Add basic code

.rubocop.yml

@@ -8,6 +8,9 @@ Layout/AccessModifierIndentation:
 Metrics/LineLength:
   Max: 120
 
+Style/Documentation:
+  Enabled: false
+
 Style/TrailingCommaInArguments:
   EnforcedStyleForMultiline: comma
 

exe/client

@@ -1,2 +1,9 @@
 #!/usr/bin/env ruby
 # frozen_string_literal: true
+
+lib = File.expand_path('../lib', __dir__).freeze
+$LOAD_PATH.unshift lib unless $LOAD_PATH.include? lib
+
+require 'main'
+
+Main.new

lib/main.rb

@@ -0,0 +1,63 @@
+# frozen_string_literal: true
+
+require 'thread'
+require 'curses'
+
+class Main
+  def self.inherited(_base)
+    raise "#{self} is final"
+  end
+
+  def self.mutex
+    (@mutex ||= Mutex.new).tap { freeze }
+  end
+
+  def initialize
+    raise "#{self.class} is singleton" unless self.class.mutex.try_lock
+    call
+  end
+
+private
+
+  def call
+    before_loop
+    loop do
+      before_iteration
+      sleep
+      after_iteration
+    end
+    after_loop
+  end
+
+  def sleep
+    super 0.01
+  end
+
+  def before_loop
+    Curses.init_screen
+    Curses.start_color
+    Curses.noecho # do no echo input
+    Curses.curs_set 0 # invisible cursor
+    Curses.timeout = 0 # non-blocking input
+    Curses.stdscr.keypad = true
+
+    Curses.init_pair 1, Curses::COLOR_WHITE, Curses::COLOR_BLACK
+    Curses.init_pair 2, Curses::COLOR_BLACK, Curses::COLOR_WHITE
+  end
+
+  def after_loop
+    Curses.close_screen
+  end
+
+  def before_iteration; end
+
+  def after_iteration
+    loop do
+      event = Curses.getch
+      break if event.nil?
+      handle event
+    end
+  end
+
+  def handle(event); end
+end