Fan-Out Fan-In

package main

import (
	"context"
	"fmt"
	"sync"
)

func gen(ctx context.Context, nums ...int) <-chan int {
	out := make(chan int)
	go func() {
		defer close(out)
		for _, n := range nums {
			select {
			case out <- n:
			case <-ctx.Done():
				return
			}
		}
	}()
	return out
}

func worker(ctx context.Context, in <-chan int) <-chan int {
	out := make(chan int)
	go func() {
		defer close(out)
		for n := range in {
			select {
			case out <- n * n:
			case <-ctx.Done():
				return
			}
		}
	}()
	return out
}

func fanIn(ctx context.Context, channels ...<-chan int) <-chan int {
	out := make(chan int)
	var wg sync.WaitGroup
	for _, ch := range channels {
		wg.Add(1)
		go func(c <-chan int) {
			defer wg.Done()
			for val := range c {
				select {
				case out <- val:
				case <-ctx.Done():
					return
				}
			}
		}(ch)
	}
	go func() {
		wg.Wait()
		close(out)
	}()
	return out
}

func main() {
	ctx, cancel := context.WithCancel(context.Background())
	defer cancel()

	nums := gen(ctx, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

	// Fan-out: 3 workers
	w1 := worker(ctx, nums)
	w2 := worker(ctx, nums)
	w3 := worker(ctx, nums)

	// Fan-in: merge results
	for val := range fanIn(ctx, w1, w2, w3) {
		fmt.Println(val)
	}
}