package domain

import (
 "crypto/rand"
 "math/big"

 "github.com/pkg/errors"
)

// 关键词：用户、店铺、商品、订单
//
// 场景描述：店铺展示商品，其价格为P、库存为N，用户（余额为Y）看到商品觉得合适，于是下单购买B个；
// 购买前，用户余额Y必须不小于P，商品库存N不小于B；购买后，用户余额减少P，库存减少B；
//
// 先不考虑并发情况，建立此时的领域模型

type User struct {
 name  string // 名称
 phone string // 电话

 balance Money // 余额
}

type Shop struct {
 name string // 名称
 addr string // 地址
}

type Product struct {
 name  string // 名称
 price Money  // 价格
 stock int    // 库存

 ownShop *Shop // 所属商铺
}

type Order struct {
 name string // 名称

 user    *User    // 用户
 product *Product // 商品
}

type Money int

func NewUser(name, phone string, bal Money) *User {
 return &User{
  name:    name,
  phone:   phone,
  balance: bal,
 }
}
func (u *User) Balance() Money {
 return u.balance
}
func (u *User) DeductBalance(amount Money) {
 if u.balance < amount {
  panic("not enough money")
 }
 u.balance -= amount
}

func NewShop(name, addr string) *Shop {
 return &Shop{
  name: name,
  addr: addr,
 }
}
func NewProduct(name string, price Money, stock int, shop *Shop) *Product {
 return &Product{
  name:    name,
  price:   price,
  stock:   stock,
  ownShop: shop,
 }
}
func (p *Product) Stock() int {
 return p.stock
}
func (p *Product) DeductStock(c int) {
 if p.stock < c {
  panic("not enough stock")
 }
 p.stock -= c
}

// NewOrder 用户对商品下单c个
func NewOrder(user *User, product *Product, c int) *Order {
 name, err := GenerateRandomString(12)
 if err != nil {
  panic(err)
 }

 user.DeductBalance(product.price * Money(c))
 product.DeductStock(c)

 return &Order{
  name:    name,
  user:    user,
  product: product,
 }
}

func (o *Order) User() *User {
 return o.user
}

func (o *Order) Product() *Product {
 return o.product
}

// GenerateRandomString 随机字符串包含有数字和大小写字母
func GenerateRandomString(n int) (string, error) {
 const (
  letters = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
 )

 return generate(n, letters)
}

func generate(n int, letters string) (string, error) {
 ret := make([]byte, n)
 for i := 0; i < n; i++ {
  num, err := rand.Int(rand.Reader, big.NewInt(int64(len(letters))))
  if err != nil {
   return "", errors.WithStack(err)
  }
  ret[i] = letters[num.Int64()]
 }

 return string(ret), nil
}