Fix vendor mapping delete behavior and update docs

internal/services/pricelist/service.go

@@ -38,6 +38,21 @@ type CreateItemInput struct {
 	Category    string
 }
 
+type PriceChangeItem struct {
+	LotName    string   `json:"lot_name"`
+	OldPrice   float64  `json:"old_price"`
+	NewPrice   *float64 `json:"new_price,omitempty"`
+	Diff       *float64 `json:"diff,omitempty"`
+	ChangeType string   `json:"change_type"` // increased, decreased, missing
+}
+
+type PriceChangeSummary struct {
+	PreviousPricelistID      uint              `json:"previous_pricelist_id,omitempty"`
+	PreviousPricelistVersion string            `json:"previous_pricelist_version,omitempty"`
+	Changed                  []PriceChangeItem `json:"changed"`
+	Missing                  []PriceChangeItem `json:"missing"`
+}
+
 func NewService(db *gorm.DB, repo *repository.PricelistRepository, componentRepo *repository.ComponentRepository, pricingSvc *pricing.Service) *Service {
 	return &Service{
 		repo:          repo,
@@ -505,3 +520,96 @@ func (s *Service) StreamItemsForExport(pricelistID uint, batchSize int, callback
 	}
 	return s.repo.StreamItemsForExport(pricelistID, batchSize, callback)
 }
+
+// BuildPriceChangeSummary compares a newly created pricelist with the previous pricelist of the same source.
+func (s *Service) BuildPriceChangeSummary(pricelistID uint) (*PriceChangeSummary, error) {
+	if s.db == nil {
+		return &PriceChangeSummary{Changed: []PriceChangeItem{}, Missing: []PriceChangeItem{}}, nil
+	}
+
+	var current models.Pricelist
+	if err := s.db.Select("id", "source", "created_at").First(&current, pricelistID).Error; err != nil {
+		return nil, fmt.Errorf("loading current pricelist: %w", err)
+	}
+
+	var previous models.Pricelist
+	err := s.db.Select("id", "version").
+		Where("source = ? AND id <> ?", current.Source, current.ID).
+		Order("created_at DESC, id DESC").
+		First(&previous).Error
+	if err != nil {
+		if errors.Is(err, gorm.ErrRecordNotFound) {
+			return &PriceChangeSummary{Changed: []PriceChangeItem{}, Missing: []PriceChangeItem{}}, nil
+		}
+		return nil, fmt.Errorf("loading previous pricelist: %w", err)
+	}
+
+	type row struct {
+		LotName string
+		Price   float64
+	}
+	var prevItems []row
+	var curItems []row
+	if err := s.db.Model(&models.PricelistItem{}).
+		Select("lot_name", "price").
+		Where("pricelist_id = ?", previous.ID).
+		Find(&prevItems).Error; err != nil {
+		return nil, fmt.Errorf("loading previous pricelist items: %w", err)
+	}
+	if err := s.db.Model(&models.PricelistItem{}).
+		Select("lot_name", "price").
+		Where("pricelist_id = ?", current.ID).
+		Find(&curItems).Error; err != nil {
+		return nil, fmt.Errorf("loading current pricelist items: %w", err)
+	}
+
+	curMap := make(map[string]float64, len(curItems))
+	for _, it := range curItems {
+		curMap[strings.TrimSpace(it.LotName)] = it.Price
+	}
+
+	changed := make([]PriceChangeItem, 0)
+	missing := make([]PriceChangeItem, 0)
+	const eps = 0.000001
+	for _, prev := range prevItems {
+		lotName := strings.TrimSpace(prev.LotName)
+		if lotName == "" {
+			continue
+		}
+		newPrice, ok := curMap[lotName]
+		if !ok {
+			missing = append(missing, PriceChangeItem{
+				LotName:    lotName,
+				OldPrice:   prev.Price,
+				NewPrice:   nil,
+				Diff:       nil,
+				ChangeType: "missing",
+			})
+			continue
+		}
+		diff := newPrice - prev.Price
+		if diff > -eps && diff < eps {
+			continue
+		}
+		newPriceCopy := newPrice
+		diffCopy := diff
+		changeType := "increased"
+		if diff < 0 {
+			changeType = "decreased"
+		}
+		changed = append(changed, PriceChangeItem{
+			LotName:    lotName,
+			OldPrice:   prev.Price,
+			NewPrice:   &newPriceCopy,
+			Diff:       &diffCopy,
+			ChangeType: changeType,
+		})
+	}
+
+	return &PriceChangeSummary{
+		PreviousPricelistID:      previous.ID,
+		PreviousPricelistVersion: previous.Version,
+		Changed:                  changed,
+		Missing:                  missing,
+	}, nil
+}