Permutations for Golang (Heap's algolithm)

permutations.go

package permutations

func Permutations(arr []int) <-chan []int {
	var fn func([]int, int)
	ch := make(chan []int)
	token := make(chan struct{}, factorial(len(arr)))

	fn = func(arr []int, n int) {
		if n == 1 {
			tmp := make([]int, len(arr))
			copy(tmp, arr)
			token <- struct{}{}
			ch <- tmp
			if len(token) == cap(token) {
				close(ch)
			}
		} else {
			for i := 0; i < n; i++ {
				fn(arr, n-1)
				if n%2 == 1 {
					arr[i], arr[n-1] = arr[n-1], arr[i]
				} else {
					arr[0], arr[n-1] = arr[n-1], arr[0]
				}
			}
		}
	}
	go fn(arr, len(arr))
	return ch
}

func factorial(n int) int {
	if n <= 1 {
		return 1
	}
	return n * factorial(n-1)
}