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c4se

#golang §69 Exercise: Web Crawler

Programming Golang

A Tour of Go 69

Exercise: Web Crawler

In this exercise you'll use Go's concurrency features to parallelize a web crawler.
Modify the Crawl function to fetch URLs in parallel without fetching the same URL twice.

Example

package main

import (
	"fmt"
)

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)
}

func crawlConcurr(url string, depth int, fetcher Fetcher,
		  ch chan func()([]string,int), quit chan int) {
	if depth <= 0 {
		quit <- 0
		return
	}
	body, urls, err := fetcher.Fetch(url)
	if err != nil {
		fmt.Println(err)
		quit <- 0
		return
	} else {
		fmt.Printf("found: %s %q\n", url, body)
		f := func()([]string,int) { return urls, depth }
		ch <- f
	}
}

var history = make(map[string]bool)

// 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:
	history[url] = true
	ch := make(chan func()([]string,int))
	quit := make(chan int)
	routinsNum := 1
	go crawlConcurr(url, depth, fetcher, ch, quit)
	for routinsNum > 0 {
		select {
		case f := <- ch:
			routinsNum --
			us, d := f()
			for _, u := range us {
				if history[u] != true {
					history[u] = true;
					routinsNum ++
					go crawlConcurr(u, d - 1, fetcher, ch, quit)
				}
			}
		case <- quit:
			routinsNum --
		}
	}
}

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/",
		},
	},
}