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)
}

type request struct {
	url   string
	depth int
}

// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func Fetch(req request, fetcher Fetcher, queue chan []request,
	finished chan bool) {
	defer func() {
		finished <- true
	}()

	var links []request

	body, urls, err := fetcher.Fetch(req.url)
	if err != nil {
		fmt.Println(err)
		return
	}
	fmt.Printf("found: %s %q\n", req.url, body)
	for _, u := range urls {
		link := request{
			url:   u,
			depth: req.depth + 1,
		}
		links = append(links, link)
	}
	queue <- links
	return
}

func Crawl(url string, depth int) {
	queue := make(chan []request)
	finished := make(chan bool)
	lookup := make(map[string]bool)
	seed := request{
		url:   url,
		depth: 1,
	}

	go Fetch(seed, fetcher, queue, finished)
	lookup[seed.url] = true
	outstanding := 1

	for outstanding > 0 {
		select {
		case newRequests := <-queue:
			for _, r := range newRequests {
				if _, ok := lookup[r.url]; ok || r.depth >= depth {
					continue
				} else {
					go Fetch(r, fetcher, queue, finished)
					outstanding++
				}
			}
		case <-finished:
			outstanding--
		}
	}
}

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

// 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/",
		},
	},
}