import (
    "flag"
    "fmt"
    "io/ioutil"
    "os"
    "path/filepath"
    "sync"
    "time"
)
//一个已经被关闭的channel不会阻塞，已经被关闭的channel会实时返回
//goroutine退出，关闭done来进行广播
var done = make(chan struct{})
//判断done是否关闭，即是否执行goroutine退出
func cancelled() bool {
    select {
    case <-done:
        return true
    default:
        return false
    }
}
//获取目录dir下的文件大小
func walkDir(dir string, wg *sync.WaitGroup, fileSizes chan<- int64) {
    defer wg.Done()
    if cancelled() {
        return
    }
    for _, entry := range dirents(dir) {
        if entry.IsDir() { //目录
            wg.Add(1)
            subDir := filepath.Join(dir, entry.Name())
            go walkDir(subDir, wg, fileSizes)
        } else {
            fileSizes <- entry.Size()
        }
    }
}
//sema is a counting semaphore for limiting concurrency in dirents
var sema = make(chan struct{}, 20)
//读取目录dir下的文件信息
func dirents(dir string) []os.FileInfo {
    select {
    case sema <- struct{}{}: //acquire token
    case <-done:
        return  nil // cancelled
    }
    defer func() { <-sema }() //release token
    entries, err := ioutil.ReadDir(dir)
    if err != nil {
        fmt.Fprintf(os.Stderr, "du: %v\n", err)
        return nil
    }
    return entries
}
//输出文件数量的大小
func printDiskUsage(nfiles, nbytes int64) {
    fmt.Printf("%d files %.1f GB\n", nfiles, float64(nbytes)/1e9)
}
//提供-v 参数会显示程序进度信息
var verbose = flag.Bool("v", false, "show verbose progress messages")
func Start() {
    flag.Parse()
    roots := flag.Args() //需要统计的目录
    if len(roots) == 0 {
        roots = []string{"."}
    }
    fileSizes := make(chan int64, 5)
    var wg sync.WaitGroup
    for _, root := range roots {
        wg.Add(1)
        go walkDir(root, &wg, fileSizes)
    }
    go func() {
        os.Stdin.Read(make([]byte, 1)) //从标准输入读取一个字符，执行goroutine退出
        close(done)
    }()
    go func() {
        wg.Wait() //等待goroutine结束
        close(fileSizes)
    }()
    var tick <-chan time.Time
    if *verbose {
        tick = time.Tick(100 * time.Millisecond) //输出时间间隔
    }
    var nfiles, nbytes int64
loop:
    for {
        select {
        case <-done:
                //to allow existing goroutines to finish
            for range fileSizes { //fileSizes关闭时，for循环会自动结束
                //Do nothing
            }
            return
        case size, ok := <-fileSizes:
            if !ok {
                break loop
            }
            nfiles++
            nbytes += size
        case <-tick:
            printDiskUsage(nfiles, nbytes)
        }
    }
    printDiskUsage(nfiles, nbytes)
}