A Tour of Go - Exercise: Web Crawler

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 SafeUrlMap struct {
	mux sync.Mutex
	v   map[string]int
}

type crawResult struct {
	url  string
	body string
}

func merge(cs ...chan crawResult) chan crawResult {
	var wg sync.WaitGroup
	out := make(chan crawResult)

	output := func(c chan crawResult) {
		for n := range c {
			out <- n
		}
		wg.Done()
	}
	wg.Add(len(cs))
	for _, c := range cs {
		go output(c)
	}

	go func() {
		wg.Wait()
		close(out)
	}()
	return out
}

func _crawl(url string, depth int, fetcher Fetcher, urlMap *SafeUrlMap) chan crawResult {
	ch := make(chan crawResult, 1)
	if depth <= 0 {
		defer close(ch)
		return ch
	}
	body, urls, err := fetcher.Fetch(url)
	fmt.Printf("fetched %s\n", url)
	if err != nil {
		fmt.Println(err)
		defer close(ch)
		return ch
	}
	ch <- crawResult{url, body}
	defer close(ch)

	channels := []chan crawResult{ch}

	for i := 0; i < len(urls); i++ {
		_url := urls[i]
		
		urlMap.mux.Lock()
		_, ok := urlMap.v[_url]
		if !ok {
			urlMap.v[_url] = 1
		}
		urlMap.mux.Unlock()
		
		if !ok {
			channels = append(channels, _crawl(_url, depth-1, fetcher, urlMap))
		}
	}

	return merge(channels...)
}

// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func Crawl(url string, depth int, fetcher Fetcher) {
	// TODO: Fetch URLs in parallel.
	// TODO: Don't fetch the same URL twice.
	// This implementation doesn't do either:
	urlMap := SafeUrlMap{v: make(map[string]int)}
	urlMap.v[url] = 1
	ch := _crawl(url, depth, fetcher, &urlMap)

	for result := range ch {
		fmt.Printf("found: %s %q\n", result.url, result.body)
	}
	return
}

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/",
		},
	},
}