Golang Web Crawler Exercise

A sample solution for the final Golang exercise to make a mock web crawler.

package main

import (
	"fmt"
)

//This covers the final exercise, "Web Crawler"

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 FetchedUrl struct {
	parent	string
	body	string
	urls 	[]string
	err		error
	depth	int
}

// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func Crawl(url string, depth int, fetcher Fetcher) {
	results := make(chan *FetchedUrl)
	alreadyFetched := make(map[string]bool)
	
	fetch   := func(url string, depth int) {
		body, urls, err := fetcher.Fetch(url)
		results <- &FetchedUrl{url, body, urls, err, depth}
	}
	
	go fetch(url, depth)
	alreadyFetched[url] = true
	
	for countFetching := 1; countFetching > 0; countFetching-- {
		result := <- results
		
		if result.err != nil { //skip bad urls
			fmt.Println(result.err)
			continue
		}
		
		fmt.Printf("found: %s %q\n", result.parent, result.body)
		
		if result.depth > 0 {
			for _, newUrl := range result.urls {
				if !alreadyFetched[newUrl] {
					countFetching++
					go fetch(newUrl, depth-1)
					alreadyFetched[newUrl] = true
				}
			}
		}
	}

	close(results)
}

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) {
	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/",
		},
	},
}