121 lines
2.2 KiB
Go
121 lines
2.2 KiB
Go
package main
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import (
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"fmt"
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"sync"
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)
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type Fetcher interface {
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// Fetch returns the body of URL and
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// a slice of URLs found on that page.
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Fetch(url string) (body string, urls []string, err error)
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}
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type SafeCounter struct {
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mu sync.Mutex
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wg sync.WaitGroup
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visited map[string]struct{}
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}
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var (
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safeCounter *SafeCounter
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)
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// Crawl uses fetcher to recursively crawl
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// pages starting with url, to a maximum of depth.
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func Crawl(url string, depth int, fetcher Fetcher) {
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// TODO: Fetch URLs in parallel.
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// TODO: Don't fetch the same URL twice.
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// This implementation doesn't do either:
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defer safeCounter.wg.Done()
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safeCounter.mu.Lock()
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defer safeCounter.mu.Unlock()
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if url != "" {
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_, ok := safeCounter.visited[url]
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if ok {
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return
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}
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safeCounter.visited[url] = struct{}{}
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}
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if depth <= 0 {
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safeCounter.wg.Done()
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return
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}
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body, urls, err := fetcher.Fetch(url)
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if err != nil {
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fmt.Println(err)
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return
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}
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fmt.Printf("found: %s %q\n", url, body)
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for _, u := range urls {
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safeCounter.wg.Add(1)
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go Crawl(u, depth-1, fetcher)
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}
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return
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}
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func main() {
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safeCounter = &SafeCounter{
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visited: make(map[string]struct{}),
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}
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safeCounter.wg.Add(1)
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go Crawl("https://golang.org/", 4, fetcher)
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safeCounter.wg.Wait()
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}
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// fakeFetcher is Fetcher that returns canned results.
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type fakeFetcher map[string]*fakeResult
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type fakeResult struct {
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body string
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urls []string
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}
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func (f fakeFetcher) Fetch(url string) (string, []string, error) {
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if res, ok := f[url]; ok {
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return res.body, res.urls, nil
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}
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return "", nil, fmt.Errorf("not found: %s", url)
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}
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// fetcher is a populated fakeFetcher.
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var fetcher = fakeFetcher{
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"https://golang.org/": &fakeResult{
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"The Go Programming Language",
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[]string{
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"https://golang.org/pkg/",
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"https://golang.org/cmd/",
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},
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},
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"https://golang.org/pkg/": &fakeResult{
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"Packages",
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[]string{
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"https://golang.org/",
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"https://golang.org/cmd/",
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"https://golang.org/pkg/fmt/",
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"https://golang.org/pkg/os/",
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},
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},
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"https://golang.org/pkg/fmt/": &fakeResult{
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"Package fmt",
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[]string{
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"https://golang.org/",
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"https://golang.org/pkg/",
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},
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},
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"https://golang.org/pkg/os/": &fakeResult{
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"Package os",
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[]string{
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"https://golang.org/",
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"https://golang.org/pkg/",
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},
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},
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}
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