Created
March 29, 2020 19:15
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package main | |
import ( | |
"fmt" | |
"sync" | |
"time" | |
) | |
// SafeUrl is safe to use concurrently. | |
type SafeUrl struct { | |
v map[string]bool | |
mux sync.Mutex | |
} | |
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, | |
urls_map *SafeUrl) { | |
if depth <= 0 { | |
return | |
} | |
urls_map.mux.Lock() | |
if _, ok := urls_map.v[url]; ok { | |
urls_map.mux.Unlock() | |
return | |
} else { | |
urls_map.v[url] = true | |
urls_map.mux.Unlock() | |
} | |
body, urls, err := fetcher.Fetch(url) | |
if err != nil { | |
fmt.Println(err) | |
return | |
} | |
fmt.Printf("found: %s %q\n", url, body) | |
for _, u := range urls { | |
go Crawl(u, depth-1, fetcher, urls_map) | |
} | |
return | |
} | |
func main() { | |
urls_map := SafeUrl{v: make(map[string]bool)} | |
go Crawl("https://golang.org/", 4, fetcher, &urls_map) | |
// This is to make sure the goroutines complete, it would be better | |
// to use a wait group. | |
time.Sleep(time.Second) | |
for k, v := range urls_map.v { | |
fmt.Println(k, v) | |
} | |
} | |
// 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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