package main

import (
	"context"
	"fmt"
	"log"
	"net/url"
	"sync"
)

func classic(startURL url.URL, maxConcurrency, maxURLs int) {
	worklist := make(chan []url.URL)
	var numPendingSends int

	numPendingSends++
	go func() {
		worklist <- []url.URL{startURL}
	}()

	// Crawl the web concurrently.
	seen := make(map[url.URL]bool)
	count := 1

	ctx, cancel := context.WithCancel(context.Background())
	var wg sync.WaitGroup
	sema := make(chan struct{}, maxConcurrency)

loop:
	for ; numPendingSends > 0; numPendingSends-- {
		batch := <-worklist
		for _, u := range batch {
			if !seen[u] {
				fmt.Printf("%d. %s\n", count, &u)
				count++

				seen[u] = true
				if len(seen) == maxURLs {
					break loop
				}

				numPendingSends++
				wg.Go(func() {
					sema <- struct{}{}
					defer func() { <-sema }()

					select {
					case <-ctx.Done():
						return
					case worklist <- getBatch(u):
					}
				})
			}
		}
	}

	cancel()
	wg.Wait()
}

func getBatch(u url.URL) []url.URL {
	doc, err := fetch(u)
	if err != nil {
		log.Print(err)
	}

	batch := findURLs(u, doc)

	return batch
}