Golang Beginner's goroutine---web crawler

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Go Tour Practice HTTPS://TOUR.GO-ZH.ORG/CONCURRENCY/10

Package Mainimport ("FMT" "Sync" "Time") type Fetcher interface {//Fetch returns the body content of the URL and places the URL found on this page into a slice. Fetch (URL string) (body string, URLs []string, err Error)}//Safecounter is safe for concurrent use. Type safeurlmap struct {v map[string]boolmux sync. Mutex}func (c *safeurlmap) Put (key string) {C.mux.lock () C.v[key] = Truec.mux.Unlock ()}func (c *safeurlmap) Contains (key s  Tring) bool {c.mux.lock () defer C.mux.unlock () _, OK: = C.v[key]return ok}type Resp struct {URL stringbody string}var urlmap *safeurlmap = &safeurlmap{v:make (map[string]bool)}//Crawl uses Fetcher to crawl pages recursively from a URL until the maximum depth is reached. Func Crawl (URL string, depth int, fetcher fetcher, ch Chan Resp) {if depth <= 0 {return}urlmap.put (URL) body, URLs, err : = Fetcher. Fetch (URL) if err! = Nil {fmt. PRINTLN (Err) return}ch <-Resp{url:url, Body:body}for _, U: = Range URLs {if urlmap.contains (u) {fmt. PRINTF ("has processed:%s\n", u) continue}go Crawl (U, depth-1, Fetcher, ch)}return}func main () {ch: = make (chan Resp) go Cr Awl ("Http://golang.org/", 4, Fetcher, ch) Boom: = time. After (3 * time. Second) for {select {case r: = <-ch:fmt. Printf ("Found:%s%q\n", R.url, r.body) boom = time. After (3 * time. Second) Case <-boom:fmt. Printf ("Time out\n") return}}}//Fakefetcher is a fetcher that returns several results. Type Fakefetcher map[string]*fakeresulttype fakeresult struct {body Stringurls []string}func (f fakefetcher) Fetch (URL St Ring) (String, []string, error) {If res, OK: = F[url]; OK {return res.body, Res.urls, Nil}return "", Nil, FMT. Errorf ("Not Found:%s", url)}//Fetcher is a populated fakefetcher. var fetcher = fakefetcher{"http://golang.org/": &fakeresult{"the Go programming Language", []string{]/HTTP/ golang.org/pkg/"," http://golang.org/cmd/",},}," http://golang.org/pkg/": &fakeresult{" Packages ", []string{] http://golang.org/"," http://golang.org/cmd/"," http://golang.org/pkg/fmt/"," http://golang.org/pkg/os/",},}," http://golang.org/pkg/fmt/": &fakeresult{" Package Fmt ", []string{" http://golang.org/"," http://golang.org/pkg/ ",},}," HTTP://GOLANG.Org/pkg/os/": &fakeresult{" Package OS ", []string{" http://golang.org/"," http://golang.org/pkg/",},},}