Golang tour / Exercise: Web Crawler

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)
}

// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func Crawl(url string, depth int, fetcher Fetcher) {
	defer wg.Done()
	if depth <= 0 {
		return
	}
	var (body string
		 urls[]string
		 err error
	)
	
	cache.mux.Lock()
	result, ok := cache.m[url]
	cache.mux.Unlock()
	if ok {
		body, urls, err = result.body, result.urls, result.err
	} else {
		body, urls, err = fetcher.Fetch(url)
		cache.mux.Lock()
		cache.m[url] = fetchResult{body: body, urls:urls, err:err}
		cache.mux.Unlock()
	}
	if err != nil {
		fmt.Println(err)
		return
	}
	fmt.Printf("found: %s %q\n", url, body)
	for _, u := range urls {
		wg.Add(1)
		go Crawl(u, depth-1, fetcher)
	}
	return
}

func main() {
	wg.Add(1)
	Crawl("http://golang.org/", 4, fetcher)
	wg.Wait()
}

type fetchResult struct {
	body string
	urls []string
	err error
}

// fakeFetcher is Fetcher that returns canned results.
type fakeFetcher map[string]*fakeResult

type fakeResult struct {
	body string
	urls []string
}

type SafeMap struct {
	m map[string]fetchResult
	mux sync.Mutex
}

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/",
		},
	},
}

var cache = SafeMap{m: make(map[string]fetchResult)}
var wg sync.WaitGroup