go-routines

package 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 Seen struct {
    mu sync.Mutex
    seen map[string]bool
}

func (s *Seen) isSeen(url string) bool {
    s.mu.Lock()
    defer s.mu.Unlock()
    if val, ok := s.seen[url]; ok {
        return val
    }
    return false
}

func (s *Seen) markSeen(url string) {
    s.mu.Lock()
    defer s.mu.Unlock()
    s.seen[url] = true
}

var ss Seen = Seen{seen: make(map[string]bool)}

// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func Crawl(url string, depth int, fetcher Fetcher, wg *sync.WaitGroup) {
    // TODO: Fetch URLs in parallel.
    // TODO: Don't fetch the same URL twice.
    // This implementation doesn't do either:
    defer wg.Done()

    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 {
        if ss.isSeen(u) {
            continue
        }
        ss.markSeen(u)
        wg.Add(1)
        go Crawl(u, depth-1, fetcher, wg)
    }
    return
}

func main() {
    var wg sync.WaitGroup
    defer wg.Wait()
    wg.Add(1)
    go Crawl("https://golang.org/", 4, fetcher, &wg)
}

// 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/",
        },
    },
}