veza/veza-backend-api/internal/core/education/service.go

package education

import (
	"context"
	"errors"
	"fmt"
	"strings"
	"time"

	"github.com/google/uuid"
	"go.uber.org/zap"
	"gorm.io/gorm"
)

// Service errors
var (
	ErrCourseNotFound       = errors.New("course not found")
	ErrLessonNotFound       = errors.New("lesson not found")
	ErrEnrollmentNotFound   = errors.New("enrollment not found")
	ErrAlreadyEnrolled      = errors.New("already enrolled in this course")
	ErrNotEnrolled          = errors.New("not enrolled in this course")
	ErrCourseNotPublished   = errors.New("course is not published")
	ErrNotCourseOwner       = errors.New("you are not the owner of this course")
	ErrReviewAlreadyExists  = errors.New("you have already reviewed this course")
	ErrInvalidRating        = errors.New("rating must be between 1 and 5")
	ErrCertificateNotFound  = errors.New("certificate not found")
	ErrCourseNotCompleted   = errors.New("course is not fully completed")
	ErrCertificateExists    = errors.New("certificate already issued for this course")
	ErrCannotEnrollOwnCourse = errors.New("cannot enroll in your own course")
	ErrSlugAlreadyTaken     = errors.New("slug is already taken")
)

// ServiceOption is a functional option for configuring the Service
type ServiceOption func(*Service)

// Service handles education business logic
type Service struct {
	db     *gorm.DB
	logger *zap.Logger
}

// NewService creates a new education service
func NewService(db *gorm.DB, logger *zap.Logger, opts ...ServiceOption) *Service {
	s := &Service{
		db:     db,
		logger: logger,
	}
	for _, opt := range opts {
		opt(s)
	}
	return s
}

// --- Course CRUD ---

// CreateCourseRequest holds the fields for creating a course
type CreateCourseRequest struct {
	Title             string       `json:"title" binding:"required"`
	Description       string       `json:"description"`
	Category          string       `json:"category"`
	Tags              []string     `json:"tags"`
	CoverImageURL     string       `json:"cover_image_url"`
	PriceCents        int          `json:"price_cents"`
	Currency          string       `json:"currency"`
	PricingModel      PricingModel `json:"pricing_model"`
	MinimumPriceCents int          `json:"minimum_price_cents"`
	Level             CourseLevel  `json:"level"`
	Language          string       `json:"language"`
}

// CreateCourse creates a new course in draft status
func (s *Service) CreateCourse(ctx context.Context, creatorID uuid.UUID, req CreateCourseRequest) (*Course, error) {
	slug := generateSlug(req.Title)

	// Ensure slug uniqueness
	var count int64
	s.db.WithContext(ctx).Model(&Course{}).Where("slug = ?", slug).Count(&count)
	if count > 0 {
		slug = fmt.Sprintf("%s-%s", slug, uuid.New().String()[:8])
	}

	currency := req.Currency
	if currency == "" {
		currency = "USD"
	}
	pricingModel := req.PricingModel
	if pricingModel == "" {
		pricingModel = PricingFixed
	}
	level := req.Level
	if level == "" {
		level = CourseLevelBeginner
	}
	language := req.Language
	if language == "" {
		language = "en"
	}

	course := &Course{
		CreatorID:         creatorID,
		Title:             req.Title,
		Slug:              slug,
		Description:       req.Description,
		Category:          req.Category,
		Tags:              req.Tags,
		CoverImageURL:     req.CoverImageURL,
		PriceCents:        req.PriceCents,
		Currency:          currency,
		PricingModel:      pricingModel,
		MinimumPriceCents: req.MinimumPriceCents,
		Status:            CourseStatusDraft,
		Level:             level,
		Language:          language,
	}

	if err := s.db.WithContext(ctx).Create(course).Error; err != nil {
		if strings.Contains(err.Error(), "duplicate") && strings.Contains(err.Error(), "slug") {
			return nil, ErrSlugAlreadyTaken
		}
		return nil, fmt.Errorf("failed to create course: %w", err)
	}

	s.logger.Info("Course created",
		zap.String("course_id", course.ID.String()),
		zap.String("creator_id", creatorID.String()),
	)

	return course, nil
}

// UpdateCourseRequest holds updatable course fields
type UpdateCourseRequest struct {
	Title             *string       `json:"title"`
	Description       *string       `json:"description"`
	Category          *string       `json:"category"`
	Tags              []string      `json:"tags"`
	CoverImageURL     *string       `json:"cover_image_url"`
	PriceCents        *int          `json:"price_cents"`
	Currency          *string       `json:"currency"`
	PricingModel      *PricingModel `json:"pricing_model"`
	MinimumPriceCents *int          `json:"minimum_price_cents"`
	Level             *CourseLevel  `json:"level"`
	Language          *string       `json:"language"`
}

