tour71.go

package main

import (
	"fmt"
	"sync"
)

var workerSize = 4

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 task struct {
	url   string
	depth int
}

type cache struct {
	m map[string]bool
	mu sync.Mutex
}

func (c *cache) visited(url string) bool {
	c.mu.Lock()
	defer c.mu.Unlock()
	if _, visited := c.m[url]; visited {
		return true
	}
	c.m[url] = true
	return false
}

func fetchTask(url string, depth int, tasks chan task, wg *sync.WaitGroup) {
	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 {
		wg.Add(1)
		tasks <- task{u, depth - 1}
	}
}

func wakeupWorker(c *cache, tasks chan task, wg *sync.WaitGroup) {
	for t := range tasks {
		if !c.visited(t.url) {
			fetchTask(t.url, t.depth, tasks, wg)
		}
		wg.Done()
	}
}

// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func Crawl(url string, depth int, fetcher Fetcher) {
	tasks := make(chan task, workerSize)
	// Shutdown workers
	defer close(tasks)

	var wg sync.WaitGroup
	cache := &cache{m:map[string]bool{url: true}}
	// Create worker pool
	for i := 0; i < workerSize; i++ {
		go wakeupWorker(cache, tasks, &wg)
	}

	// Run the first fetch task
	fetchTask(url, depth, tasks, &wg)

	wg.Wait()
}

func main() {
	Crawl("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://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/",
		},
	},
}