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// +build OMIT
package main
import (
"fmt"
"sync"
)
// gen sends the values in nums on the returned channel, then closes it.
func gen(nums ...int) <-chan int {
out := make(chan int, len(nums))
for _, n := range nums {
out <- n
}
close(out)
return out
}
// sq receives values from in, squares them, and sends them on the returned
// channel, until in is closed. Then sq closes the returned channel.
func sq(in <-chan int) <-chan int {
out := make(chan int)
go func() {
for n := range in {
out <- n * n
}
close(out)
}()
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.
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 is closed or it receives a value
// from done, then output calls wg.Done.
output := func(c <-chan int) {
for n := range c {
select {
case out <- n:
case <-done: // HL
}
}
wg.Done()
}
// ... 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() {
in := gen(2, 3)
// Distribute the sq work across two goroutines that both read from in.
c1 := sq(in)
c2 := sq(in)
// Consume the first value from output.
done := make(chan struct{}, 2) // HL
out := merge(done, c1, c2)
fmt.Println(<-out) // 4 or 9
// Tell the remaining senders we're leaving.
done <- struct{}{} // HL
done <- struct{}{} // HL
}
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