// UpdateCourse updates an existing course
func (s *Service) UpdateCourse(ctx context.Context, creatorID, courseID uuid.UUID, req UpdateCourseRequest) (*Course, error) {
	course, err := s.getCourseByOwner(ctx, courseID, creatorID)
	if err != nil {
		return nil, err
	}

	updates := map[string]interface{}{}
	if req.Title != nil {
		updates["title"] = *req.Title
	}
	if req.Description != nil {
		updates["description"] = *req.Description
	}
	if req.Category != nil {
		updates["category"] = *req.Category
	}
	if req.Tags != nil {
		updates["tags"] = req.Tags
	}
	if req.CoverImageURL != nil {
		updates["cover_image_url"] = *req.CoverImageURL
	}
	if req.PriceCents != nil {
		updates["price_cents"] = *req.PriceCents
	}
	if req.Currency != nil {
		updates["currency"] = *req.Currency
	}
	if req.PricingModel != nil {
		updates["pricing_model"] = *req.PricingModel
	}
	if req.MinimumPriceCents != nil {
		updates["minimum_price_cents"] = *req.MinimumPriceCents
	}
	if req.Level != nil {
		updates["level"] = *req.Level
	}
	if req.Language != nil {
		updates["language"] = *req.Language
	}

	if len(updates) > 0 {
		if err := s.db.WithContext(ctx).Model(course).Updates(updates).Error; err != nil {
			return nil, fmt.Errorf("failed to update course: %w", err)
		}
	}

	return course, nil
}

// PublishCourse sets a course to published status
func (s *Service) PublishCourse(ctx context.Context, creatorID, courseID uuid.UUID) (*Course, error) {
	course, err := s.getCourseByOwner(ctx, courseID, creatorID)
	if err != nil {
		return nil, err
	}

	now := time.Now()
	course.Status = CourseStatusPublished
	course.PublishedAt = &now

	if err := s.db.WithContext(ctx).Save(course).Error; err != nil {
		return nil, fmt.Errorf("failed to publish course: %w", err)
	}

	s.logger.Info("Course published",
		zap.String("course_id", courseID.String()),
	)

	return course, nil
}

// ArchiveCourse sets a course to archived status
func (s *Service) ArchiveCourse(ctx context.Context, creatorID, courseID uuid.UUID) (*Course, error) {
	course, err := s.getCourseByOwner(ctx, courseID, creatorID)
	if err != nil {
		return nil, err
	}

	course.Status = CourseStatusArchived
	if err := s.db.WithContext(ctx).Save(course).Error; err != nil {
		return nil, fmt.Errorf("failed to archive course: %w", err)
	}

	return course, nil
}

// GetCourse returns a course by ID (public, for any user)
func (s *Service) GetCourse(ctx context.Context, courseID uuid.UUID) (*Course, error) {
	var course Course
	err := s.db.WithContext(ctx).First(&course, "id = ?", courseID).Error
	if err != nil {
		if errors.Is(err, gorm.ErrRecordNotFound) {
			return nil, ErrCourseNotFound
		}
		return nil, fmt.Errorf("failed to get course: %w", err)
	}
	return &course, nil
}

// GetCourseBySlug returns a course by its slug
func (s *Service) GetCourseBySlug(ctx context.Context, slug string) (*Course, error) {
	var course Course
	err := s.db.WithContext(ctx).First(&course, "slug = ?", slug).Error
	if err != nil {
		if errors.Is(err, gorm.ErrRecordNotFound) {
			return nil, ErrCourseNotFound
		}
		return nil, fmt.Errorf("failed to get course by slug: %w", err)
	}
	return &course, nil
}

