tour_of_go_71.go

package main

import (
	"fmt"
	"sync"
)

type Page struct {
	url, body string
}

type Finished struct{}

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)
}

type SyncURLs struct {
	mutex *sync.Mutex
	urls  map[string]bool
}

func (self *SyncURLs) contains(url string) bool {
	_, ok := self.urls[url]
	return ok
}

func (self *SyncURLs) add(url string) {
	defer self.mutex.Unlock()
	self.mutex.Lock()
	self.urls[url] = true
}

// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func Crawl(url string, depth int, fetcher Fetcher, ch chan Page, quit chan Finished, knownUrls SyncURLs) {

	// Notify I've completed
	defer func() { quit <- Finished{} }()

	// TODO: Don't fetch the same URL twice.
	if depth <= 0 {
		return
	}

	if knownUrls.contains(url) {
		return
	}
	knownUrls.add(url)

	// Fetch URL
	body, urls, err := fetcher.Fetch(url)
	if err != nil {
		fmt.Println(err)
		return
	}

	// Scatter goroutines
	ch <- Page{url, body}
	finished := make(chan Finished)
	for _, u := range urls {
		go Crawl(u, depth-1, fetcher, ch, finished, knownUrls)
	}

	// Wait for all children completing
	// TODO: use channel or WaitGroup()
	for i := 0; i < len(urls); i++ {
		<-finished
	}

	return
}

func main() {
	ch, finished := make(chan Page), make(chan Finished)
	go Crawl("http://golang.org/", 4, fetcher, ch, finished, SyncURLs{new(sync.Mutex), map[string]bool{}})
	// var result Page
	for {
		select {
		case result := <-ch:
			fmt.Printf("found: %s %s\n", result.url, result.body)
		case <-finished:
			return
		}
	}
}

// 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/",
		},
	},
}