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
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
 
p mapping
p mapping[1].new
 

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