// ListPublishedCourses returns published courses with optional filters
func (s *Service) ListPublishedCourses(ctx context.Context, category *string, level *CourseLevel, language *string, limit, offset int) ([]Course, int64, error) {
	if limit <= 0 || limit > 100 {
		limit = 20
	}
	if offset < 0 {
		offset = 0
	}

	query := s.db.WithContext(ctx).Where("status = ?", string(CourseStatusPublished))
	if category != nil && *category != "" {
		query = query.Where("category = ?", *category)
	}
	if level != nil {
		query = query.Where("level = ?", string(*level))
	}
	if language != nil && *language != "" {
		query = query.Where("language = ?", *language)
	}

	var total int64
	query.Model(&Course{}).Count(&total)

	var courses []Course
	err := query.Order("published_at DESC").Limit(limit).Offset(offset).Find(&courses).Error
	if err != nil {
		return nil, 0, fmt.Errorf("failed to list courses: %w", err)
	}
	return courses, total, nil
}

// ListCreatorCourses returns all courses for a creator
func (s *Service) ListCreatorCourses(ctx context.Context, creatorID uuid.UUID, limit, offset int) ([]Course, int64, error) {
	if limit <= 0 || limit > 100 {
		limit = 20
	}
	if offset < 0 {
		offset = 0
	}

	query := s.db.WithContext(ctx).Where("creator_id = ?", creatorID)

	var total int64
	query.Model(&Course{}).Count(&total)

	var courses []Course
	err := query.Order("created_at DESC").Limit(limit).Offset(offset).Find(&courses).Error
	if err != nil {
		return nil, 0, fmt.Errorf("failed to list creator courses: %w", err)
	}
	return courses, total, nil
}

// DeleteCourse soft-deletes a course
func (s *Service) DeleteCourse(ctx context.Context, creatorID, courseID uuid.UUID) error {
	course, err := s.getCourseByOwner(ctx, courseID, creatorID)
	if err != nil {
		return err
	}

	if err := s.db.WithContext(ctx).Delete(course).Error; err != nil {
		return fmt.Errorf("failed to delete course: %w", err)
	}
	return nil
}

// --- Lesson CRUD ---

// CreateLessonRequest holds the fields for creating a lesson
type CreateLessonRequest struct {
	Title         string `json:"title" binding:"required"`
	Description   string `json:"description"`
	VideoFilePath string `json:"video_file_path"`
	DurationSeconds int  `json:"duration_seconds"`
	IsPreviewFree bool   `json:"is_preview_free"`
}

// CreateLesson adds a lesson to a course
func (s *Service) CreateLesson(ctx context.Context, creatorID, courseID uuid.UUID, req CreateLessonRequest) (*Lesson, error) {
	if _, err := s.getCourseByOwner(ctx, courseID, creatorID); err != nil {
		return nil, err
	}

	// Get next order index
	var maxOrder int
	s.db.WithContext(ctx).Model(&Lesson{}).Where("course_id = ?", courseID).
		Select("COALESCE(MAX(order_index), 0)").Scan(&maxOrder)

	lesson := &Lesson{
		CourseID:       courseID,
		OrderIndex:     maxOrder + 1,
		Title:          req.Title,
		Description:    req.Description,
		VideoFilePath:  req.VideoFilePath,
		DurationSeconds: req.DurationSeconds,
		IsPreviewFree:  req.IsPreviewFree,
	}

	err := s.db.WithContext(ctx).Transaction(func(tx *gorm.DB) error {
		if err := tx.Create(lesson).Error; err != nil {
			return fmt.Errorf("failed to create lesson: %w", err)
		}
		// Update course counters
		return tx.Model(&Course{}).Where("id = ?", courseID).
			UpdateColumns(map[string]interface{}{
				"lesson_count":          gorm.Expr("lesson_count + 1"),
				"total_duration_seconds": gorm.Expr("total_duration_seconds + ?", req.DurationSeconds),
			}).Error
	})
	if err != nil {
		return nil, err
	}

	return lesson, nil
}

// UpdateLessonRequest holds updatable lesson fields
type UpdateLessonRequest struct {
	Title           *string `json:"title"`
	Description     *string `json:"description"`
	VideoFilePath   *string `json:"video_file_path"`
	DurationSeconds *int    `json:"duration_seconds"`
	IsPreviewFree   *bool   `json:"is_preview_free"`
}

