package main

import (
	"context"
	"flag"
	"fmt"
	"strconv"
	"strings"
	"sync"
	"sync/atomic"
)

var workers int
var endpoints string
var urlsDone int64
var urlsError int64

func main() {
	flag.IntVar(&workers, "workers", 5, "number of workers")
	flag.StringVar(&endpoints, "endpoints", "1,2,3,a,b", "list of endpoints comma delimited")

	// parse
	flag.Parse()

	// pass wg as pointer
	wg := &sync.WaitGroup{}

	// to quit all workers when all messages have been treated.
	ctx, cancel := context.WithCancel(context.Background())
	tasks := make(chan string)
	defer close(tasks)
	done := make(chan bool)
	defer close(done)

    // good practice
    wg.Add(workers)
	for i := 0; i < workers; i++ {
		go worker(ctx, wg, tasks, done)
	}

	msgs := strings.Split(endpoints, ",")
	go func() {
		for _, endpoint := range msgs {
			tasks <- endpoint
		}
	}()

	// wait for all messages to be treated
	for i := 0; i < len(msgs); i++ {
		<-done
	}
	// signal workers to quit.
	cancel()
	// wait for workers to finish gracefully.
	wg.Wait()
	fmt.Printf("work done. successfully=%d errors=%d", urlsDone, urlsError)
}

func worker(ctx context.Context, wg *sync.WaitGroup, urls chan string, done chan bool) {
	// The wg is still necessary. In case there's going to be cleanup when `ctx.Done` receives.
	// You want `main` to wait until clean up is done for all workers.
	defer wg.Done()

	for {
		select {
		case msg := <-urls:
			//_, err := http.Get(msg)
			// for convenience
			_, err := strconv.ParseInt(msg, 10, 32)
			if err != nil {
				// atomically update shared variables
				atomic.AddInt64(&urlsError, 1)
			} else {
				atomic.AddInt64(&urlsDone, 1)
			}
			done <- true
		case <-ctx.Done():
			// quit worker.
			// cleanup if necessary
			return
		}
	}
}