目录

• 原题目
• 中文版题目
• 初始代码
• 先看懂原始代码
• 我的改进方法

原题目

• Exercise: Web Crawler
• In this exercise you'll use Go's concurrency features to parallelize a web crawler.
• Modify the Crawl function to fetch URLs in parallel without fetching the same URL twice.
• Hint: you can keep a cache of the URLs that have been fetched on a map, but maps alone are not safe for concurrent use!

中文版题目

• 练习：Web 爬虫
• 在这个练习中，我们将会使用 Go 的并发特性来并行化一个 Web 爬虫。
• 修改 Crawl 函数来并行地抓取 URL，并且保证不重复。
• 提示：你可以用一个 map 来缓存已经获取的 URL，但是要注意 map 本身并不是并发安全的！

初始代码

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)
}

// 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:
    if depth <= 0 {
        return
    }
    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 {
        Crawl(u, depth-1, fetcher)
    }
    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
}

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/",
        },
    },
}

先看懂原始代码

• 原始代码本来就可以运行
• 结果也没有错
• 但是有两点缺陷：
  • 其一就是没有并发
  • 其二是结果有重复值，而操作上也的确是做了重复的工作
• 这些导致了低效率

我的改进方法

• 使用channel quit来确保当前goroutine会在执行完其他goroutine后才结束
• 使用visited[]来标记哪些已经访问，哪些没有
• visited是各个goroutine竞争的变量，因此使用Mutex来限制visited的访问
• 代码：

ackage 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)
}

type Visitor struct {
    visited map[string]bool
    mux sync.Mutex
}

// 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:
    
    quit := make(chan bool)
    var v Visitor
    v.visited = make(map[string]bool)
    go MyCrawl(url, depth, fetcher, quit, &v)
    <-quit
}

func MyCrawl(url string, depth int, fetcher Fetcher, quit chan bool, v *Visitor) {
    if depth <= 0 {
        quit <- true
        return
    }
    v.mux.Lock()
    if !v.visited[url] {
        body, urls, err := fetcher.Fetch(url)
        if err != nil {
            fmt.Println(err)
            v.visited[url] = true
            v.mux.Unlock()
            quit <- true
            return
        }
        v.visited[url] = true
        fmt.Printf("found: %s %q\n", url, body)
        v.mux.Unlock()
        for _, u := range urls {
            childquit := make(chan bool)
            go MyCrawl(u, depth-1, fetcher, childquit, v)
            <-childquit
        }
    } else {
        v.mux.Unlock()
    }
    quit <- true
}

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
}

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/",
        },
    },
}

• 笔者曾查过许多网上的代码，但大多都不能满足，并发，去重的要求，许多都只是为了用上channel和Mutex而去编写的代码，所以就只能自己写了
• 这道问题涉及程序中的许多细节问题，欢迎读者质疑我，我也找个时间放上github