// UpdateLesson updates a lesson
func (s *Service) UpdateLesson(ctx context.Context, creatorID, courseID, lessonID uuid.UUID, req UpdateLessonRequest) (*Lesson, error) {
	if _, err := s.getCourseByOwner(ctx, courseID, creatorID); err != nil {
		return nil, err
	}

	var lesson Lesson
	err := s.db.WithContext(ctx).First(&lesson, "id = ? AND course_id = ?", lessonID, courseID).Error
	if err != nil {
		if errors.Is(err, gorm.ErrRecordNotFound) {
			return nil, ErrLessonNotFound
		}
		return nil, fmt.Errorf("failed to get lesson: %w", err)
	}

	oldDuration := lesson.DurationSeconds
	updates := map[string]interface{}{}
	if req.Title != nil {
		updates["title"] = *req.Title
	}
	if req.Description != nil {
		updates["description"] = *req.Description
	}
	if req.VideoFilePath != nil {
		updates["video_file_path"] = *req.VideoFilePath
		updates["transcoding_status"] = string(TranscodingPending)
	}
	if req.DurationSeconds != nil {
		updates["duration_seconds"] = *req.DurationSeconds
	}
	if req.IsPreviewFree != nil {
		updates["is_preview_free"] = *req.IsPreviewFree
	}

	if len(updates) > 0 {
		err = s.db.WithContext(ctx).Transaction(func(tx *gorm.DB) error {
			if err := tx.Model(&lesson).Updates(updates).Error; err != nil {
				return err
			}
			// Update course duration if changed
			if req.DurationSeconds != nil && *req.DurationSeconds != oldDuration {
				diff := *req.DurationSeconds - oldDuration
				return tx.Model(&Course{}).Where("id = ?", courseID).
					UpdateColumn("total_duration_seconds", gorm.Expr("total_duration_seconds + ?", diff)).Error
			}
			return nil
		})
		if err != nil {
			return nil, fmt.Errorf("failed to update lesson: %w", err)
		}
	}

	return &lesson, nil
}

// DeleteLesson removes a lesson
func (s *Service) DeleteLesson(ctx context.Context, creatorID, courseID, lessonID uuid.UUID) error {
	if _, err := s.getCourseByOwner(ctx, courseID, creatorID); err != nil {
		return err
	}

	var lesson Lesson
	err := s.db.WithContext(ctx).First(&lesson, "id = ? AND course_id = ?", lessonID, courseID).Error
	if err != nil {
		if errors.Is(err, gorm.ErrRecordNotFound) {
			return ErrLessonNotFound
		}
		return fmt.Errorf("failed to get lesson: %w", err)
	}

	return s.db.WithContext(ctx).Transaction(func(tx *gorm.DB) error {
		if err := tx.Delete(&lesson).Error; err != nil {
			return err
		}
		return tx.Model(&Course{}).Where("id = ?", courseID).
			UpdateColumns(map[string]interface{}{
				"lesson_count":          gorm.Expr("lesson_count - 1"),
				"total_duration_seconds": gorm.Expr("total_duration_seconds - ?", lesson.DurationSeconds),
			}).Error
	})
}

// GetCourseLessons returns all lessons for a course in order
func (s *Service) GetCourseLessons(ctx context.Context, courseID uuid.UUID) ([]Lesson, error) {
	var lessons []Lesson
	err := s.db.WithContext(ctx).
		Where("course_id = ?", courseID).
		Order("order_index ASC").
		Find(&lessons).Error
	if err != nil {
		return nil, fmt.Errorf("failed to get lessons: %w", err)
	}
	return lessons, nil
}

// ReorderLessons updates the order of lessons
type ReorderLessonsRequest struct {
	LessonIDs []uuid.UUID `json:"lesson_ids" binding:"required"`
}

func (s *Service) ReorderLessons(ctx context.Context, creatorID, courseID uuid.UUID, req ReorderLessonsRequest) error {
	if _, err := s.getCourseByOwner(ctx, courseID, creatorID); err != nil {
		return err
	}

	return s.db.WithContext(ctx).Transaction(func(tx *gorm.DB) error {
		for i, id := range req.LessonIDs {
			if err := tx.Model(&Lesson{}).
				Where("id = ? AND course_id = ?", id, courseID).
				Update("order_index", i+1).Error; err != nil {
				return fmt.Errorf("failed to reorder lesson %s: %w", id, err)
			}
		}
		return nil
	})
}

// --- Enrollments ---

// EnrollRequest holds the parameters for enrolling in a course
type EnrollRequest struct {
	PricePaidCents int    `json:"price_paid_cents"`
	Currency       string `json:"currency"`
}

