package main

import (
    "fmt"
    "sync"
)

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

func (c *SafeCounter) Inc(key string) {
    c.mux.Lock()
    defer c.mux.Unlock()
    c.v[key]++
}

func (c *SafeCounter) Value(key string) int {
    c.mux.Lock()
    defer c.mux.Unlock()
    return c.v[key]
}

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 maximum of depth.
func Crawl(url string, depth int, fetcher Fetcher, c SafeCounter) {
    // TODO: Fetch URLs in parallel.
    // TODO: Don't fetch the same URL twice.
    // This implementation doesn't do either.
    if depth <= 0 {
        return
    }
    if c.Value(url) > 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 {
        go Crawl(u, depth-1, fetcher, c)
    }
}

func main() {
    c := SafeCounter{v: make(map[string]int)}
    Crawl("https://www.golang.com/", 4, fetcher, c)
}

type fakeFetcher map[string]*fakeResult

// fakeFetcher is Fetcher that returns canned results.
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/",
		},
	},
}