package main

import (
    "fmt"
    "sync"
)

// 三个协程交替打印 cat dog fish
var repeatCount = 100

func main() {
    // wg 用来防止主协程提前先退出
    wg := &sync.WaitGroup{}
    wg.Add(3)

    chCat := make(chan struct{}, 1)
    defer close(chCat)
    chDog := make(chan struct{}, 1)
    defer close(chDog)
    chFish := make(chan struct{}, 1)
    defer close(chFish)

    go printAnimal(chCat, chDog, "cat", wg)
    go printAnimal(chDog, chFish, "dog", wg)
    go printAnimal(chFish, chCat, "fish", wg)
    chCat <- struct{}{}
    wg.Wait()
}

// wg 需要传指针
func printAnimal(in, out chan struct{}, s string, wg *sync.WaitGroup) {
    count := 0
    for {
        <-in
        count++
        fmt.Println(s)
        out <- struct{}{}

        if count >= repeatCount {
            wg.Done()
            return
        }

    }
}