internet-cafe.go

package main

import (
	"log"
	"math/rand"
	"sync"
	"time"
)

// http://whipperstacker.com/2015/10/05/3-trivial-concurrency-exercises-for-the-confused-newbie-gopher/
// Solution 3: Internet Cafe
func main() {
	rand.Seed(time.Now().UnixNano())

	numTourists := 25
	maxOnline := 8

	var wg sync.WaitGroup
	wg.Add(numTourists)

	done := make(chan interface{}, maxOnline)
	defer close(done)

	online := func(done chan interface{}, wg *sync.WaitGroup, i int) {
		kv := make(map[int]int, 1)
	loop:
		select {
		case done <- i:
			log.Printf("Tourist %d is online.\n", i)
			duration := 5 + rand.Intn(10)
			time.Sleep(time.Duration(duration) * time.Second)
			log.Printf("Tourist %d is done, having spent %d seconds online.\n", i, duration)
			<-done
			wg.Done()
		default:
			_, ok := kv[i]
			if !ok {
				kv[i] = i
				log.Printf("Tourist %d waiting for turn.\n", i)
			}
			goto loop
		}

	}

	for i := 0; i < numTourists; i++ {
		go online(done, &wg, i+1)
	}

	wg.Wait()

	log.Println("The place is empty, let's close up and go to the beach!")
}