// Enroll enrolls a user in a course
func (s *Service) Enroll(ctx context.Context, userID, courseID uuid.UUID, req EnrollRequest) (*CourseEnrollment, error) {
	// Get course
	course, err := s.GetCourse(ctx, courseID)
	if err != nil {
		return nil, err
	}

	if course.Status != CourseStatusPublished {
		return nil, ErrCourseNotPublished
	}

	if course.CreatorID == userID {
		return nil, ErrCannotEnrollOwnCourse
	}

	// Check existing enrollment
	var existing int64
	s.db.WithContext(ctx).Model(&CourseEnrollment{}).
		Where("user_id = ? AND course_id = ?", userID, courseID).
		Count(&existing)
	if existing > 0 {
		return nil, ErrAlreadyEnrolled
	}

	currency := req.Currency
	if currency == "" {
		currency = course.Currency
	}

	enrollment := &CourseEnrollment{
		UserID:              userID,
		CourseID:            courseID,
		PurchasedPriceCents: req.PricePaidCents,
		Currency:            currency,
		Status:              EnrollmentActive,
		PurchasedAt:         time.Now(),
	}

	err = s.db.WithContext(ctx).Transaction(func(tx *gorm.DB) error {
		if err := tx.Create(enrollment).Error; err != nil {
			if strings.Contains(err.Error(), "duplicate") {
				return ErrAlreadyEnrolled
			}
			return fmt.Errorf("failed to create enrollment: %w", err)
		}
		return tx.Model(&Course{}).Where("id = ?", courseID).
			UpdateColumn("enrollment_count", gorm.Expr("enrollment_count + 1")).Error
	})
	if err != nil {
		return nil, err
	}

	s.logger.Info("User enrolled in course",
		zap.String("user_id", userID.String()),
		zap.String("course_id", courseID.String()),
	)

	return enrollment, nil
}

// GetUserEnrollments returns all enrollments for a user
func (s *Service) GetUserEnrollments(ctx context.Context, userID uuid.UUID, limit, offset int) ([]CourseEnrollment, int64, error) {
	if limit <= 0 || limit > 100 {
		limit = 20
	}
	if offset < 0 {
		offset = 0
	}

	query := s.db.WithContext(ctx).Where("user_id = ?", userID)

	var total int64
	query.Model(&CourseEnrollment{}).Count(&total)

	var enrollments []CourseEnrollment
	err := query.Order("purchased_at DESC").Limit(limit).Offset(offset).Find(&enrollments).Error
	if err != nil {
		return nil, 0, fmt.Errorf("failed to get enrollments: %w", err)
	}
	return enrollments, total, nil
}

// GetEnrollment returns a specific enrollment
func (s *Service) GetEnrollment(ctx context.Context, userID, courseID uuid.UUID) (*CourseEnrollment, error) {
	var enrollment CourseEnrollment
	err := s.db.WithContext(ctx).First(&enrollment, "user_id = ? AND course_id = ?", userID, courseID).Error
	if err != nil {
		if errors.Is(err, gorm.ErrRecordNotFound) {
			return nil, ErrNotEnrolled
		}
		return nil, fmt.Errorf("failed to get enrollment: %w", err)
	}
	return &enrollment, nil
}

// --- Progress ---

// UpdateProgressRequest holds progress update data
type UpdateProgressRequest struct {
	WatchedPercentage       int `json:"watched_percentage"`
	WatchedDurationSeconds  int `json:"watched_duration_seconds"`
	PlaybackPositionSeconds int `json:"playback_position_seconds"`
}

// UpdateProgress updates or creates lesson progress
func (s *Service) UpdateProgress(ctx context.Context, userID, lessonID uuid.UUID, req UpdateProgressRequest) (*LessonProgress, error) {
	// Get the lesson to find the course
	var lesson Lesson
	err := s.db.WithContext(ctx).First(&lesson, "id = ?", lessonID).Error
	if err != nil {
		if errors.Is(err, gorm.ErrRecordNotFound) {
			return nil, ErrLessonNotFound
		}
		return nil, fmt.Errorf("failed to get lesson: %w", err)
	}

	// Get enrollment
	enrollment, err := s.GetEnrollment(ctx, userID, lesson.CourseID)
	if err != nil {
		return nil, err
	}

	// Clamp percentage
	percentage := req.WatchedPercentage
	if percentage < 0 {
		percentage = 0
	}
	if percentage > 100 {
		percentage = 100
	}

