feat: decide when a conditional questionnaire question is shown

src/Model/Questionnaire/Condition.php

@@ -19,4 +19,26 @@ readonly class Condition
         public string|array|int $value,
     ) {
     }
+
+    /**
+     * Whether the controlling question's answer satisfies this condition. An unanswered question
+     * satisfies none: a follow-up stays hidden until there is something to follow up on.
+     *
+     * This says nothing about whether the controlling question is itself shown; that is
+     * Definition::isVisible().
+     *
+     * @param array<string, mixed> $answers keyed by question key
+     */
+    public function isMet(array $answers): bool
+    {
+        $answer = $answers[$this->question] ?? null;
+
+        return match ($this->operator) {
+            ConditionOperator::EQUALS => $answer === $this->value,
+            ConditionOperator::IN => in_array($answer, $this->value, true),
+            // A rating arrives as an int from the form and from stored JSON alike; anything else
+            // is not a rating.
+            ConditionOperator::LTE => is_int($answer) && $answer <= $this->value,
+        };
+    }
 }

src/Model/Questionnaire/Definition.php

@@ -40,4 +40,27 @@ readonly class Definition
     {
         return $this->getQuestions()[$key] ?? null;
     }
+
+    /**
+     * Whether a question is shown for these answers: it has no show_if, or its condition is met
+     * and the question it depends on is shown in turn. A hidden question's answer counts for
+     * nothing, even if one was given before it was hidden - otherwise a follow-up could stay open
+     * on an answer the respondent can no longer see.
+     *
+     * Terminates because a show_if may only name an earlier question (QuestionnaireCatalog).
+     *
+     * @param array<string, mixed> $answers keyed by question key
+     */
+    public function isVisible(Question $question, array $answers): bool
+    {
+        if (null === $question->showIf) {
+            return true;
+        }
+
+        $controlling = $this->getQuestion($question->showIf->question);
+
+        return null !== $controlling
+            && $this->isVisible($controlling, $answers)
+            && $question->showIf->isMet($answers);
+    }
 }