长安链 DApp 开发必学 Go 21：爬虫练习

众所周知，谷歌的本质是一家大型的爬虫公司，爬虫要提高爬取网站的效率，就需要使用并发爬取。

这又是 Go 擅长的领域了。直接上代码：

package main

import (
	"fmt"
	"sync"
	"time"
)

// 声明两个全局变量，一把互斥锁，一个去重用的 Map
var mu sync.Mutex
var v map[string]int = make(map[string]int)

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) {
	if depth <= 0 {
		return
	}
        
	mu.Lock()    // 上锁，要操作去重 Map 了
	if v[url] == 1 {
		fmt.Printf("dedup: %s\n", url)
		return
	} else {
		v[url] = 1
	}
	mu.Unlock()  // 操作完了，别忘了解锁

	body, urls, err := fetcher.Fetch(url)
	if err != nil {
		fmt.Println(err)
		return
	}
	fmt.Printf("found: %s %q\n", url, body)
	for _, u := range urls {
		go Crawl(u, depth-1, fetcher)  // 调用协程进行并发爬取
	}

	return
}

func main() {
	Crawl("https://golang.org/", 4, fetcher)
	time.Sleep(time.Second)  // 等待协程运行完毕

}

// 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/",
		},
	},
}

哦了，Go 教程到此为止了。

少年，利剑在手，目标是星辰大海！！！