halp

package main

import (
    "fmt"
    "sync"
)

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

// Inc increments the counter for the given key.
func (c *SafeMap) Add(key, value string) {
    c.mux.Lock()
    // Lock so only one goroutine at a time can access the map c.v.
    c.v[key] = value
    c.mux.Unlock()
}

// Value returns the current value of the counter for the given key.
func (c *SafeMap) Value(key string) (res string,ok bool) {
    c.mux.Lock()
    // Lock so only one goroutine at a time can access the map c.v.
    defer c.mux.Unlock()
    res, ok = c.v[key]
    return
}


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)
}

var theMap = SafeMap{v: make(map[string]string)}

// 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:
    _, ok := theMap.Value(url)

    if ok || depth == 0 {
        return
    }

    body, urls, err := fetcher.Fetch(url)
    theMap.Add(url, body)

    if err != nil {
        fmt.Println(err)
        return
    } else {
        fmt.Printf("Found: url=%s : %s \n", url , body)
    }

    for _,v := range urls {
        go Crawl(v, depth -1 , fetcher)
    }
}

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/",
        },
    },
}