class Patient
    def id=(id)
        @id = id
    end
    def name=(name)
        @name = name
    end
    def surname=(surname)
        @surname = surname
    end
    def exams
        @exams ||= Hash.new
    end
end

class Exam
    def initialize(image_id)
        @image_id = image_id
        @signed = false
    end

    def sign
        #rebuild image to add signature
        @signed = true
    end
end

class Event
    def initialize(arguments={})
        @arguments = arguments
    end

    def arguments
        @arguments
    end
end

class PatientCreated < Event
    def apply_to(subject); subject.id = arguments[:id]; end;
end
class PatientNameChanged < Event
    def apply_to(subject); subject.name = arguments[:name]; end;
end
class PatientSurnameChanged < Event
    def apply_to(subject); subject.surname = arguments[:surname]; end;
end
class PatientExamCreated < Event
    def apply_to(subject); subject.exams[arguments[:exam_id]] = Exam.new(arguments[:image_id]); end;
end
class PatientExamSigned < Event
    def apply_to(subject); subject.exams[arguments[:exam_id]].sign; end;
end

class PatientStateBuilder
    def initialize(events)
        @events = events
    end

    def build
        Patient patient = Patient.new
        @events.each { |event| patient = event.apply_to(patient) }
        patient
    end
end