feat(vocab): enhance vocabService and VocabLessonView for improved exercise handling

- Updated VocabService to include validation for synthetic exercise IDs and added methods for generating multiple-choice exercises based on chapter lexemes.
- Implemented a seeded shuffle function to randomize distractor options in multiple-choice questions, ensuring varied user experiences.
- Modified VocabLessonView to adjust target calculations for lesson goals and unlock attempts, increasing the maximum limits for better flexibility in user progress tracking.

backend/services/vocabService.js

@@ -58,12 +58,14 @@ export default class VocabService {
     }
 
     const sanitized = {};
+    const synMcId = /^syn-\d+-\d+-l2r$/;
     Object.entries(value).slice(0, 200).forEach(([exerciseId, answer]) => {
-      if (!/^\d+$/.test(String(exerciseId))) {
+      const idStr = String(exerciseId);
+      if (!/^\d+$/.test(idStr) && !synMcId.test(idStr)) {
         return;
       }
       if (Array.isArray(answer)) {
-        sanitized[exerciseId] = this._sanitizeStringArray(answer, {
+        sanitized[idStr] = this._sanitizeStringArray(answer, {
           maxItems: 12,
           maxLength: 200,
           keepEmpty: true
@@ -71,11 +73,11 @@ export default class VocabService {
         return;
       }
       if (typeof answer === 'string') {
-        sanitized[exerciseId] = this._sanitizeShortString(answer, 200);
+        sanitized[idStr] = this._sanitizeShortString(answer, 200);
         return;
       }
       if (typeof answer === 'number' && Number.isFinite(answer)) {
-        sanitized[exerciseId] = Math.trunc(answer);
+        sanitized[idStr] = Math.trunc(answer);
       }
     });
 
@@ -87,13 +89,15 @@ export default class VocabService {
       return {};
     }
 
+    const synMcId = /^syn-\d+-\d+-l2r$/;
     const sanitized = {};
     Object.entries(value).slice(0, 200).forEach(([exerciseId, result]) => {
-      if (!/^\d+$/.test(String(exerciseId)) || !result || typeof result !== 'object' || Array.isArray(result)) {
+      const idStr = String(exerciseId);
+      if ((!/^\d+$/.test(idStr) && !synMcId.test(idStr)) || !result || typeof result !== 'object' || Array.isArray(result)) {
         return;
       }
 
-      sanitized[exerciseId] = {
+      sanitized[idStr] = {
         correct: Boolean(result.correct),
         correctAnswer: this._sanitizeShortString(result.correctAnswer, 400),
         alternatives: this._sanitizeStringArray(result.alternatives, { maxItems: 8, maxLength: 200 }),
@@ -1961,6 +1965,246 @@ export default class VocabService {
     return { success: true };
   }
 
