temp.go

package main

import (
	"fmt"
	"math/rand"
	"sync"
	"sync/atomic"
	"time"
)

type DoneUrlsMux struct {
	v   map[string]bool
	mux sync.Mutex
}

func (c *DoneUrlsMux) Set(key string) (succ bool) {
	c.mux.Lock()
	defer c.mux.Unlock()

	if _, found := c.v[key]; found {
		succ = false
		return
	} else {
		c.v[key] = true
		succ = true
		return
	}
}

type Counter struct {
	count *int64
	done  chan bool
}

func (c *Counter) Init() (succ bool) {
	if nil == c.count {
		c.count = &[]int64{1}[0]
		succ = true
	} else {
		succ = false
	}
	return
}

func (c *Counter) Add(delta int) {
	if atomic.AddInt64(c.count, int64(delta)); 0 == *c.count {
		c.done <- true
	}
}

type Fetcher interface {
	Fetch(url string) (body string, urls []string, err error)
}

var counter = Counter{done: make(chan bool, 1)}
var doneUrlsMux = DoneUrlsMux{v: make(map[string]bool)}

func Crawl(url string, depth int, fetcher Fetcher) {
	if (counter.Init()) {
		defer func () {
			<-counter.done
		}()
	}

	defer counter.Add(-1)

	if depth <= 0 {
		return
	}

	if succ := doneUrlsMux.Set(url); false == succ {
		return
	}

	body, urls, err := fetcher.Fetch(url)
	
	if nil != err {
		fmt.Println(err)
		return
	}
		
	fmt.Printf("found: %s %q\n", url, body)

	counter.Add(len(urls))
	for _, url := range urls {
		go Crawl(url, depth-1, fetcher)
	}
}

func main() {
	Crawl("https://golang.org/", 4, fetcher)
}

type fakeFetcher map[string]*fakeResult

type fakeResult struct {
	body string
	urls []string
}

func (f fakeFetcher) Fetch(url string) (string, []string, error) {
	if d, e := time.ParseDuration(fmt.Sprintf("%vms", rand.Intn(10)+1)); nil == e {
		time.Sleep(d)
	} else {
		panic("Invalid duration")
	}

	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/",
		},
	},
}