Non-destructively computing size of buffered channel of struct{} (Go)

countbuffer.go

package main

import "fmt"

func main() {
	r := make(chan struct{}, 2)
	r <- struct{}{}
	r <- struct{}{}

	s := size(r, 0)

	// 2
	fmt.Println(s)
	
	// Still 2
	fmt.Println(s)

	<-r
	s = size(r, 0)

	// 1
	fmt.Println(s)
}

// size computes the number of elements buffered inside r. When first calling, s must be 0.
// When the size is finally computed, that figure, before being returned, is used to refill the
// buffered channel with its original contents. This of course works because everything is
// simply a struct{} :)
func size(r chan struct{}, s int) int {
	select {
	case <-r:
		return size(r, s+1)
	default:
		// Put everything back.
		for range s {
			r <- struct{}{}
		}

		return s
	}
}