My solution to http://tour.golang.org/#71

a-tour-of-go-71.go

package main

import (
	"fmt"
)

type Fetcher interface {
	// Fetch returns the body of URL and
	// a slice of URLs found on that result.
	Fetch(url string) (body string, urls []string, err error)
}

// Crawl uses fetcher to recursively crawl
// results starting with url, to a maximum of depth.
func Crawl(url string, depth int, fetcher Fetcher) {
	result := make(chan Result)
	quit := make(chan int)
	fetchedUrls := make(map[string]bool)
	nowFetching := 0

	crawl := func (url string, depth int) {
		nowFetching += 1
		fetchedUrls[url] = true
		go fetch(url, depth, fetcher, result, quit)
	}

	crawl(url, depth)

	for {
		select {
		case res := <-result:
			if res.err != nil {
				fmt.Println(res.err)
				break
			}
			fmt.Printf("found: %s %q\n", res.url, res.body)
			for _, u := range res.urls {
				if !fetchedUrls[u] {
					crawl(u, res.depth - 1)
				}
			}
		case <-quit:
			if nowFetching -= 1; nowFetching == 0 {
				return
			}
		}
	}

	return
}

func fetch(url string, depth int, fetcher Fetcher,
	result chan Result, quit chan int) {
	if depth <= 0 {
		quit <- 0
		return
	}

	body, urls, err := fetcher.Fetch(url)
	result <- Result{url, body, depth, urls, err}
	quit <- 0
}

type Result struct {
	url   string
	body  string
	depth int
	urls  []string
	err   error
}

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/",
		},
	},
}