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January 5, 2016 22:12
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My (second) 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 request struct { | |
| url string | |
| depth int | |
| } | |
| // Crawl uses fetcher to recursively crawl | |
| // pages starting with url, to a maximum of depth. | |
| func Fetch(req request, fetcher Fetcher, queue chan []request, | |
| finished chan bool) { | |
| defer func() { | |
| finished <- true | |
| }() | |
| var links []request | |
| body, urls, err := fetcher.Fetch(req.url) | |
| if err != nil { | |
| fmt.Println(err) | |
| return | |
| } | |
| fmt.Printf("found: %s %q\n", req.url, body) | |
| for _, u := range urls { | |
| link := request{ | |
| url: u, | |
| depth: req.depth + 1, | |
| } | |
| links = append(links, link) | |
| } | |
| queue <- links | |
| return | |
| } | |
| func Crawl(url string, depth int) { | |
| queue := make(chan []request) | |
| finished := make(chan bool) | |
| lookup := make(map[string]bool) | |
| seed := request{ | |
| url: url, | |
| depth: 1, | |
| } | |
| go Fetch(seed, fetcher, queue, finished) | |
| lookup[seed.url] = true | |
| outstanding := 1 | |
| for outstanding > 0 { | |
| select { | |
| case newRequests := <-queue: | |
| for _, r := range newRequests { | |
| if _, ok := lookup[r.url]; ok || r.depth >= depth { | |
| continue | |
| } else { | |
| go Fetch(r, fetcher, queue, finished) | |
| outstanding++ | |
| } | |
| } | |
| case <-finished: | |
| outstanding-- | |
| } | |
| } | |
| } | |
| func main() { | |
| Crawl("http://golang.org/", 4) | |
| } | |
| // 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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