Created
October 12, 2016 01:05
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A Tour of Go Exercise: Web Crawler
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package main | |
import ( | |
"fmt" | |
"sync" | |
) | |
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) | |
} | |
// Crawl uses fetcher to recursively crawl | |
// pages starting with url, to a maximum of depth. | |
func Crawl(url string, depth int, fetcher Fetcher) { | |
fetched := make(map[string]bool) | |
crawl(url, depth, fetcher, fetched) | |
return | |
} | |
func crawl(url string, depth int, fetcher Fetcher, fetched map[string]bool) { | |
if depth <= 0 { | |
return | |
} | |
if v, ok := fetched[url]; ok && v { | |
return | |
} | |
body, urls, err := fetcher.Fetch(url) | |
if err != nil { | |
fmt.Println(err) | |
return | |
} | |
fmt.Printf("found: %s %q\n", url, body) | |
fetched[url] = true | |
var wg sync.WaitGroup | |
for _, u := range urls { | |
wg.Add(1) | |
go func(u string) { | |
defer wg.Done() | |
crawl(u, depth-1, fetcher, fetched) | |
}(u) | |
} | |
wg.Wait() | |
return | |
} | |
func main() { | |
Crawl("http://golang.org/", 4, fetcher) | |
} | |
// 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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Hi,
I have a question about the code:
What guaranties you that the map
fetched
won't be accessed conccurently and be corrupted ?Plus I think your variable
var wg sync.WaitGroup
will be redeclared everytimecrawl
is called, even though it won't cause any problem but I think your intent was maybe to use the samewg
variable, as it's name suggest it is for waiting for a group of related routines to end their work not for creating a waitGroup for each routine.