package ints

// Faculty returns the faculty of i.
func Faculty(i int) int {
	if i < 2 {
		return 1
	}
	return i * Faculty(i-1)
}

// Faculty64 returns the faculty of i.
func Faculty64(i int64) int64 {
	if i < 2 {
		return 1
	}
	return i * Faculty64(i-1)
}

// Permutations returns all possible permutations of the the set integers.
func Permutations(set []int) [][]int {
	var n = len(set)
	if n == 0 {
		return nil
	}
	var res = make([][]int, 0, Faculty(n))
	var p = append([]int(nil), set...)

	var c = make([]int, n)
	res = append(res, append([]int(nil), p...))
	i := 0
	for i < n {
		if c[i] < i {
			if i%2 == 0 {
				p[0], p[i] = p[i], p[0]
			} else {
				p[c[i]], p[i] = p[i], p[c[i]]
			}
			res = append(res, append([]int(nil), p...))
			c[i] = c[i] + 1
			i = 0
		} else {
			c[i] = 0
			i++
		}
	}
	return res
}