Sleeping Barbers

barbers.go

// I was inspired by @goinggodotnet to implement the sleeping barbers problem
// https://twitter.com/goinggodotnet/status/1323664013264375809
//
// https://en.wikipedia.org/wiki/Sleeping_barber_problem

package main

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

type Customer struct {
	Name string
}

var Sleepytime = &Customer{"Napping..."}

func NewCustomer() *Customer {
	firstName := firstNames[rand.Intn(len(firstNames))]
	surName := surNames[rand.Intn(len(surNames))]
	return &Customer{firstName + " " + surName}
}

func (c *Customer) CutFinished() {
	log.Println(c.Name, "likes the haircut!")
	atomic.AddUint32(&numCuts, 1)
}

func (c *Customer) Leave() {
	log.Println(c.Name, "walked out the door")
	atomic.AddUint32(&numLeft, 1)
}

type Barber struct {
	Name    string
	cutting chan *Customer
}

func NewBarber() *Barber {
	firstName := firstNames[rand.Intn(len(firstNames))]
	surName := surNames[rand.Intn(len(surNames))]
	return &Barber{
		Name:    firstName + " " + surName,
		cutting: make(chan *Customer, 1),
	}
}

func (b *Barber) Work(w *WaitingRoom) {
	for {
		c, ok := w.FindCustomer()
		if !ok {
			log.Println(b.Name, "is taking a nap")
			b.cutting <- Sleepytime
		} else {
			log.Println(b.Name, "is cutting", c.Name)
			b.cutting <- c
		}
		dur := time.Second * time.Duration(rand.Intn(5))
		time.Sleep(dur)

		// get out of the chair
		<-b.cutting

		if ok {
			c.CutFinished()
			c.Leave()
		}
	}
}

func (b *Barber) Checkin(c *Customer) bool {
	select {
	case b.cutting <- c:
		return true
	default:
		return false
	}
}

//////

type WaitingRoom struct {
	seats chan *Customer
}

func NewWaitingRoom(nc int) *WaitingRoom {
	return &WaitingRoom{seats: make(chan *Customer, nc)}
}

func (r *WaitingRoom) FindSeat(c *Customer) bool {
	select {
	case r.seats <- c:
		// there's a chair free
		log.Println(c.Name, "sat down in the waiting room")
		return true
	default:
		// all the chairs are full
		log.Println(c.Name, "left because the waiting room is full")
		return false
	}
}

func (r *WaitingRoom) FindCustomer() (*Customer, bool) {
	select {
	case c, ok := <-r.seats:
		log.Println(c.Name, "called in from the waiting room")
		// there was a customer waiting
		return c, ok
	default:
		log.Println("no one in the waiting room")
		// no one waiting
		return nil, false
	}
}

////////

type Shop struct {
	barbers     []*Barber
	waitingRoom *WaitingRoom
}

func NewShop(numChairs, numBarbers int) *Shop {
	s := &Shop{
		barbers:     make([]*Barber, 0, numBarbers),
		waitingRoom: NewWaitingRoom(numChairs),
	}
	for i := 0; i < numBarbers; i++ {
		b := NewBarber()
		go b.Work(s.waitingRoom)
	}

	return s
}

func (s *Shop) GetHaircut(c *Customer) {
	atomic.AddUint32(&numEntered, 1)
	// check the barbers
	for _, b := range s.barbers {
		if b.Checkin(c) {
			// got a cut right away!
			return
		}
	}
	if s.waitingRoom.FindSeat(c) {
		// got a seat to wait
		return
	}

	c.Leave()
}

////////
var (
	numEntered, numCuts, numLeft uint32
)

func statsRunner() {
	for range time.Tick(time.Second * 5) {
		e := atomic.LoadUint32(&numEntered)
		c := atomic.LoadUint32(&numCuts)
		l := atomic.LoadUint32(&numLeft)
		log.Printf("STATS: %d entered, %d left, %d cuts", e, l, c)
	}
}

func main() {
	numChairs := flag.Int("c", 4, "`number` of chairs in waiting room")
	numBarbers := flag.Int("b", 1, "`number` of barbers")
	flag.Parse()

	s := NewShop(*numChairs, *numBarbers)

	///////////
	go statsRunner()
	for {
		// 0-5s random entry times
		dur := time.Second * time.Duration(rand.Intn(5))
		time.Sleep(dur)

		c := NewCustomer()
		go s.GetHaircut(c)
	}
}

names.go

package main

// extracted from https://blogs.ancestry.com/cm/whats-the-most-popular-surname-in-your-state/
var surNames = []string{"Anderson", "Brown", "Chavez", "Garcia", "Hernandez",
	"Johnson", "Jones", "Kim", "Lee", "Lopez", "Martinez", "Miller", "Nelson",
	"Olson", "Smith", "Sullivan", "Williams", "Wong"}

// top 25 male and female names from 2000
// extracted from http://www.babynames.it/top100/1/year-2000.html
var firstNames = []string{
	"Jacob", "Emily", // 1st
	"Michael", "Hannah", // 2nd
	"Matthew", "Madison", // 3rd
	"Joshua", "Ashley", // 4th
	"Christopher", "Sarah", // 5th
	"Nicholas", "Alexis", // 6th
	"Andrew", "Samantha", // 7th
	"Joseph", "Jessica", // 8th
	"Daniel", "Taylor", // 9th
	"Tyler", "Elizabeth", // 10th
	"William", "Lauren", // 11th
	"Brandon", "Alyssa", // 12th
	"Ryan", "Kayla", // 13th
	"John", "Abigail", // 14th
	"Zachary", "Brianna", // 15th
	"David", "Olivia", // 16th
	"Anthony", "Emma", // 17th
	"James", "Megan", // 18th
	"Justin", "Grace", // 19th
	"Alexander", "Victoria", // 20th
	"Jonathan", "Rachel", // 21st
	"Christian", "Anna", // 22nd
	"Austin", "Sydney", // 23rd
	"Dylan", "Destiny", // 24th
	"Ethan", "Morgan", // 25th
}