A Tour of Go: Exercise: Web Crawler without Mutex (https://tour.golang.org/concurrency/10)

go-tour-web-crawler-without-mutex.go

package main

import (
	"fmt"
)

type Fetcher interface {
	Fetch(url string) (body string, urls []string, err error)
}

func main() {
	cache := make(map[string]bool)
	ch_queue := make(chan string, 10)
	ch_result := make(chan string, 10)
	num_running := 0

	ch_queue <- "https://golang.org/"
	for {
		select {
		case url := <-ch_queue:
			if _, ok := cache[url]; !ok {
				cache[url] = true
				num_running += 1
				go func(url string) {
					body, urls, err := fetcher.Fetch(url)
					if err == nil {
						for _, u := range urls {
							ch_queue <- u
						}
						ch_result <- fmt.Sprintf("found: %s %q\n", url, body)
					} else {
						ch_result <- fmt.Sprintln(err)
					}
				}(url)
			}
		case res := <-ch_result:
			num_running -= 1
			fmt.Println(res)
		}
		if num_running == 0 && len(ch_result) == 0 && len(ch_queue) == 0 {
			return
		}
	}
}

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)
}

var fetcher = fakeFetcher{
	"https://golang.org/": &fakeResult{
		"The Go Programming Language",
		[]string{
			"https://golang.org/pkg/",
			"https://golang.org/cmd/",
		},
	},
	"https://golang.org/pkg/": &fakeResult{
		"Packages",
		[]string{
			"https://golang.org/",
			"https://golang.org/cmd/",
			"https://golang.org/pkg/fmt/",
			"https://golang.org/pkg/os/",
		},
	},
	"https://golang.org/pkg/fmt/": &fakeResult{
		"Package fmt",
		[]string{
			"https://golang.org/",
			"https://golang.org/pkg/",
		},
	},
	"https://golang.org/pkg/os/": &fakeResult{
		"Package os",
		[]string{
			"https://golang.org/",
			"https://golang.org/pkg/",
		},
	},
}