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)
}

// 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:
	ch := make(chan result)
	cache := SafeCache{v: make(map[string]*fakeResult)}
	
	var crawl func(url string, depth int, fetcher Fetcher, ch chan result)
	crawl = func(url string, depth int, fetcher Fetcher, ch chan result) {
		if depth <= 0 {
			return
		}
		cache.mu.Lock()
		if it := cache.v[url]; it == nil {
			body, urls, err := fetcher.Fetch(url)
			if err != nil {
				fmt.Println(err)
				cache.mu.Unlock()
				return
			}
			cache.v[url] = &fakeResult{body, urls}
			cache.mu.Unlock()
			ch <- result{url, body}
			for _, u := range urls {
				crawl(u, depth-1, fetcher, ch)
			}
		} else {
			cache.mu.Unlock()
		}
	}
	
	fetch := func(url string, depth int, fetcher Fetcher, ch chan result) {
		defer close(ch)
		crawl(url, depth, fetcher, ch)
	}
	
	go fetch(url, depth, fetcher, ch)
	
	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
}

type result struct {
	body string
	url string
}

// SafeCache is safe to use concurrently.
type SafeCache struct {
	mu sync.Mutex
	v  map[string]*fakeResult
}

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/",
		},
	},
}