+  _seededShuffle(items, seed) {
+    const arr = items.slice();
+    let t = (Number(seed) >>> 0) ^ 0x6a09e667;
+    const rnd = () => {
+      t ^= t << 13;
+      t ^= t >>> 17;
+      t ^= t << 5;
+      return (t >>> 0) / 4294967296;
+    };
+    for (let i = arr.length - 1; i > 0; i -= 1) {
+      const j = Math.floor(rnd() * (i + 1));
+      [arr[i], arr[j]] = [arr[j], arr[i]];
+    }
+    return arr;
+  }
+
+  _buildDeterministicChapterLexemeMcOptions(correct, distractorSource, seed) {
+    const norm = (s) => this._normalizeTextAnswer(s);
+    const nc = norm(correct);
+    const filtered = distractorSource.filter((t) => norm(t) !== nc && String(t || '').trim());
+    const shuffled = this._seededShuffle(filtered, (seed ^ 0x9e3779b9) >>> 0);
+    const picks = [];
+    for (const p of shuffled) {
+      if (picks.length >= 3) {
+        break;
+      }
+      picks.push(p);
+    }
+    let pad = 1;
+    while (picks.length < 3) {
+      picks.push(`(${pad})`);
+      pad += 1;
+    }
+    const ordered = this._seededShuffle([correct, ...picks], seed >>> 0);
+    const correctAnswer = ordered.findIndex((o) => String(o) === String(correct));
+    return { options: ordered, correctAnswer: correctAnswer >= 0 ? correctAnswer : 0 };
+  }
+
+  _lexemePairCoveredByMultipleChoice(exerciseList, learning, reference) {
+    const nl = this._normalizeTextAnswer(learning);
+    const nr = this._normalizeTextAnswer(reference);
+    if (!nl || !nr) {
+      return false;
+    }
+    for (const ex of exerciseList) {
+      if (Number(ex.exerciseTypeId) !== 2) {
+        continue;
+      }
+      const qd = typeof ex.questionData === 'string' ? JSON.parse(ex.questionData) : ex.questionData;
+      if (!qd || qd.type !== 'multiple_choice') {
+        continue;
+      }
+      const prompt = this._normalizeTextAnswer(qd.question || qd.text || '');
+      if (!prompt.includes(nl)) {
+        continue;
+      }
+      const ad = typeof ex.answerData === 'string' ? JSON.parse(ex.answerData) : ex.answerData;
+      const options = Array.isArray(qd.options) ? qd.options : [];
+      let idx = ad?.correctAnswer;
+      if (Array.isArray(idx)) {
+        idx = idx[0];
+      }
+      if (idx === undefined && ad?.correct !== undefined) {
+        idx = Array.isArray(ad.correct) ? ad.correct[0] : ad.correct;
+      }
+      if (idx === undefined || options[Number(idx)] === undefined) {
+        continue;
+      }
+      const correctOpt = this._normalizeTextAnswer(options[Number(idx)]);
+      if (correctOpt === nr) {
+        return true;
+      }
+    }
+    return false;
+  }
+
+  _buildSyntheticLexemeMcExercisePlain(lessonId, row, exerciseNumber) {
+    const learning = String(row.learning || '').trim();
+    const reference = String(row.reference || '').trim();
+    if (!learning || !reference) {
+      return null;
+    }
+    const seed = (Number(row.id) * 100003 + Number(lessonId)) >>> 0;
+    const { options, correctAnswer } = this._buildDeterministicChapterLexemeMcOptions(
+      reference,
+      row.allReferences || [],
+      seed
+    );
+    const questionData = {
+      type: 'multiple_choice',
+      question: `Was bedeutet „${learning}“?`,
+      options,
+      randomizeDistractors: false
+    };
+    const answerData = {
+      type: 'multiple_choice',
+      correctAnswer
+    };
+    return {
+      id: `syn-${lessonId}-${row.id}-l2r`,
+      lessonId,
+      exerciseTypeId: 2,
+      exerciseType: { id: 2, name: 'multiple_choice' },
+      exerciseNumber,
+      title: `Kapitel-Vokabel: ${learning}`,
+      instruction: 'Wähle die passende Übersetzung.',
+      questionData,
+      answerData,
+      explanation: null,
+      createdByUserId: 0
+    };
+  }
+
+  async _fetchChapterLexemeRowsForMc(chapterId) {
+    const id = Number.parseInt(chapterId, 10);
+    if (!Number.isFinite(id)) {
+      return [];
+    }
+    const rows = await sequelize.query(
+      `
+      SELECT
+        cl.id,
+        l1.text AS learning,
+        l2.text AS reference
+      FROM community.vocab_chapter_lexeme cl
+      JOIN community.vocab_lexeme l1 ON l1.id = cl.learning_lexeme_id
+      JOIN community.vocab_lexeme l2 ON l2.id = cl.reference_lexeme_id
+      WHERE cl.chapter_id = :chapterId
+      ORDER BY cl.id ASC
+      `,
+      {
+        replacements: { chapterId: id },
+        type: sequelize.QueryTypes.SELECT
+      }
+    );
+    return rows;
+  }
+
+  async _mergeSyntheticChapterLexemeMcExercises(plainLesson, grammarExercises) {
+    const list = Array.isArray(grammarExercises) ? [...grammarExercises] : [];
+    if (!plainLesson?.chapterId) {
+      return list;
+    }
+    if (plainLesson.lessonType === 'review' || plainLesson.lessonType === 'vocab_review') {
+      return list;
+    }
+    const rows = await this._fetchChapterLexemeRowsForMc(plainLesson.chapterId);
+    if (!rows.length) {
+      return list;
+    }
+    const allReferences = rows.map((r) => r.reference).filter(Boolean);
+    let maxNum = list.reduce((m, ex) => Math.max(m, Number(ex.exerciseNumber) || 0), 0);
+    const augmentedRows = rows.map((r) => ({ ...r, allReferences }));
+    for (const row of augmentedRows) {
+      if (this._lexemePairCoveredByMultipleChoice(list, row.learning, row.reference)) {
+        continue;
+      }
+      maxNum += 1;
+      const ex = this._buildSyntheticLexemeMcExercisePlain(plainLesson.id, row, maxNum);
+      if (ex) {
+        list.push(ex);
+      }
+    }
+    return list;
+  }
+
+  async _checkSyntheticLexemeMcAnswer(user, lessonId, chapterLexemeId, userAnswer) {
+    const lesson = await VocabCourseLesson.findByPk(lessonId, {
+      include: [{ model: VocabCourse, as: 'course' }]
+    });
+    if (!lesson) {
+      const err = new Error('Exercise not found');
+      err.status = 404;
+      throw err;
+    }
+    if (lesson.course.ownerUserId !== user.id && !lesson.course.isPublic) {
+      const err = new Error('Access denied');
+      err.status = 403;
+      throw err;
+    }
+    const enrollment = await VocabCourseEnrollment.findOne({
+      where: { userId: user.id, courseId: lesson.courseId }
+    });
+    if (!enrollment) {
+      const err = new Error('Not enrolled in this course');
+      err.status = 403;
+      throw err;
+    }
+    if (!lesson.chapterId) {
+      const err = new Error('Exercise not found');
+      err.status = 404;
+      throw err;
+    }
+    const rows = await this._fetchChapterLexemeRowsForMc(lesson.chapterId);
+    const row = rows.find((r) => Number(r.id) === Number(chapterLexemeId));
+    if (!row) {
+      const err = new Error('Exercise not found');
+      err.status = 404;
+      throw err;
+    }
+    const learning = String(row.learning || '').trim();
+    const reference = String(row.reference || '').trim();
+    if (!learning || !reference) {
+      const err = new Error('Exercise not found');
+      err.status = 404;
+      throw err;
+    }
+    const allReferences = rows.map((r) => r.reference).filter(Boolean);
+    const seed = (Number(row.id) * 100003 + Number(lessonId)) >>> 0;
+    const { options, correctAnswer } = this._buildDeterministicChapterLexemeMcOptions(reference, allReferences, seed);
+    const questionData = {
+      type: 'multiple_choice',
+      question: `Was bedeutet „${learning}“?`,
+      options,
+      randomizeDistractors: false
+    };
+    const answerData = {
+      type: 'multiple_choice',
+      correctAnswer
+    };
+    const isCorrect = this._checkAnswer(answerData, questionData, userAnswer, 2);
+    const correctIdx = Number(correctAnswer);
+    const correctAnswerText = options[correctIdx];
+    const alternatives = options.filter((_, idx) => idx !== correctIdx);
+
+    return {
+      correct: isCorrect,
+      correctAnswer: correctAnswerText || null,
+      alternatives,
+      explanation: null,
+      progress: {
+        attempts: 1,
+        correctAttempts: isCorrect ? 1 : 0,
+        lastAttemptAt: new Date(),
+        completed: Boolean(isCorrect),
+        completedAt: isCorrect ? new Date() : null
+      }
+    };
+  }
+
   async getLesson(hashedUserId, lessonId) {
     const user = await this._getUserByHashedId(hashedUserId);
     const lesson = await VocabCourseLesson.findByPk(lessonId, {
@@ -2016,7 +2260,12 @@ export default class VocabService {
       plainLesson.reviewLessons = await this._getReviewLessons(plainLesson.courseId, plainLesson.lessonNumber);
       plainLesson.reviewVocabExercises = plainLesson.previousLessonExercises || [];
     }
-    
+
+    plainLesson.grammarExercises = await this._mergeSyntheticChapterLexemeMcExercises(
+      plainLesson,
+      plainLesson.grammarExercises || []
+    );
+
     plainLesson.didactics = this._buildLessonDidactics(plainLesson);
     plainLesson.pedagogy = this._buildLessonPedagogy(plainLesson);
     plainLesson.progress = this._serializeLessonProgress(progress, plainLesson);
@@ -2772,7 +3021,9 @@ export default class VocabService {
       order: [['exerciseNumber', 'ASC']]
     });
 
