import "container/ring"

// Use a goroutine to receive values from `out` and store them
// in an auto-expanding buffer, so that sending to `out` never blocks.
// Return a channel which serves as a sending proxy to to `out`.
func sendproxy(out chan<- int) chan<- int {
	in := make(chan int, 100)
	go func() {
		n := 1000 // the allocated length of the circular queue
		first := ring.New(n)
		last := first
		var c chan<- int
		var e int
		var ok bool
		for {
			select {
			case e = <-in:
				for added := 0; added < 1000; added++ {
					if closed(in) {
						in = nil
						break
					}
					last.Value = e
					if last.Next() == first {
						// buffer full: expand it
						last.Link(ring.New(n))
						n *= 2
					}
					last = last.Next()
					if e, ok = <-in; !ok {
						break
					}
				}
				c = out // enable output
				e = first.Value.(int)
			case c <- e:
				for {
					first = first.Next()
					if first == last {
						// buffer empty: disable output
						if in == nil {
							close(out)
							return
						}
						c = nil
						break
					}
					e = first.Value.(int)
					if ok = c <- e; !ok {
						break
					}
				}
			}
		}
	}()
	return in
}