6_46.rb

# ...
class Bicycle
  attr_reader :size, :chain, :tire_size

  def initialize(**opts)
    @size       = opts[:size]
    @chain      = opts[:chain]      || default_chain
    @tire_size  = opts[:tire_size]  || default_tire_size
    post_initialize(opts)
  end

  def spares
    {tire_size: tire_size,
     chain:     chain}.merge(local_spares)
  end

  def default_tire_size
    raise NotImplementedError
  end

  # subclasses may override
  def post_initialize(opts)
  end

  def local_spares
    {}
  end

  def default_chain
    "11-speed"
  end
end

class RoadBike < Bicycle
  attr_reader :tape_color

  def post_initialize(opts)
    @tape_color = opts[:tape_color]
  end

  def local_spares
    {tape_color: tape_color}
  end

  def default_tire_size
    "23"
  end
end

class MountainBike < Bicycle
  attr_reader :front_shock, :rear_shock

  def post_initialize(opts)
    @front_shock = opts[:front_shock]
    @rear_shock  = opts[:rear_shock]
  end

  def local_spares
    {front_shock: front_shock}
  end

  def default_tire_size
    "2.1"
  end
end

road_bike =
  RoadBike.new(
    size:       'M',
    tape_color: 'red' )

puts road_bike.tire_size      # => 23
puts road_bike.chain          # => 11-speed
puts road_bike.spares
# => {:tire_size=>"23", :chain=>"11-speed", :tape_color=>"red"}

mountain_bike =
  MountainBike.new(
    size:         'S',
    front_shock:  'Manitou',
    rear_shock:   'Fox')

puts mountain_bike.tire_size  # => 2.1
puts mountain_bike.chain      # => 11-speed
puts mountain_bike.spares
# => {:tire_size=>"2.1", :chain=>"11-speed", :front_shock=>"Manitou"}


class RecumbentBike < Bicycle
  attr_reader :flag

  def post_initialize(opts)
    @flag = opts[:flag]
  end

  def local_spares
    {flag: flag}
  end

  def default_chain
    '10-speed'
  end

  def default_tire_size
    '28'
  end
end

bent =
  RecumbentBike.new(
    size: "M",
    flag: 'tall and orange')

puts bent.spares
# => {:tire_size=>"28", :chain=>"10-speed", :flag=>"tall and orange"}