Go Tour Exercise: Web Crawler

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)
}

// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func Crawl(url string, depth int, fetcher Fetcher, end chan bool) {
    //channel to exchange urls been processed between go routines
	used_urls := make(chan map[string]bool)
	//channnel to exchange urls been crawling or crawled between go routines
	crawled_urls := make(chan map[string]bool)
	//map to searlized transmit urls which are processed
	used_urls_map := map[string]bool{url: true}
	//map to searlized transmmit urls which are processing or processed
	//true for processed, false for processing
	crawled_urls_map := map[string]bool{url: false}
	go _Crawl(url, depth, fetcher, used_urls, crawled_urls, end)
	//send map to channel crawled first or goroutines will fail, because code in line 31
	crawled_urls <- crawled_urls_map
	used_urls <- used_urls_map
	return
}

func _Crawl(url string, depth int, fetcher Fetcher, used_urls chan map[string]bool, crawled_urls chan map[string]bool, end chan bool) {
	crawled_urls_map := <-crawled_urls

	if depth <= 0 {
		crawled_urls_map[url] = true
		return
	}

	body, urls, err := fetcher.Fetch(url)

	used_urls_map := <-used_urls
	used_urls_map[url] = true

	if err != nil {
		fmt.Printf("not found: %s\n", url)
		crawled_urls_map[url] = true
		return
	}

	fmt.Printf("found: %s %q\n", url, body)

	crawled_urls_map[url] = true

	for _, u := range urls {
		if _, ok := used_urls_map[u]; !ok {
			used_urls_map[u] = true
			crawled_urls_map[u] = false
			go _Crawl(u, depth-1, fetcher, used_urls, crawled_urls, end)
			crawled_urls <- crawled_urls_map
			used_urls <- used_urls_map
		}
	}

	all_checked := true

	for u, _ := range crawled_urls_map {
		all_checked = all_checked && crawled_urls_map[u]
	}

	if all_checked {
		end <- true
	}
}

func main() {
	end := make(chan bool, 1)
	Crawl("http://golang.org/", 4, fetcher, end)
	<-end
}

// 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/",
		},
	},
}