package main

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

func main() {
	for i := range merge(source(5), source(10), source(20)) {
		fmt.Printf("%20d\n", i)
	}
}

func source(c int) <-chan int {
	out := make(chan int)
	go func() {
		vals := make([]int, c)
		src := rand.New(rand.NewSource(time.Now().UnixNano()))
		for i := 0; i < c; i++ {
			vals[i] = src.Int()
		}

		sort.Ints(vals)

		for _, i := range vals {
			out <- i
		}
		close(out)
	}()
	return out
}

func merge(cs ...<-chan int) <-chan int {
	wg := sync.WaitGroup{}
	out := make(chan int)

	output := func(c <-chan int) {
		for n := range c {
			out <- n
		}
		wg.Done()
	}

	wg.Add(len(cs))
	for _, c := range cs {
		go output(c)
	}
	go func() {
		wg.Wait()
		close(out)
	}()
	return out
}