-    return exercises.map(e => e.get({ plain: true }));
+    const plainLesson = lesson.get({ plain: true });
+    const list = exercises.map((e) => e.get({ plain: true }));
+    return await this._mergeSyntheticChapterLexemeMcExercises(plainLesson, list);
   }
 
   async getGrammarExercise(hashedUserId, exerciseId) {
@@ -2802,6 +3053,16 @@ export default class VocabService {
 
   async checkGrammarExerciseAnswer(hashedUserId, exerciseId, userAnswer) {
     const user = await this._getUserByHashedId(hashedUserId);
+    const exIdStr = String(exerciseId ?? '');
+    const synMatch = /^syn-(\d+)-(\d+)-l2r$/.exec(exIdStr);
+    if (synMatch) {
+      return this._checkSyntheticLexemeMcAnswer(
+        user,
+        Number(synMatch[1]),
+        Number(synMatch[2]),
+        userAnswer
+      );
+    }
     const exercise = await VocabGrammarExercise.findByPk(exerciseId, {
       include: [
         { model: VocabCourseLesson, as: 'lesson', include: [{ model: VocabCourse, as: 'course' }] }
@@ -3149,14 +3410,18 @@ export default class VocabService {
   async getGrammarExerciseProgress(hashedUserId, lessonId) {
     const user = await this._getUserByHashedId(hashedUserId);
     const exercises = await this.getGrammarExercisesForLesson(hashedUserId, lessonId);
-    
-    const exerciseIds = exercises.map(e => e.id);
-    const progress = await VocabGrammarExerciseProgress.findAll({
-      where: {
-        userId: user.id,
-        exerciseId: { [Op.in]: exerciseIds }
-      }
-    });
+
+    const numericExerciseIds = exercises
+      .map((e) => e.id)
+      .filter((id) => /^\d+$/.test(String(id)));
+    const progress = numericExerciseIds.length
+      ? await VocabGrammarExerciseProgress.findAll({
+          where: {
+            userId: user.id,
+            exerciseId: { [Op.in]: numericExerciseIds }
+          }
+        })
+      : [];
 
     const progressMap = new Map(progress.map(p => [p.exerciseId, p.get({ plain: true })]));