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) {
	// DONE: Fetch URLs in parallel.
	// DONE: Don't fetch the same URL twice.
	ch := make(chan Fetched)
	n := 1
	cache := map[string]bool{url: true}
	go Fetch(url, depth, fetcher, ch)

	for n > 0 {
		n--
		fetched := <-ch
		if fetched.err != nil {
			fmt.Println(fetched.err)
			continue
		}
		fmt.Printf("found: %s %q\n", fetched.url, fetched.body)
		if fetched.depth > 1 {
			for _, u := range fetched.urls {
				if !cache[u] {
					n++
					cache[u] = true
					go Fetch(u, fetched.depth-1, fetcher, ch)
				}
			}
		}
	}
}

func Fetch(url string, depth int, fetcher Fetcher, ch chan Fetched) {
	body, urls, err := fetcher.Fetch(url)
	ch <- Fetched{url, depth, body, urls, err}
}

type Fetched struct {
	url   string
	depth int
	body  string
	urls  []string
	err   error
}

func main() {
	Crawl("http://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{
	"http://golang.org/": &fakeResult{
		"The Go Programming Language",
		[]string{
			"http://golang.org/pkg/",
			"http://golang.org/cmd/",
		},
	},
	"http://golang.org/pkg/": &fakeResult{
		"Packages",
		[]string{
			"http://golang.org/",
			"http://golang.org/cmd/",
			"http://golang.org/pkg/fmt/",
			"http://golang.org/pkg/os/",
		},
	},
	"http://golang.org/pkg/fmt/": &fakeResult{
		"Package fmt",
		[]string{
			"http://golang.org/",
			"http://golang.org/pkg/",
		},
	},
	"http://golang.org/pkg/os/": &fakeResult{
		"Package os",
		[]string{
			"http://golang.org/",
			"http://golang.org/pkg/",
		},
	},
}