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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) | |
} | |
type Cache struct { | |
visited map[string]bool | |
mux sync.Mutex | |
} | |
// Crawl uses fetcher to recursively crawl | |
// pages starting with url, to a maximum of depth. | |
func Crawl(url string, depth int, fetcher Fetcher, | |
ch chan response, cache Cache) { | |
defer close(ch) | |
if depth <= 0 { | |
return | |
} | |
cache.mux.Lock() | |
if cache.visited[url] { | |
cache.mux.Unlock() | |
return | |
} | |
cache.visited[url] = true | |
cache.mux.Unlock() | |
body, urls, err := fetcher.Fetch(url) | |
if err != nil { | |
fmt.Println(err) | |
return | |
} | |
ch <- response{url, body} | |
result := make([]chan response, len(urls)) | |
for i, u := range urls { | |
result[i] = make(chan response) | |
go Crawl(u, depth-1, fetcher, result[i], cache) | |
} | |
for i := range result { | |
for resp := range result[i] { | |
ch <- resp | |
} | |
} | |
return | |
} | |
func main() { | |
var ch = make(chan response) | |
go Crawl("http://golang.org/", 4, fetcher, ch, | |
cache, Cache{visited: make(map[string] bool)}) | |
for resp := range ch { | |
fmt.Printf("found: %s %q\n", resp.url, resp.body) | |
} | |
} | |
type response struct { | |
url string | |
body string | |
} | |
// 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/", | |
}, | |
}, | |
} |
I ended up writing something similar but I suppose it has a clearer logic. The above code looks correct to me through.
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, c chan bool) {
defer close(c)
if depth <= 0 {
return
}
mtx.Lock()
found := cache[url]
mtx.Unlock()
if found {
return
}
mtx.Lock()
cache[url] = true
mtx.Unlock()
body, urls, err := fetcher.Fetch(url)
if err != nil {
fmt.Println(err)
return
}
fmt.Printf("found: %s %q\n", url, body)
var chans []chan bool
for i, u := range urls {
chans = append(chans, make(chan bool))
go Crawl(u, depth-1, fetcher, chans[i])
}
for _, cc := range chans {
<- cc
}
return
}
func main() {
c := make(chan bool)
go Crawl("https://golang.org/", 4, fetcher, c)
<- c
}
// 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)
}
var mtx sync.Mutex
var cache = make(map[string] bool)
// 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/",
},
},
}
with highlight
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, c chan bool) {
defer close(c)
if depth <= 0 {
return
}
mtx.Lock()
found := cache[url]
mtx.Unlock()
if found {
return
}
mtx.Lock()
cache[url] = true
mtx.Unlock()
body, urls, err := fetcher.Fetch(url)
if err != nil {
fmt.Println(err)
return
}
fmt.Printf("found: %s %q\n", url, body)
var chans []chan bool
for i, u := range urls {
chans = append(chans, make(chan bool))
go Crawl(u, depth-1, fetcher, chans[i])
}
for _, cc := range chans {
<- cc
}
return
}
func main() {
c := make(chan bool)
go Crawl("https://golang.org/", 4, fetcher, c)
<- c
}
// 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)
}
var mtx sync.Mutex
var cache = make(map[string] bool)
// 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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@cedarmora But in fact the channels passed to each recursive calls are distinct, made from make([]chan response). There will not be any channel closed twice.