NULL: The Billion Dollar Mistake

final class City {
    public function interactionBetween($somePerson, $anotherPerson) {
        if ($this->meetingProbability() < random()) {
            return null; //no interaction
        } else {
            return new PersonToPersonInteraction($somePerson, $anotherPerson);
        }
    }
}

final class PersonToPersonInteraction {
    public function propagate($aVirus) {
        if ($this->somePerson->isInfectedWith($aVirus) && $aVirus->infectionProbability() > random()) {
            $this->anotherPerson->getInfectedWith($aVirus);
        }
    }
}

$covid19 = new Virus();
$janeDoe = new Person();
$johnSmith = new Person();
$wuhan = new City();

$interaction = $wuhan->interactionBetween($johnSmith, $janeDoe);
if ($interaction != null) {
    $interaction->propagate($covid19);
}

/* In this example we modeled the interaction between an infected person and a healthy one.
Jane is healthy but might be infected if Virus R0 applies to her.*/

Interface SocialInteraction{
    public function propagate($aVirus);
}

final class SocialDistancing implements SocialInteraction {
    public function propagate($aVirus) { //Do nothing !!!!
    }
}

final class PersonToPersonInteraction implements SocialInteraction {
    public function propagate($aVirus) {
        if ($this->somePerson->isInfectedWith($aVirus) && $aVirus->infectionProbability() > random()) {
            $this->anotherPerson->getInfectedWith($aVirus);
        }
    }
}

final class City {

    public function interactionBetween($aPerson, $anotherPerson) {
        return new SocialDistancing(); // The cities are smart enough to implement social distancing to model Person to Person interactions
    }
}

$covid19 = new Virus();
$janeDoe = new Person();
$johnSmith = new Person();
$wuhan = new City();

$interaction = $wuhan->interactionBetween($johnSmith, $janeDoe);
$interaction->propagate($covid19);

/* Jane will not be affected since the interaction prevents from propagating the virus

Interface Visitable {
    public function accept($aVisitor);
}

final class Date implements Visitable {
    public function accept($aVisitor) {
        $aVisitor->visitDate($this);
    }
}

final class DateNotPresent implements Visitable {
    public function accept($aVisitor) {
        $aVisitor->visitDateNotPresent($this);
    }
}

final class AverageCalculator {
    private $count = 0;
    private $ageSum = 0;

    public function visitDate($aDate) {
        $this->count++;
        $this->ageSum += today() - $aDate;
    }

    public function visitDateNotPresent($aDate) {
    }

    public function average() {
        if ($this->count == 0)
            return 0;
        else
            return $this->ageSum / $this->count;
    }
}

function averagePatientsAge($patients) {
    $calculator = new AverageCalculator();
    foreach ($patients as $patient)
        $patient->birthDate()->accept($calculator);
    return $calculator->average();
}