Last exercise in go tour https://tour.golang.org/concurrency/10

web-crawler.go

package main

import (
	"fmt"
	"sync"
)

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 CacheError string

func (e CacheError) Error() string {
	return fmt.Sprintf("no record in cache for %v", string(e))
}

type CrawlerCacheRecord struct {
	body string
	urls []string
}

type CrawlerCache struct {
	record map[string]*CrawlerCacheRecord
	mux    sync.Mutex
}

func (cache *CrawlerCache) add(key, body string, urls []string) {
	cache.mux.Lock()
	cache.record[key] = &CrawlerCacheRecord{urls: make([]string, 1), body: ""}
	cache.record[key].body = body
	cache.record[key].urls = urls
	cache.mux.Unlock()
}

func (cache *CrawlerCache) get(key string) (string, []string, error) {
	cache.mux.Lock()
	defer cache.mux.Unlock()
	record, ok := cache.record[key]
	if !ok {
		return "", nil, CacheError(key)
	}
	return record.body, record.urls, nil
}

// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func Crawl(url string, depth int, fetcher Fetcher) {

	cache := CrawlerCache{record: make(map[string]*CrawlerCacheRecord)}

	fin := make(chan string)
	defer func() { <-fin }()

	cv := 0
	var crawler func(u string, d int)
	crawler = func(u string, d int) {
		defer func() {
			cv--
			if cv == 0 {
				fin <- "done!"
			}
		}()

		if d <= 0 {
			return
		}
		var body string
		var urls []string
		var err error
		if body, urls, err = cache.get(u); err != nil {
			body, urls, err = fetcher.Fetch(u)
			if err != nil {
				fmt.Println(err)
				cache.add(u, body, urls)
				return
			}
			cache.add(u, body, urls)
		}

		fmt.Printf("found: %s %q\n", u, body)
		for _, uu := range urls {
			cv++
			go crawler(uu, d-1)
		}
		return
	}
	cv++
	go crawler(url, depth)
	return
}

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) {
	//fmt.Println("hi from fetcher")
	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/",
		},
	},
}