_SRC

#+BEGIN_SRC elixir
  defmodule KVServer do
    require Logger
    @max_len_msg 32

    def start(speak_server_pid, port) do
      {:ok, socket} =
        :gen_tcp.listen(port, [:binary, packet: :line, active: false, reuseaddr: true])

      loop_acceptor(socket, speak_server_pid)
    end

    defp loop_acceptor(socket, speak_server_pid) do
      {:ok, client} = :gen_tcp.accept(socket)
      Task.start_link(fn -> serve(client, speak_server_pid) end)

      loop_acceptor(socket, speak_server_pid)
    end

    defp serve(socket, speak_server_pid) do
      msg = socket
      |> read_line()
      |> String.trim()

      if valid_msg(msg) do
        send(speak_server_pid, msg)
      end

      serve(socket, speak_server_pid)
    end

    defp read_line(socket) do
      {:ok, data} = :gen_tcp.recv(socket, 0)
      data
    end

    defp valid_msg(msg), do: String.length(msg) < @max_len_msg && String.match?(msg, ~r/^[A-Za-z ]+$/)
  end

  android = :"android@a02364151-23.bluezone.usu.edu"

  Node.connect(android)
  speak_server_pid = Node.spawn(android, &SpeakServer.loop/0)

  KVServer.start(speak_server_pid, 42069)
#+END_SRC

This demo shows how we can:
+ Connect nodes running Elixir
+ Spawn processes on nodes and inter process communication
+ Basic Elixir constructs (pattern matching, atoms, function calls, referencing functions)

* CheSSH
With a very brief and quick exploration into concurrency with Elixir, we can now explore the architecture of CheSSH,
and the hardware cluster it runs on:

[[./pis.jpeg]]