Implement quiz attempt start service

src/main/java/com/example/questionbank/service/quiz/QuizAttemptService.java

@@ -0,0 +1,70 @@
+package com.example.questionbank.service.quiz;
+
+import com.example.questionbank.domain.ExamAttempt;
+import com.example.questionbank.domain.Question;
+import com.example.questionbank.domain.Subject;
+import com.example.questionbank.domain.User;
+import com.example.questionbank.domain.enumeration.AttemptStatus;
+import com.example.questionbank.repository.ExamAttemptRepository;
+import com.example.questionbank.repository.QuestionRepository;
+import com.example.questionbank.repository.SubjectRepository;
+import com.example.questionbank.repository.UserRepository;
+import com.example.questionbank.web.rest.dto.ExamAttemptStartResult;
+import java.time.Instant;
+import java.util.List;
+import org.springframework.http.HttpStatus;
+import org.springframework.stereotype.Service;
+import org.springframework.transaction.annotation.Transactional;
+import org.springframework.web.server.ResponseStatusException;
+
+@Service
+public class QuizAttemptService {
+
+    private final ExamAttemptRepository attemptRepository;
+    private final SubjectRepository subjectRepository;
+    private final QuestionRepository questionRepository;
+    private final UserRepository userRepository;
+
+    public QuizAttemptService(
+        ExamAttemptRepository attemptRepository,
+        SubjectRepository subjectRepository,
+        QuestionRepository questionRepository,
+        UserRepository userRepository
+    ) {
+        this.attemptRepository = attemptRepository;
+        this.subjectRepository = subjectRepository;
+        this.questionRepository = questionRepository;
+        this.userRepository = userRepository;
+    }
+
+    @Transactional
+    public ExamAttemptStartResult start(Long subjectId, String login) {
+        Subject subject = subjectRepository
+            .findById(subjectId)
+            .orElseThrow(() -> new ResponseStatusException(HttpStatus.NOT_FOUND, "과목을 찾을 수 없습니다."));
+
+        if (!Boolean.TRUE.equals(subject.getActive())) {
+            throw new ResponseStatusException(HttpStatus.BAD_REQUEST, "비활성 과목은 응시할 수 없습니다.");
+        }
+
+        List<Question> activeQuestions = questionRepository.findAllBySubjectIdAndActiveTrue(subjectId);
+
+        if (activeQuestions.isEmpty()) {
+            throw new ResponseStatusException(HttpStatus.CONFLICT, "출제된 활성 문항이 없는 과목입니다.");
+        }
+
+        User user = userRepository
+            .findOneByLogin(login)
+            .orElseThrow(() -> new ResponseStatusException(HttpStatus.NOT_FOUND, "사용자를 찾을 수 없습니다."));
+
+        ExamAttempt attempt = new ExamAttempt();
+        attempt.setSubject(subject);
+        attempt.setUser(user);
+        attempt.setStartedAt(Instant.now());
+        attempt.setStatus(AttemptStatus.IN_PROGRESS);
+        attempt.setScore(0);
+
+        ExamAttempt saved = attemptRepository.save(attempt);
+        return new ExamAttemptStartResult(saved.getId(), saved.getStartedAt());
+    }
+}