echo server

.formatter.exs

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+# Used by "mix format"
+[
+  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
+]

.gitignore

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+# The directory Mix will write compiled artifacts to.
+/_build/
+
+# If you run "mix test --cover", coverage assets end up here.
+/cover/
+
+# The directory Mix downloads your dependencies sources to.
+/deps/
+
+# Where third-party dependencies like ExDoc output generated docs.
+/doc/
+
+# Ignore .fetch files in case you like to edit your project deps locally.
+/.fetch
+
+# If the VM crashes, it generates a dump, let's ignore it too.
+erl_crash.dump
+
+# Also ignore archive artifacts (built via "mix archive.build").
+*.ez
+
+# Ignore package tarball (built via "mix hex.build").
+protohackers-*.tar
+
+# Temporary files, for example, from tests.
+/tmp/

README.md

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+# Protohackers
+
+Implementations of protocols from https://protohackers.com/
+

lib/protohackers.ex

@@ -0,0 +1,18 @@
+defmodule Protohackers do
+  @moduledoc """
+  Documentation for `Protohackers`.
+  """
+
+  @doc """
+  Hello world.
+
+  ## Examples
+
+      iex> Protohackers.hello()
+      :world
+
+  """
+  def hello do
+    :world
+  end
+end

lib/protohackers/application.ex

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+defmodule Protohackers.Application do
+  # See https://hexdocs.pm/elixir/Application.html
+  # for more information on OTP Applications
+  @moduledoc false
+
+  use Application
+
+  @impl true
+  def start(_type, _args) do
+    children = [
+      {Protohackers.EchoServer, port: 5001},
+      {Protohackers.PrimeTimeServer, port: 5002}
+    ]
+
+    # See https://hexdocs.pm/elixir/Supervisor.html
+    # for other strategies and supported options
+    opts = [strategy: :one_for_one, name: Protohackers.Supervisor]
+    Supervisor.start_link(children, opts)
+  end
+end

lib/protohackers/echo_server.ex

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+defmodule Protohackers.EchoServer do
+  use GenServer
+
+  require Logger
+
+  @spec start_link(keyword()) :: GenServer.on_start()
+  def start_link(opts) do
+    GenServer.start_link(__MODULE__, opts)
+  end
+
+  defstruct [:listen_socket, :supervisor]
+
+  @impl true
+  def init(opts) do
+    port = Keyword.fetch!(opts, :port)
+    {:ok, supervisor} = Task.Supervisor.start_link(max_children: 100)
+
+    listen_options = [
+      ifaddr: {0, 0, 0, 0},
+      mode: :binary,
+      active: false,
+      reuseaddr: true,
+      exit_on_close: false
+    ]
+
+    case :gen_tcp.listen(port, listen_options) do
+      {:ok, listen_socket} ->
+        Logger.info("Started echo server on port #{port}")
+        state = %__MODULE__{listen_socket: listen_socket, supervisor: supervisor}
+        {:ok, state, {:continue, :accept}}
+
+      {:error, reason} ->
+        {:stop, reason}
+    end
+  end
+
+  @impl true
+  def handle_continue(:accept, %__MODULE__{} = state) do
+    case :gen_tcp.accept(state.listen_socket) do
+      {:ok, socket} ->
+        Task.Supervisor.start_child(state.supervisor, fn -> handle_connection(socket) end)
+        {:noreply, state, {:continue, :accept}}
+
+      {:error, reason} ->
+        {:stop, reason}
+    end
+  end
+
+  ## Helpers
+
+  defp handle_connection(socket) do
+    case recv_until_closed(socket, _buffer = "", _buffered_size = 0) do
+      {:ok, data} -> :gen_tcp.send(socket, data)
+      {:error, reason} -> Logger.error("Failed to receive data: #{inspect(reason)}")
+    end
+
+    :gen_tcp.close(socket)
+  end
+
+  @limit _100_kb = 1024 * 100
+  defp recv_until_closed(socket, buffer, buffered_size) do
+    case :gen_tcp.recv(socket, 0, 10_000) do
+      {:ok, data} when buffered_size + byte_size(data) > @limit -> {:error, :buffer_overflow}
+      {:ok, data} -> recv_until_closed(socket, [buffer, data], buffered_size + byte_size(data))
+      {:error, :closed} -> {:ok, buffer}
+      {:error, reason} -> {:error, reason}
+    end
+  end
+end

