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A Tour of Go: Exercise: Web Crawler, https://go-tour-jp.appspot.com/concurrency/10
package main
import (
"fmt"
"sync"
)
type FetchedUrl struct {
url map[string]bool
mux sync.Mutex
}
func (f *FetchedUrl) Add(url string) {
f.mux.Lock()
f.url[url] = true
f.mux.Unlock()
}
func (f *FetchedUrl) Exists(url string) bool {
f.mux.Lock()
defer f.mux.Unlock()
return f.url[url]
}
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, fetchedUrl FetchedUrl) {
// TODO: Fetch URLs in parallel.
// TODO: Don't fetch the same URL twice.
// This implementation doesn't do either:
if fetchedUrl.Exists(url) {
return
}
if depth <= 0 {
return
}
ch := make(chan []string)
go Fetch(url, ch)
fetchedUrl.Add(url)
urls := <-ch
for _, u := range urls {
Crawl(u, depth-1, fetcher, fetchedUrl)
}
return
}
func Fetch(url string, ch chan []string) {
body, urls, err := fetcher.Fetch(url)
if err != nil {
fmt.Println(err)
close(ch)
return
}
fmt.Printf("found: %s %q\n", url, body)
ch <- urls
close(ch)
}
func main() {
fetchedUrl := FetchedUrl{url: make(map[string]bool)}
Crawl("http://golang.org/", 4, fetcher, fetchedUrl)
}
// 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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