My solution to Go Tour #70 - Web Crawler

gotour-70.go

package main

import (
	"fmt"
	"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)
}

type Todo struct {
	url   string
	depth int
}

func getPage(url string, depth int, todolist chan Todo, live chan int) {
	body, urls, err := fetcher.Fetch(url)
	if err != nil {
		fmt.Println(err)
		live <- -1
		return
	}
	fmt.Printf("found: %s %q\n", url, body)
	for _, u := range urls {
		todolist <- Todo{u, depth + 1}
	}
	live <- -1
}

func Crawl(url string, depth int, fetcher Fetcher) {
	todolist := make(chan Todo, 1)
	live := make(chan int)
	livecount := 0
	visited := make(map[string]bool)

	todolist <- Todo{url, 0}

	for {
		select {
		case todo := <-todolist:
			if todo.depth <= depth && !visited[todo.url] {
				livecount++
				visited[todo.url] = true
				go getPage(todo.url, todo.depth, todolist, live)
			}
		default:
			select {
			case c := <-live:
				livecount += c
			default:
				time.Sleep(1)
			}
		}
		if livecount == 0 {
			break
		}
	}
}

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/",
		},
	},
}

xoutput.txt

found: http://golang.org/ "The Go Programming Language"
found: http://golang.org/pkg/ "Packages"
found: http://golang.org/pkg/fmt/ "Package fmt"
found: http://golang.org/pkg/os/ "Package os"
not found: http://golang.org/cmd/