Go Web Crawler

Crawler.go

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)
}

type FetchResult struct {
	url  string
	body string
	err  error
}

type IUniqueReservationMap interface {
	Reserve(key string) chan bool
}

type UniqueReservationMap struct {
	isReserved *map[string] bool
	lockAttempts chan lockAttempt
}

type lockAttempt struct {
	key string
	success chan bool
}

func (reservationMap UniqueReservationMap) Reserve(key string) chan bool {
	attempt := lockAttempt{key, make(chan bool)}
	reservationMap.lockAttempts <- attempt
	return attempt.success
}

func createUniqueReservationMap() IUniqueReservationMap {
	isReserved := make(map[string] bool)
	reservationMap := UniqueReservationMap{&isReserved, make(chan lockAttempt)}
	go func() {
		for {
			select {
			case attempt := <- reservationMap.lockAttempts:
				attempt.success <- !isReserved[attempt.key]
				isReserved[attempt.key] = true
			}
		}
	}()
	return reservationMap
}

// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func Crawl(url string, depth int, fetcher Fetcher, results chan<- FetchResult) {
	crawlRequests := make(chan string, int)
	urlReservations := make(map[string] bool)
	fetchAndCrawl := func (url string, depth int, results chan<- FetchResult) {
		if depth <= 0 {
			return
		}
		
		body, urls, err := fetcher.Fetch(url)
		results <- FetchResult{url, body, err}

		if len(urls) == 0 {
			done <- true
			return
		}

		subResults := make([]chan FetchResult, len(urls))
		for i, url := range urls {
			subResults[i] = crawl(url, depth-1, fetcher)
		}	

		for _, resultChannel := range subResults {
			result <- resultChannel
		}
		results.close()
	}
	
	go func() {
		for {
			select {
				case crawlRequest <- crawlRequests:
					if crawlRequests[crawlRequest] == nil { break }
					go fetchAndCrawl(...crawlRequest, results)
			}
		}
	}()
}

func main() {
	results := make(chan FetchResult)
	go Crawl("https://golang.org/", 4, fetcher, results)
	for result := range results {
		if result.err != nil {
			fmt.Println(result.err)
			continue
		}
		fmt.Printf("found: %s %q\n", result.url, result.body)
	}
	
	fmt.Printf("done!")
}

// 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/",
		},
	},
}