package main 
import (
	"fmt"
	"time"
	"sync"
	"sync/atomic"
)
 
func main() {
	test1()
	test2()
}
 
func test1() {
	var wg sync.WaitGroup
	count := 0
	t := time.Now()
	for i := 0 ; i < 50000 ; i++ {
		wg.Add(1)
		go func(wg *sync.WaitGroup,i int) {
			count++ //count不是并发安全的
			wg.Done()
		}(&wg,i)
	}
 
	wg.Wait()
	fmt.Println(time.Now().Sub(t))
	fmt.Println("count====>",count) //count的值<50000
	fmt.Println("exit")
} 
 
func test2() {
	var wg sync.WaitGroup
	count := int64(0)
	t := time.Now()
	for i := 0 ; i < 50000 ; i++ {
		wg.Add(1)
		go func(wg *sync.WaitGroup,i int) {
			atomic.AddInt64(&count,1) //原子操作
			wg.Done()
		}(&wg,i)
	}
 
	wg.Wait()
	fmt.Println(time.Now().Sub(t))
	fmt.Println("count====>",count) //count的值为50000
	fmt.Println("exit")
}

18.0485ms
count====> 46621
exit
16.0418ms
count====> 50000
exit

var (
sema = make(chan struct{}, 1) // a binary semaphore guarding balance
balance int
)
func Deposit(amount int) {
sema <- struct{}{} // acquire token
balance = balance + amount
<-sema // release token
}
func Balance() int {
sema <- struct{}{} // acquire token
b := balance
<-sema // release token
return b
}

import "sync"
var (
mu sync.Mutex // guards balance
balance int
)
func Deposit(amount int) {
mu.Lock()
balance = balance + amount
mu.Unlock()
}
func Balance() int {
mu.Lock()
b := balance
mu.Unlock()
return b
}

var x, y int
go func() {
x = 1 // A1
fmt.Print("y:", y, " ") // A2
}()
go func() {
y = 1 // B1
fmt.Print("x:", x, " ") // B2
}()