A solution to "A Tour of Go #73: Web Crawler"

go.tour.webcrawler.go

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)
}

// Keep track of already crawled urls.
var store map[string]bool

// An edge in the crawling tree is made of an
// url and the current depth of the edge.
type Edge struct {
	depth int
	url   string
}

// _crawl fetches a given edge, retrieve the next edges and 
// feeds them to the channel.
func _crawl(edge Edge, fetcher Fetcher, Edges chan []Edge) {
	url := edge.url
	body, urls, err := fetcher.Fetch(url)
	if err != nil {
		fmt.Println(err)
	} else {
		fmt.Printf("found: %s %q\n", url, body)
	}
	// Create edges from the new urls and the updated depth.
	edges := make([]Edge, len(urls))
	for i, u := range urls {
		edges[i] = Edge{edge.depth + 1, u}
	}
	// Feed the list of edges to the channel.
	Edges <- edges
}

func Crawl(url string, depth int, fetcher Fetcher) {
	// Channel of pending list of edges.
	Edges := make(chan []Edge)

	// Crawl level 0.
	go _crawl(Edge{0, url}, fetcher, Edges)
	store[url] = true

	// Crawl next levels.
	band := 1 // track the number of outstanding goroutines
	for band > 0 {
		band--
		next := <-Edges // next list of edges to process
		for _, edge := range next {
			// Process the edge if it is a new url and if the max depth
			// has not been reached yet.
			if _, seen := store[edge.url]; !seen && edge.depth < depth {
				store[edge.url] = true
				band++
				go _crawl(edge, fetcher, Edges)
			}
		}
	}
}

func main() {
	store = make(map[string]bool)
	Crawl("http://golang.org/", 3, 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/woot",
		},
	},
	"http://golang.org/pkg/os/": &fakeResult{
		"Package os",
		[]string{
			"http://golang.org/",
			"http://golang.org/pkg/",
		},
	},
	// Add more examples to verify that the depth
	// limit is indeed enforced
	"http://golang.org/pkg/woot": &fakeResult{
		"Woot Woot!",
		[]string{
			"http://golang.org/",
			"http://golang.org/pkg/bla",
		},
	},
	"http://golang.org/pkg/bla": &fakeResult{
		"Bla Bla",
		[]string{
			"http://golang.org/",
		},
	},
}