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My (first) answer to "The Final Problem": https://tour.golang.org/concurrency/9
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| package main | |
| import ( | |
| "fmt" | |
| ) | |
| type Fetcher interface { | |
| // Fetch returns the body of URL and | |
| // a slice of URLs found on that page. | |
| Fetch(url string) (body string, urls []string, err error) | |
| } | |
| type Accessor interface { | |
| Get(key string) (in bool) | |
| Set(key string, value interface{}) | |
| } | |
| type lookupTable struct { | |
| table map[string]bool | |
| sem chan bool | |
| } | |
| func (t *lookupTable) Get(key string) (in bool) { | |
| defer func() { | |
| t.sem <- true | |
| }() | |
| <-t.sem | |
| if _, present := t.table[key]; present { | |
| in = true | |
| } else { | |
| in = false | |
| } | |
| return | |
| } | |
| func (t *lookupTable) Set(key string, value interface{}) { | |
| defer func() { | |
| t.sem <- true | |
| }() | |
| <-t.sem | |
| t.table[key] = value.(bool) | |
| return | |
| } | |
| // Crawl uses fetcher to recursively crawl | |
| // pages starting with url, to a maximum of depth. | |
| func Crawl(url string, depth int, accessor Accessor, fetcher Fetcher, | |
| sem chan bool) { | |
| defer func() { | |
| sem <- true | |
| }() | |
| if depth <= 0 { | |
| return | |
| } | |
| if accessor.Get(url) { | |
| return | |
| } | |
| body, urls, err := fetcher.Fetch(url) | |
| if err != nil { | |
| fmt.Println(err) | |
| return | |
| } | |
| accessor.Set(url, true) | |
| fmt.Printf("found: %s %q\n", url, body) | |
| var sems []chan bool | |
| for _, u := range urls { | |
| sem := make(chan bool, 1) | |
| go Crawl(u, depth-1, accessor, fetcher, sem) | |
| sems = append(sems, sem) | |
| } | |
| for _, v := range sems { | |
| <-v | |
| } | |
| return | |
| } | |
| func main() { | |
| sem := make(chan bool) | |
| urlLookup := &lookupTable{ | |
| table: make(map[string]bool), | |
| sem: make(chan bool, 1), | |
| } | |
| urlLookup.sem <- true | |
| go Crawl("http://golang.org/", 4, urlLookup, fetcher, sem) | |
| <-sem | |
| } | |
| // fakeFetcher is Fetcher that returns canned results. | |
| type fakeFetcher map[string]*fakeResult | |
| type fakeResult struct { | |
| body string | |
| urls []string | |
| } | |
| func (f fakeFetcher) Fetch(url string) (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/", | |
| }, | |
| }, | |
| } |
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