	// Upsert progress
	var progress LessonProgress
	err = s.db.WithContext(ctx).
		Where("user_id = ? AND lesson_id = ?", userID, lessonID).
		First(&progress).Error

	if errors.Is(err, gorm.ErrRecordNotFound) {
		progress = LessonProgress{
			UserID:                  userID,
			LessonID:               lessonID,
			EnrollmentID:           enrollment.ID,
			WatchedPercentage:       percentage,
			WatchedDurationSeconds:  req.WatchedDurationSeconds,
			PlaybackPositionSeconds: req.PlaybackPositionSeconds,
		}
		if percentage >= 90 {
			progress.IsCompleted = true
			now := time.Now()
			progress.CompletedAt = &now
		}
		if err := s.db.WithContext(ctx).Create(&progress).Error; err != nil {
			return nil, fmt.Errorf("failed to create progress: %w", err)
		}
	} else if err != nil {
		return nil, fmt.Errorf("failed to get progress: %w", err)
	} else {
		updates := map[string]interface{}{
			"watched_percentage":        percentage,
			"watched_duration_seconds":  req.WatchedDurationSeconds,
			"playback_position_seconds": req.PlaybackPositionSeconds,
		}
		if percentage >= 90 && !progress.IsCompleted {
			updates["is_completed"] = true
			now := time.Now()
			updates["completed_at"] = now
		}
		if err := s.db.WithContext(ctx).Model(&progress).Updates(updates).Error; err != nil {
			return nil, fmt.Errorf("failed to update progress: %w", err)
		}
	}

	return &progress, nil
}

// GetCourseProgress returns all lesson progress for a user in a course
func (s *Service) GetCourseProgress(ctx context.Context, userID, courseID uuid.UUID) ([]LessonProgress, error) {
	enrollment, err := s.GetEnrollment(ctx, userID, courseID)
	if err != nil {
		return nil, err
	}

	var progress []LessonProgress
	err = s.db.WithContext(ctx).
		Where("enrollment_id = ?", enrollment.ID).
		Find(&progress).Error
	if err != nil {
		return nil, fmt.Errorf("failed to get course progress: %w", err)
	}
	return progress, nil
}

// --- Certificates ---

// IssueCertificate issues a completion certificate for a fully completed course
func (s *Service) IssueCertificate(ctx context.Context, userID, courseID uuid.UUID) (*Certificate, error) {
	enrollment, err := s.GetEnrollment(ctx, userID, courseID)
	if err != nil {
		return nil, err
	}

	// Check if certificate already exists
	var existingCount int64
	s.db.WithContext(ctx).Model(&Certificate{}).
		Where("user_id = ? AND course_id = ?", userID, courseID).
		Count(&existingCount)
	if existingCount > 0 {
		return nil, ErrCertificateExists
	}

	// Check all lessons are completed
	var lessonCount int64
	s.db.WithContext(ctx).Model(&Lesson{}).Where("course_id = ?", courseID).Count(&lessonCount)

	var completedCount int64
	s.db.WithContext(ctx).Model(&LessonProgress{}).
		Where("enrollment_id = ? AND is_completed = true", enrollment.ID).
		Count(&completedCount)

	if lessonCount == 0 || completedCount < lessonCount {
		return nil, ErrCourseNotCompleted
	}

	code := fmt.Sprintf("VEZA-CERT-%s-%s", courseID.String()[:8], uuid.New().String()[:8])

	cert := &Certificate{
		UserID:          userID,
		CourseID:        courseID,
		EnrollmentID:    enrollment.ID,
		CertificateCode: code,
		IssueDate:       time.Now(),
		Status:          CertificateActive,
	}

	if err := s.db.WithContext(ctx).Create(cert).Error; err != nil {
		return nil, fmt.Errorf("failed to issue certificate: %w", err)
	}

	s.logger.Info("Certificate issued",
		zap.String("user_id", userID.String()),
		zap.String("course_id", courseID.String()),
		zap.String("certificate_code", code),
	)

	return cert, nil
}

// GetCertificate returns a certificate by code (public verification)
func (s *Service) GetCertificate(ctx context.Context, code string) (*Certificate, error) {
	var cert Certificate
	err := s.db.WithContext(ctx).First(&cert, "certificate_code = ?", code).Error
	if err != nil {
		if errors.Is(err, gorm.ErrRecordNotFound) {
			return nil, ErrCertificateNotFound
		}
		return nil, fmt.Errorf("failed to get certificate: %w", err)
	}
	return &cert, nil
}

