package main

import (
"fmt"
"sync"
)

const maxConcurrency = 5 // Max number of concurrency

func doSomething(a int) {
    fmt.Println("Doing something...",a)
    return
}

func main() {
    var ch = make(chan int, 50) // This number 50 can be anything as long as it's larger than maxConcurrency
    var wg sync.WaitGroup

    // This starts max number of goroutines that wait for something to do
    wg.Add(maxConcurrency)
    for i:=0; i<maxConcurrency; i++ {
        go func() {
            for {
                a, ok := <-ch
                if !ok { // if there is nothing to do and the channel has been closed then end the goroutine
                    wg.Done()
                    return
                }
                doSomething(a) // do something
            }
        }()
    }

    // Add the jobs to the channel, which is acts as a queue
    for i:=0; i<50; i++ {
        ch <- i // add i to the queue
    }

    close(ch) // This tells the goroutines there's nothing else to do
    wg.Wait() // Wait for the threads to finish
}