chore: refactored project according to clean architecture

internal/domain/order.go

@@ -0,0 +1,32 @@
+package domain
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+)
+
+var (
+	ErrOrderAlreadyExist = errors.New("order already exist")
+	ErrOrderNotFound     = errors.New("order not found")
+	ErrInvalidOrderData  = errors.New("invalid order data")
+)
+
+type Order struct {
+	ID       string
+	Item     string
+	Quantity int32
+}
+
+func (o *Order) Validate() error {
+	if strings.TrimSpace(o.Item) == "" {
+		return fmt.Errorf("%w: item cannot be empty", ErrInvalidOrderData)
+	}
+	if o.Quantity <= 0 {
+		return fmt.Errorf("%w: quantity must be positive", ErrInvalidOrderData)
+	}
+	if o.ID == "" {
+		return fmt.Errorf("%w: ID cannot be empty", ErrInvalidOrderData)
+	}
+	return nil
+}