location.rb

class Location
  attr_reader :x, :y

  def self.find_available_location
    begin
      location = Location.new(rand(1..10), rand(1..10))
    end until location.empty?

    location
  end


  def initialize(x, y)
    @x = x
    @y = y
  end


  def empty?
    User.where(x: x, y: y).empty?
  end


  def on_board?
    x.in?(1..10) && y.in?(1..10)
  end


  # Compute the same value for the same coordinates.
  def hash
    [ x, y ].hash
  end


  # Allow to use locations as hash keys.
  def eql?(other_object)
    hash == other_object.hash
  end


  # Allow locations to match same-coordinates locations in arrays.
  def ==(other_object)
    hash == other_object.hash
  end


  class NotOnBoard < StandardError; end
  class NotAround < StandardError; end
  class AlreadyOccupied < StandardError; end
end

user.rb

class User < ActiveRecord::Base
  has_secure_password

  validates_length_of :name, within: 2..15
  validates_length_of :password, minimum: 2, on: :create


  def self.all_grouped_by_location
    User.all.index_by(&:location)
  end


  def self.generate_persistence_token
    begin
      persistence_token = SecureRandom.hex(8)
    end until User.where(persistence_token: persistence_token).empty?

    persistence_token
  end


  def location
    return nil if x.blank? || y.blank?
    Location.new(x, y)
  end


  def location=(location)
    self.x = location.x
    self.y = location.y

    location
  end


  def around_locations
    [
      Location.new(x - 1, y - 1),
      Location.new(    x, y - 1),
      Location.new(x + 1, y - 1),

      Location.new(x - 1, y),
      Location.new(x + 1, y),

      Location.new(x - 1, y + 1),
      Location.new(    x, y + 1),
      Location.new(x + 1, y + 1)
    ]
  end


  def move(x, y)
    target_location = Location.new(x, y)

    raise Location::NotOnBoard unless target_location.on_board?
    raise Location::NotAround unless target_location.in?(around_locations)
    raise Location::AlreadyOccupied unless target_location.empty?

    update_attributes(x: x, y: y)
    target_location
  end
end