// GetUserCertificates returns all certificates for a user
func (s *Service) GetUserCertificates(ctx context.Context, userID uuid.UUID) ([]Certificate, error) {
	var certs []Certificate
	err := s.db.WithContext(ctx).
		Where("user_id = ?", userID).
		Order("issue_date DESC").
		Find(&certs).Error
	if err != nil {
		return nil, fmt.Errorf("failed to get certificates: %w", err)
	}
	return certs, nil
}

// --- Reviews ---

// CreateReviewRequest holds the fields for creating a review
type CreateReviewRequest struct {
	Rating  int    `json:"rating" binding:"required"`
	Title   string `json:"title"`
	Content string `json:"content" binding:"required"`
}

// CreateReview creates a course review
func (s *Service) CreateReview(ctx context.Context, userID, courseID uuid.UUID, req CreateReviewRequest) (*CourseReview, error) {
	if req.Rating < 1 || req.Rating > 5 {
		return nil, ErrInvalidRating
	}

	// Must be enrolled
	enrollment, err := s.GetEnrollment(ctx, userID, courseID)
	if err != nil {
		return nil, err
	}

	// Check existing review
	var existing int64
	s.db.WithContext(ctx).Model(&CourseReview{}).
		Where("user_id = ? AND course_id = ?", userID, courseID).
		Count(&existing)
	if existing > 0 {
		return nil, ErrReviewAlreadyExists
	}

	review := &CourseReview{
		CourseID:     courseID,
		UserID:       userID,
		EnrollmentID: enrollment.ID,
		Rating:       req.Rating,
		Title:        req.Title,
		Content:      req.Content,
		Status:       ReviewApproved,
	}

	err = s.db.WithContext(ctx).Transaction(func(tx *gorm.DB) error {
		if err := tx.Create(review).Error; err != nil {
			if strings.Contains(err.Error(), "duplicate") {
				return ErrReviewAlreadyExists
			}
			return fmt.Errorf("failed to create review: %w", err)
		}
		// Update course review count and average
		return tx.Model(&Course{}).Where("id = ?", courseID).
			UpdateColumns(map[string]interface{}{
				"review_count": gorm.Expr("review_count + 1"),
				"average_rating": gorm.Expr(
					"(SELECT COALESCE(AVG(rating), 0) FROM course_reviews WHERE course_id = ? AND deleted_at IS NULL AND status = 'approved')",
					courseID,
				),
			}).Error
	})
	if err != nil {
		return nil, err
	}

	return review, nil
}

// GetCourseReviews returns reviews for a course
func (s *Service) GetCourseReviews(ctx context.Context, courseID uuid.UUID, limit, offset int) ([]CourseReview, int64, error) {
	if limit <= 0 || limit > 100 {
		limit = 20
	}
	if offset < 0 {
		offset = 0
	}

	query := s.db.WithContext(ctx).Where("course_id = ? AND status = ?", courseID, string(ReviewApproved))

	var total int64
	query.Model(&CourseReview{}).Count(&total)

	var reviews []CourseReview
	err := query.Order("created_at DESC").Limit(limit).Offset(offset).Find(&reviews).Error
	if err != nil {
		return nil, 0, fmt.Errorf("failed to get reviews: %w", err)
	}
	return reviews, total, nil
}

// --- Helpers ---

func (s *Service) getCourseByOwner(ctx context.Context, courseID, creatorID uuid.UUID) (*Course, error) {
	var course Course
	err := s.db.WithContext(ctx).First(&course, "id = ?", courseID).Error
	if err != nil {
		if errors.Is(err, gorm.ErrRecordNotFound) {
			return nil, ErrCourseNotFound
		}
		return nil, fmt.Errorf("failed to get course: %w", err)
	}
	if course.CreatorID != creatorID {
		return nil, ErrNotCourseOwner
	}
	return &course, nil
}

func generateSlug(title string) string {
	slug := strings.ToLower(title)
	slug = strings.Map(func(r rune) rune {
		if (r >= 'a' && r <= 'z') || (r >= '0' && r <= '9') {
			return r
		}
		if r == ' ' || r == '-' || r == '_' {
			return '-'
		}
		return -1
	}, slug)
	// Remove consecutive dashes
	for strings.Contains(slug, "--") {
		slug = strings.ReplaceAll(slug, "--", "-")
	}
	slug = strings.Trim(slug, "-")
	if slug == "" {
		slug = uuid.New().String()[:8]
	}
	return slug
}