package main

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

func main() {
	p := NewPool(5)

	wg := sync.WaitGroup{}

	for i := 0; i < 10; i++ {
		wg.Add(1)
		go func() {
			n := p.Get()
			fmt.Println("start working on: ", n)
			time.Sleep(time.Duration(n) * time.Second)
			fmt.Println("done working on: ", n)
			p.Put(n)
			wg.Done()
		}()
	}

	wg.Wait()
}

type pool struct {
	c    chan int
	free chan int
}

func NewPool(size int) *pool {
	p := &pool{
		c:    make(chan int, size),
		free: make(chan int, size),
	}

	// put 5 tokens onto the free channel
	for i := 0; i < size; i++ {
		p.free <- 1
	}

	return p
}

func (p *pool) Get() (n int) {
	select {
	case n = <-p.c:
	case <-p.free:
		// take a token
		// block if we've used up all available tokens
		// tokens don't get put back.
		fmt.Println("(get) default free")
		n = rand.Intn(9) + 1
	}

	return
}

func (p *pool) Put(n int) {
	select {
	case p.c <- n:
	default:
		fmt.Println("This should never be called", n)
	}
}