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A solution for the Web Crawler concurrency exercise in the Tour of Go (https://go.dev/tour/concurrency/10)
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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) | |
} | |
type History struct { | |
mu sync.Mutex | |
set map[string]struct{} | |
} | |
func (hist *History) HasSeen(url string) bool { | |
hist.mu.Lock() | |
_, seen := hist.set[url] | |
hist.set[url] = struct{}{} | |
hist.mu.Unlock() | |
return seen | |
} | |
var hist History = History{set: make(map[string]struct{})} | |
// Crawl uses fetcher to recursively crawl | |
// pages starting with url, to a maximum of depth. | |
func Crawl(url string, depth int, fetcher Fetcher, ret chan string) { | |
defer close(ret) | |
if depth <= 0 || hist.HasSeen(url) { | |
return | |
} | |
body, urls, err := fetcher.Fetch(url) | |
if err != nil { | |
ret <-err.Error() | |
return | |
} | |
ret <-fmt.Sprintf("found: %s %q", url, body) | |
result := make([]chan string, len(urls)) | |
for i, u := range urls { | |
result[i] = make(chan string) | |
go Crawl(u, depth-1, fetcher, result[i]) | |
} | |
for i := range result { | |
for s := range result[i] { | |
ret <-s | |
} | |
} | |
} | |
func main() { | |
result := make(chan string) | |
go Crawl("https://golang.org/", 4, fetcher, result) | |
for s := range result { | |
fmt.Println(s) | |
} | |
} | |
// 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{ | |
"https://golang.org/": &fakeResult{ | |
"The Go Programming Language", | |
[]string{ | |
"https://golang.org/pkg/", | |
"https://golang.org/cmd/", | |
}, | |
}, | |
"https://golang.org/pkg/": &fakeResult{ | |
"Packages", | |
[]string{ | |
"https://golang.org/", | |
"https://golang.org/cmd/", | |
"https://golang.org/pkg/fmt/", | |
"https://golang.org/pkg/os/", | |
}, | |
}, | |
"https://golang.org/pkg/fmt/": &fakeResult{ | |
"Package fmt", | |
[]string{ | |
"https://golang.org/", | |
"https://golang.org/pkg/", | |
}, | |
}, | |
"https://golang.org/pkg/os/": &fakeResult{ | |
"Package os", | |
[]string{ | |
"https://golang.org/", | |
"https://golang.org/pkg/", | |
}, | |
}, | |
} |
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