Refactor answer checking logic in VocabService to support multiple exercise types

- Updated the _checkAnswer method to handle both multiple choice and gap fill exercises more effectively.
- Enhanced the extraction of correct answers and alternatives based on exercise type, improving accuracy in answer validation.
- Added JSON parsing for answer and question data to ensure compatibility with various input formats.
- Improved fallback mechanisms for answer checking to accommodate different data structures.

backend/services/vocabService.js

@@ -1285,7 +1285,7 @@ export default class VocabService {
     }
 
     // Überprüfe Antwort (vereinfachte Logik - kann je nach Übungstyp erweitert werden)
-    const isCorrect = this._checkAnswer(exercise.answerData, userAnswer, exercise.exerciseTypeId);
+    const isCorrect = this._checkAnswer(exercise.answerData, exercise.questionData, userAnswer, exercise.exerciseTypeId);
 
     // Speichere Fortschritt
     const [progress, created] = await VocabGrammarExerciseProgress.findOrCreate({
@@ -1321,11 +1321,36 @@ export default class VocabService {
     const answerData = typeof exercise.answerData === 'string' 
       ? JSON.parse(exercise.answerData) 
       : exercise.answerData;
+    const questionData = typeof exercise.questionData === 'string'
+      ? JSON.parse(exercise.questionData)
+      : exercise.questionData;
     
-    const correctAnswer = Array.isArray(answerData.correct) 
-      ? answerData.correct[0] 
-      : answerData.correct;
-    const alternatives = answerData.alternatives || [];
+    let correctAnswer = null;
+    let alternatives = [];
+
+    // Für Multiple Choice: Extrahiere die richtige Antwort aus dem Index
+    if (exercise.exerciseTypeId === 2 && answerData.correctAnswer !== undefined) {
+      const options = questionData?.options || [];
+      const correctIndex = answerData.correctAnswer;
+      if (options[correctIndex] !== undefined) {
+        correctAnswer = options[correctIndex];
+      }
+      // Alternativen sind alle anderen Optionen
+      alternatives = options.filter((opt, idx) => idx !== correctIndex);
+    }
+    // Für Gap Fill: Extrahiere aus answers Array
+    else if (exercise.exerciseTypeId === 1 && answerData.answers) {
+      correctAnswer = Array.isArray(answerData.answers) 
+        ? answerData.answers.join(', ') 
+        : answerData.answers;
+    }
+    // Fallback: Versuche correct oder correctAnswer
+    else {
+      correctAnswer = Array.isArray(answerData.correct) 
+        ? answerData.correct[0] 
+        : (answerData.correct || answerData.correctAnswer);
+      alternatives = answerData.alternatives || [];
+    }
 
     return {
       correct: isCorrect,
@@ -1336,28 +1361,50 @@ export default class VocabService {
     };
   }
 
-  _checkAnswer(answerData, userAnswer, exerciseTypeId) {
+  _checkAnswer(answerData, questionData, userAnswer, exerciseTypeId) {
     // Vereinfachte Antwortprüfung - kann je nach Übungstyp erweitert werden
-    if (!answerData || !userAnswer) return false;
+    if (!answerData || userAnswer === undefined || userAnswer === null) return false;
 
-    // Für Multiple Choice: Prüfe ob userAnswer eine der richtigen Antworten ist
+    // Parse JSON strings
+    const parsedAnswerData = typeof answerData === 'string' ? JSON.parse(answerData) : answerData;
+    const parsedQuestionData = typeof questionData === 'string' ? JSON.parse(questionData) : questionData;
+
+    // Für Multiple Choice: Prüfe ob userAnswer (Index) mit correctAnswer (Index) übereinstimmt
     if (exerciseTypeId === 2) { // multiple_choice
-      const correctAnswers = Array.isArray(answerData.correct) ? answerData.correct : [answerData.correct];
-      return correctAnswers.includes(userAnswer);
+      const correctIndex = parsedAnswerData.correctAnswer !== undefined 
+        ? parsedAnswerData.correctAnswer 
+        : (parsedAnswerData.correct !== undefined ? parsedAnswerData.correct : null);
+      if (correctIndex === null) return false;
+      // userAnswer ist der Index (0, 1, 2, ...)
+      return Number(userAnswer) === Number(correctIndex);
     }
 
     // Für Lückentext: Normalisiere und vergleiche
     if (exerciseTypeId === 1) { // gap_fill
       const normalize = (str) => String(str || '').trim().toLowerCase();
-      const correctAnswers = Array.isArray(answerData.correct) ? answerData.correct : [answerData.correct];
-      const normalizedUserAnswer = normalize(userAnswer);
-      return correctAnswers.some(correct => normalize(correct) === normalizedUserAnswer);
+      const correctAnswers = parsedAnswerData.answers || parsedAnswerData.correct || [];
+      const correctAnswersArray = Array.isArray(correctAnswers) ? correctAnswers : [correctAnswers];
+      
+      // userAnswer ist ein Array von Antworten
+      if (Array.isArray(userAnswer)) {
+        if (userAnswer.length !== correctAnswersArray.length) return false;
+        return userAnswer.every((ans, idx) => {
+          const correct = correctAnswersArray[idx];
+          return normalize(ans) === normalize(correct);
+        });
+      } else {
+        // Fallback: Einzelne Antwort
+        const normalizedUserAnswer = normalize(userAnswer);
+        return correctAnswersArray.some(correct => normalize(correct) === normalizedUserAnswer);
+      }
     }
 
     // Für andere Typen: einfacher String-Vergleich (kann später erweitert werden)
     const normalize = (str) => String(str || '').trim().toLowerCase();
-    const correctAnswers = Array.isArray(answerData.correct) ? answerData.correct : [answerData.correct];
-    return correctAnswers.some(correct => normalize(correct) === normalize(userAnswer));
+    const correctAnswers = parsedAnswerData.correct || parsedAnswerData.correctAnswer || [];
+    const correctAnswersArray = Array.isArray(correctAnswers) ? correctAnswers : [correctAnswers];
+    const normalizedUserAnswer = normalize(userAnswer);
+    return correctAnswersArray.some(correct => normalize(correct) === normalizedUserAnswer);
   }
 
   async getGrammarExerciseProgress(hashedUserId, lessonId) {