Exercise: Web Crawler

exercise-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 SharedState struct {
	messages chan string
	visited map[string]interface{}
	mux sync.Mutex
	wg sync.WaitGroup
}

func CrawlImpl(state *SharedState, url string, depth int, fetcher Fetcher) {
	defer state.wg.Done()
	if depth <= 0 {
		return
	}
	
	state.mux.Lock()
	
	if _, ok := state.visited[url]; !ok {
		state.visited[url] = nil
		body, urls, err := fetcher.Fetch(url)
		if err != nil { 
			state.mux.Unlock()
			return
		}
        state.messages <- fmt.Sprintf("%s found: %s", url, body)
		state.mux.Unlock()
		
		for _, url = range urls {
			if _, ok = state.visited[url]; !ok {
				state.wg.Add(1)
				go CrawlImpl(state, url, depth - 1, fetcher)
			}
		}
	} else {
		state.mux.Unlock()
	}
}

// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func Crawl(url string, depth int, fetcher Fetcher) {
	state := SharedState {
        messages: make(chan string),
		visited: make(map[string]interface{}),
	}
	state.wg.Add(1)
	go func() {
		state.wg.Wait()
		close(state.messages)
	}()
	go CrawlImpl(&state, url, depth, fetcher)
	
    for m := range state.messages {
        fmt.Println(m)
    }
}

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/",
		},
	},
	"http://golang.org/cmd/": &fakeResult{
		"Commands",
		[]string{
			"http://golang.org/",
		},
	},
}