feat: move entity and usecase to root folder

entity/book.go

@@ -0,0 +1,41 @@
+package entity
+
+import (
+	"time"
+)
+
+//Book data
+type Book struct {
+	ID        ID
+	Title     string
+	Author    string
+	Pages     int
+	Quantity  int
+	CreatedAt time.Time
+	UpdatedAt time.Time
+}
+
+//NewBook create a new book
+func NewBook(title string, author string, pages int, quantity int) (*Book, error) {
+	b := &Book{
+		ID:        NewID(),
+		Title:     title,
+		Author:    author,
+		Pages:     pages,
+		Quantity:  quantity,
+		CreatedAt: time.Now(),
+	}
+	err := b.Validate()
+	if err != nil {
+		return nil, ErrInvalidEntity
+	}
+	return b, nil
+}
+
+//Validate validate book
+func (b *Book) Validate() error {
+	if b.Title == "" || b.Author == "" || b.Pages <= 0 || b.Quantity <= 0 {
+		return ErrInvalidEntity
+	}
+	return nil
+}