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package main
import (
"fmt"
"sync"
)
// gen sends the values in nums on the returned channel, then closes it.
func gen(done <-chan struct{}, nums ...int) <-chan int {
out := make(chan int, len(nums))
for _, n := range nums {
// We ignore done here because these sends cannot block.
out <- n
}
close(out)
return out
}
// sq receives values from in, squares them, and sends them on the returned
// channel, until in or done is closed. Then sq closes the returned channel.
func sq(done <-chan struct{}, in <-chan int) <-chan int {
out := make(chan int)
go func() {
defer close(out) // HL
for n := range in {
select {
case out <- n * n:
case <-done:
return // HL
}
}
}()
return out
}
// merge receives values from each input channel and sends them on the returned
// channel. merge closes the returned channel after all the input values have
// been sent or after done is closed.
func merge(done <-chan struct{}, cs ...<-chan int) <-chan int {
var wg sync.WaitGroup
out := make(chan int)
// Start an output goroutine for each input channel in cs. output
// copies values from c to out until c or done is closed, then calls
// wg.Done.
output := func(c <-chan int) {
defer wg.Done() // HL
for n := range c {
select {
case out <- n:
case <-done:
return // HL
}
}
}
// ... the rest is unchanged ...
wg.Add(len(cs))
for _, c := range cs {
go output(c)
}
// Start a goroutine to close out once all the output goroutines are
// done. This must start after the wg.Add call.
go func() {
wg.Wait()
close(out)
}()
return out
}
func main() {
// Set up a done channel that's shared by the whole pipeline,
// and close that channel when this pipeline exits, as a signal
// for all the goroutines we started to exit.
done := make(chan struct{}) // HL
defer close(done) // HL
in := gen(done, 2, 3)
// Distribute the sq work across two goroutines that both read from in.
c1 := sq(done, in)
c2 := sq(done, in)
// Consume the first value from output.
out := merge(done, c1, c2)
fmt.Println(<-out) // 4 or 9
// done will be closed by the deferred call. // HL
}
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