A Tour of Go Exercise: Web Crawler

exercise-web-crawler.go

package main

import (
	"fmt"
	"sync"
	"time"
)

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)
}

// Struct to handle results
type fetchedUrls struct {
	urls map[string]*fetchedUrl
	mux sync.Mutex
}

// Inner results url struct
type fetchedUrl struct {
	body string
	err error
}

// Add method to our fetched URLs
func (u *fetchedUrls) Add(url string, body string, err error) {
	u.mux.Lock()
	if _, ok := u.urls[url]; ok == false {
		u.urls[url] = &fetchedUrl{body: body, err: err}
	}
	u.mux.Unlock()
}

// Method to check if url is present in our result
func (u *fetchedUrls) Check(url string) error {
	u.mux.Lock()
	if _, ok := u.urls[url]; ok == true {
		defer u.mux.Unlock()
		return fmt.Errorf("Already processed: %s", url)
	}
	defer u.mux.Unlock()
	return nil
}

// Crawl also receives results variable
func Crawl(url string, depth int, fetcher Fetcher, fetched *fetchedUrls) {
	if depth <= 0 {
		return
	}
  // Check if we have url in our results
	if fetched.Check(url) != nil {
		return
	}
	body, urls, err := fetcher.Fetch(url)
	if err != nil {
    // Add error to results
		fetched.Add(url, "", err)
		return
	}
  // Add fetched url to results
	fetched.Add(url, body, nil)
	for _, u := range urls {
    // Perform another crawl asyncroniously
		go Crawl(u, depth-1, fetcher, fetched)
	}
	return
}

func main() {
	urls := fetchedUrls{urls: make(map[string]*fetchedUrl)}
  // Run crawler in go routine
	go Crawl("http://golang.org/", 4, fetcher, &urls)
  // Right now I don't know how to handle go routines results without sleep timer
	time.Sleep(time.Second)
  
  // Display results
	for k, url := range urls.urls {
		if url.err != nil {
			fmt.Println(url.err)
		} else {
			fmt.Printf("found %s %q\n", k, url.body)
		}
	}
}

// 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/",
		},
	},
}