module Protocol
  class Packet
    class << self
      attr_reader :packet_id
    end
  end
 
  class A < Packet
    @packet_id = 1
  end
 
  class B < Packet
    @packet_id = 2
  end
 
  class NotAPacket
  end
 
  Mapping = Protocol.constants.each_with_object({}) do |constant_name, result|
    klass = Protocol.const_get(constant_name)
    next unless klass.is_a?(Class)
    next unless klass < Protocol::Packet
    result[klass.packet_id] = klass
  end
end
 
p Protocol::Mapping
p Protocol::Mapping[1].new

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