A Tour of Go - Exercise: Web Crawler

main.go

package main

// https://play.golang.org/p/hc-EOu8nYJ

import (
	"fmt"
	"sync"
)

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 OldCrawl(url string, depth int, fetcher Fetcher) {
	// TODO: Fetch URLs in parallel.
	// TODO: Don't fetch the same URL twice.
	// This implementation doesn't do either:
	if depth <= 0 {
		return
	}
	body, urls, err := fetcher.Fetch(url)
	if err != nil {
		fmt.Println(err)
		return
	}
	fmt.Printf("found: %s %q\n", url, body)
	for _, u := range urls {
		OldCrawl(u, depth-1, fetcher)
	}
	return
}

type SafeMap struct {
	done map[string]bool
	mux  sync.Mutex
}

func (m *SafeMap) Add(key string) {
	m.mux.Lock()
	defer m.mux.Unlock()
	m.done[key] = true
}

func (m *SafeMap) Contains(key string) bool {
	m.mux.Lock()
	defer m.mux.Unlock()
	return m.done[key]
}

func Crawl(url string, depth int, fetcher Fetcher, visited SafeMap) {
	// TODO: Fetch URLs in parallel.
	// TODO: Don't fetch the same URL twice.
	// This implementation doesn't do either:
	fmt.Printf("starting crawl %s %d\n", url, depth)
	if depth <= 0 {
		return
	}
	visited.Add(url)

	res := make(chan string)
	go func(out chan string) {
		defer close(res)
		_, urls, err := fetcher.Fetch(url)
		if err != nil {
			fmt.Println("error")
			return
		}
		for _, u := range urls {
			if visited.Contains(u) == false {
				out <- u
			}
		}
	}(res)

	var wg sync.WaitGroup
	defer wg.Wait()
	for u := range res {
		wg.Add(1)
		go func(next string) {
			Crawl(next, depth-1, fetcher, visited)
			wg.Done()
		}(u)
	}
	return
}

func OuterCrawl(url string, depth int, fetcher Fetcher) {
	s := SafeMap{done: make(map[string]bool)}
	Crawl(url, depth, fetcher, s)
}

func main() {
	OuterCrawl("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://gogogo/",
			"http://golang.org/cmd/",
		},
	},
	"http://golang.org/pkg/": &fakeResult{
		"Packages",
		[]string{
			"http://golang.org/",
			"http://gogogo/",
			"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/",
		},
	},
	"http://gogogo/": &fakeResult{
		"GoGoGo",
		[]string{
			"http://google.com/",
			"http://github.com/",
		},
	},
}