you must remember where you've been. for example, you might:
a. convert each candidate URL to a canonical form (absolute path)
b. look for canonical url in a map before visiting. If it was not there,
insert and visit, if it was there, do nothing
this is enough.
On Sun, Sep 24, 2017 at 4:05 AM, <gdz...@gmail.com> wrote:
> Hi I am learning Golang concurrency and trying to build a simple Website
> crawler. I managed to crawl all the links of the pages of any depth of
> website. But I still have one problem to tackle: how to avoid crawling
> visited links that are previously crawled?
>
> Here is my code. Hope you guys can shed some light. Thank you in advance.
>
> package main
> import (
> "fmt"
> "log"
> "net/http"
> "os"
> "strings"
>
> "golang.org/x/net/html")
>
> func main() {
> if len(os.Args) != 2 {
> fmt.Println("Usage: crawl [URL].")
> }
>
> url := os.Args[1]
> if !strings.HasPrefix(url, "http://";) {
> url = "http://"; + url
> }
>
> for link := range newCrawl(url, 1) {
> fmt.Println(link)
> }}
>
> func newCrawl(url string, num int) chan string {
> ch := make(chan string, 20)
>
> go func() {
> crawl(url, 1, ch)
> close(ch)
> }()
>
> return ch}
>
> func crawl(url string, n int, ch chan string) {
> if n < 1 {
> return
> }
> resp, err := http.Get(url)
> if err != nil {
> log.Fatalf("Can not reach the site. Error = %v\n", err)
> os.Exit(1)
> }
>
> b := resp.Body
> defer b.Close()
>
> z := html.NewTokenizer(b)
>
> nextN := n - 1
> for {
> token := z.Next()
>
> switch token {
> case html.ErrorToken:
> return
> case html.StartTagToken:
> current := z.Token()
> if current.Data != "a" {
> continue
> }
> result, ok := getHrefTag(current)
> if !ok {
> continue
> }
>
> hasProto := strings.HasPrefix(result, "http")
> if hasProto {
> done := make(chan struct{})
> go func() {
> crawl(result, nextN, ch)
> close(done)
> }()
> <-done
> ch <- result
> }
> }
> }}
>
> func getHrefTag(token html.Token) (result string, ok bool) {
> for _, a := range token.Attr {
> if a.Key == "href" {
> result = a.Val
> ok = true
> break
> }
> }
> return}
>
> --
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>
--
Michael T. Jones
michael.jo...@gmail.com
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