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@davidcrawford
Created February 22, 2015 20:22
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Golang Tour of Go: Web Crawler
package main
import (
"fmt"
)
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 FetchResult struct {
Url string
Body string
Urls []string
Depth int
Error 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)
fetchAsync := func(url string, depth int, ch chan *FetchResult) {
body, urls, err := fetcher.Fetch(url)
ch <- &FetchResult{url, body, urls, depth, err}
}
resultChan := make(chan *FetchResult)
fetched[url] = true
go fetchAsync(url, 1, resultChan)
waitingFor := 1
for waitingFor > 0 {
result := <-resultChan
waitingFor--
if result.Error != nil {
fmt.Println(result.Error)
continue
}
fmt.Printf("Found %s %q\n", result.Url, result.Body)
if result.Depth < depth {
for _, url := range result.Urls {
if !fetched[url] {
fetched[url] = true
go fetchAsync(url, result.Depth + 1, resultChan)
waitingFor++
}
}
}
}
}
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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