Solution to Exercise: Web Crawler (https://tour.golang.org/concurrency/10)

exercise-web-crawler.go

package main

import (
	"fmt"
	"sync"
	"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 FetcherState int

const (
	RUNNING FetcherState = 1
	DONE FetcherState = 2
	ERROR FetcherState = 3
)

type SafeCache struct {
	lock sync.Mutex
	cache map[string]bool
}

func NewSafeCache() SafeCache {
	return SafeCache{cache: make(map[string]bool)}
}

func (sc *SafeCache) trySet (url string) bool {
	sc.lock.Lock()
	defer sc.lock.Unlock()
	
	if sc.cache[url] == true {
		return false
	} else {
		sc.cache[url] = true
		return true
	}
}

func doCrawl(url string, depth int, fetcher Fetcher, ch chan FetcherState, sc *SafeCache) {	
	if !sc.trySet(url) {
		fmt.Printf("%v already crawled\n", url)
		return
	}
	
	if depth <= 0 {
		fmt.Println("reach depth limit")
		return
	}
	
	ch <- RUNNING
	
	body, urls, err := fetcher.Fetch(url)
	if err != nil {
		fmt.Printf("fetch failed: %v\n", err)
		ch <- ERROR
		return
	}

	fmt.Printf("found: %s %q\n", url, body)
	for _, u := range urls {
		go doCrawl(u, depth-1, fetcher, ch, sc)
	}
	
	time.Sleep(50 * time.Millisecond)
	ch <- DONE
}

// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func Crawl(url string, depth int, fetcher Fetcher) {
	ch := make(chan FetcherState)
	sc := NewSafeCache()
	
	go doCrawl(url, depth, fetcher, ch, &sc)
	
	runningTasks := 0
	
OuterLoop:
	for s := range ch {
		switch s {
			case RUNNING:
				runningTasks++
			
			case ERROR, DONE:
				runningTasks--
				if runningTasks == 0 {
					break OuterLoop
				}
		}
	}
	
	fmt.Println("Crawling finished")
}

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