mirror of
https://github.com/gopl-zh/gopl-zh.github.com.git
synced 2024-11-24 15:18:57 +00:00
65 lines
1.2 KiB
Go
65 lines
1.2 KiB
Go
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
|
|
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
|
|
|
|
// See page 241.
|
|
|
|
// Crawl2 crawls web links starting with the command-line arguments.
|
|
//
|
|
// This version uses a buffered channel as a counting semaphore
|
|
// to limit the number of concurrent calls to links.Extract.
|
|
package main
|
|
|
|
import (
|
|
"fmt"
|
|
"log"
|
|
"os"
|
|
|
|
"gopl.io/ch5/links"
|
|
)
|
|
|
|
//!+sema
|
|
// tokens is a counting semaphore used to
|
|
// enforce a limit of 20 concurrent requests.
|
|
var tokens = make(chan struct{}, 20)
|
|
|
|
func crawl(url string) []string {
|
|
fmt.Println(url)
|
|
tokens <- struct{}{} // acquire a token
|
|
list, err := links.Extract(url)
|
|
<-tokens // release the token
|
|
|
|
if err != nil {
|
|
log.Print(err)
|
|
}
|
|
return list
|
|
}
|
|
|
|
//!-sema
|
|
|
|
//!+
|
|
func main() {
|
|
worklist := make(chan []string)
|
|
var n int // number of pending sends to worklist
|
|
|
|
// Start with the command-line arguments.
|
|
n++
|
|
go func() { worklist <- os.Args[1:] }()
|
|
|
|
// Crawl the web concurrently.
|
|
seen := make(map[string]bool)
|
|
for ; n > 0; n-- {
|
|
list := <-worklist
|
|
for _, link := range list {
|
|
if !seen[link] {
|
|
seen[link] = true
|
|
n++
|
|
go func(link string) {
|
|
worklist <- crawl(link)
|
|
}(link)
|
|
}
|
|
}
|
|
}
|
|
}
|
|
|
|
//!-
|