import (
    "fmt"
    "runtime"
    "sync"
    "time"
)

// Pool Goroutine Pool
type Pool struct {
    queue chan int
    wg    *sync.WaitGroup
}

// New 新建一個協程池
func NewPool(size int) *Pool {
    if size <= 0 {
        size = 1
    }
    return &Pool{
        queue: make(chan int, size),
        wg:    
 
&sync.WaitGroup{},
    }
}

// Add 新增一個執行
func (p *Pool) Add(delta int) {
    // delta為正數就新增
    for i := 0; i < delta; i++ {
        p.queue <- 1
    }
    // delta為負數就減少
    for i := 0; i > delta; i-- {
        <-p.queue
    }
    p.wg.Add(delta)
}

// Done 執行完成減一
func (p *Pool) Done(i int) {
 
    <-p.queue
    p.wg.Done()
}

// Wait 等待Goroutine執行完畢
func (p *Pool) Wait() {
    p.wg.Wait()
}

func Run() {
    pool := NewPool(2)
    fmt.Println("the NumGoroutine begin is:", runtime.NumGoroutine())
    for i := 0; i < 6; i++ {
        pool.Add(1)
        go func(i int) {
            time.Sleep(
 
2*time.Second)
            fmt.Println("the NumGoroutine continue is:", i, runtime.NumGoroutine())
            pool.Done(i)
        }(i)
    }
    pool.Wait()
    fmt.Println("the NumGoroutine done is:", runtime.NumGoroutine())
}