How do you implement fan-out and fan-in patterns with Go goroutines?

Fan-out: one goroutine distributes work to multiple worker goroutines. Fan-in: multiple goroutines send results back to a single aggregator. Together they form Go's most common concurrency idiom for parallel pipelines.

// Fan-out / Fan-in pipeline
func generate(nums ...int) <-chan int {
    out := make(chan int)
    go func() {
        defer close(out)
        for _, n := range nums { out <- n }
    }()
    return out
}

func square(in <-chan int) <-chan int {
    out := make(chan int)
    go func() {
        defer close(out)
        for n := range in { out <- n * n }
    }()
    return out
}

// Fan-out: start N workers on the same input channel
func fanOut(in <-chan int, workers int) []<-chan int {
    outs := make([]<-chan int, workers)
    for i := 0; i < workers; i++ {
        outs[i] = square(in) // each reads from the shared input
    }
    return outs
}

// Fan-in: merge N result channels into one
func fanIn(ctx context.Context, channels ...<-chan int) <-chan int {
    var wg sync.WaitGroup
    merged := make(chan int)

    forward := func(c <-chan int) {
        defer wg.Done()
        for n := range c {
            select {
            case merged <- n:
            case <-ctx.Done(): return
            }
        }
    }
    wg.Add(len(channels))
    for _, c := range channels { go forward(c) }

    go func() { wg.Wait(); close(merged) }()
    return merged
}

// Wire it up
nums := generate(1, 2, 3, 4, 5)
ctx, cancel := context.WithCancel(context.Background())
defer cancel()
results := fanIn(ctx, fanOut(nums, 3)...)
for r := range results { fmt.Println(r) }

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