lib/protohackers/prime_time_server.ex

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+defmodule Protohackers.PrimeTimeServer do
+  use GenServer
+
+  require Logger
+
+  @spec start_link(keyword()) :: GenServer.on_start()
+  def start_link(opts) do
+    GenServer.start_link(__MODULE__, opts)
+  end
+
+  defstruct [:listen_socket, :supervisor]
+
+  @impl true
+  def init(opts) do
+    port = Keyword.fetch!(opts, :port)
+    {:ok, supervisor} = Task.Supervisor.start_link(max_children: 100)
+
+    listen_options = [
+      ifaddr: {0, 0, 0, 0},
+      mode: :binary,
+      active: false,
+      reuseaddr: true,
+      exit_on_close: false
+    ]
+
+    case :gen_tcp.listen(port, listen_options) do
+      {:ok, listen_socket} ->
+        Logger.info("Started prime time server on port #{port}")
+        state = %__MODULE__{listen_socket: listen_socket, supervisor: supervisor}
+        {:ok, state, {:continue, :accept}}
+
+      {:error, reason} ->
+        {:stop, reason}
+    end
+  end
+
+  @impl true
+  def handle_continue(:accept, %__MODULE__{} = state) do
+    case :gen_tcp.accept(state.listen_socket) do
+      {:ok, socket} ->
+        Task.Supervisor.start_child(state.supervisor, fn -> handle_connection(socket) end)
+        {:noreply, state, {:continue, :accept}}
+
+      {:error, reason} ->
+        {:stop, reason}
+    end
+  end
+
+  ## Helpers
+
+  defp handle_connection(socket) do
+    case recv_until_closed(socket, _buffer = "", _buffered_size = 0) do
+      {:ok, data} -> :gen_tcp.send(socket, data)
+      {:error, reason} -> Logger.error("Failed to receive data: #{inspect(reason)}")
+    end
+
+    :gen_tcp.close(socket)
+  end
+
+  @limit _100_kb = 1024 * 100
+  defp recv_until_closed(socket, buffer, buffered_size) do
+    case :gen_tcp.recv(socket, 0, 10_000) do
+      {:ok, data} when buffered_size + byte_size(data) > @limit -> {:error, :buffer_overflow}
+      {:ok, data} -> recv_until_closed(socket, [buffer, data], buffered_size + byte_size(data))
+      {:error, :closed} -> {:ok, buffer}
+      {:error, reason} -> {:error, reason}
+    end
+  end
+end

mix.exs

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+defmodule Protohackers.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :protohackers,
+      version: "0.1.0",
+      elixir: "~> 1.16",
+      start_permanent: Mix.env() == :prod,
+      deps: deps()
+    ]
+  end
+
+  # Run "mix help compile.app" to learn about applications.
+  def application do
+    [
+      extra_applications: [:logger],
+      mod: {Protohackers.Application, []}
+    ]
+  end
+
+  # Run "mix help deps" to learn about dependencies.
+  defp deps do
+    [
+      # {:dep_from_hexpm, "~> 0.3.0"},
+      # {:dep_from_git, git: "https://github.com/elixir-lang/my_dep.git", tag: "0.1.0"}
+    ]
+  end
+end

test/protohackers_test.exs

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+defmodule ProtohackersTest do
+  use ExUnit.Case
+  doctest Protohackers
+
+  test "greets the world" do
+    assert Protohackers.hello() == :world
+  end
+end

test/test_helper.exs

@@ -0,0 +1 @@
+ExUnit.start()