add strategy

15_strategy/README.md

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+# 策略模式
+
+定义一系列算法，让这些算法在运行时可以互换，使得分离算法，符合开闭原则。
+

15_strategy/strategy.go

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+package strategy
+
+import "fmt"
+
+type PaymentContext struct {
+	Name, CardID string
+	Money        int
+	payment      PaymentStrategy
+}
+
+func NewPaymentContext(name, cardid string, money int, payment PaymentStrategy) *PaymentContext {
+	return &PaymentContext{
+		Name:    name,
+		CardID:  cardid,
+		Money:   money,
+		payment: payment,
+	}
+}
+
+func (p *PaymentContext) Pay() {
+	p.payment.Pay(p)
+}
+
+type PaymentStrategy interface {
+	Pay(*PaymentContext)
+}
+
+type Cash struct{}
+
+func (*Cash) Pay(ctx *PaymentContext) {
+	fmt.Printf("Pay $%d to %s by cash", ctx.Money, ctx.Name)
+}
+
+type Bank struct{}
+
+func (*Bank) Pay(ctx *PaymentContext) {
+	fmt.Printf("Pay $%d to %s by bank account %s", ctx.Money, ctx.Name, ctx.CardID)
+
+}

15_strategy/strategy_test.go

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+package strategy
+
+func ExamplePayByCash() {
+	ctx := NewPaymentContext("Ada", "", 123, &Cash{})
+	ctx.Pay()
+	// Output:
+	// Pay $123 to Ada by cash
+}
+
+func ExamplePayByBank() {
+	ctx := NewPaymentContext("Bob", "0002", 888, &Bank{})
+	ctx.Pay()
+	// Output:
+	// Pay $888 to Bob by bank account 0002
+}