permutations added

README.md

-# ia04
+# Quelques utilitaires IA04
 

go.mod

+module gitlab.utc.fr/lagruesy/ia04
+
+go 1.16

utils/permutations.go

+// Package utils provides some usefull tools for IA04
+package utils
+
+// ReverseIntSlice reverses the given slice
+func ReverseIntSlice(a []int) {
+	n := len(a)
+	for i := 0; i < n/2; i++ {
+		a[i], a[n-i-1] = a[n-i-1], a[i]
+	}
+}
+
+// Factorial returns the product of all positive integers less than or equal to n
+func Factorial(n int) int {
+	res := 1
+	for i := 0; i <= n; i++ {
+		res *= i
+	}
+	return res
+}
+
+// FirstPermutation returns the slice of int [0, 1, ..., n]
+func FirstPermutation(n int) (a []int) {
+	a = make([]int, n)
+	for i := 0; i < n; i++ {
+		a[i] = i
+	}
+	return
+}
+
+// NextPermutation returns the next lexicographical permutation of a slice a
+// The result is stored in a new slice
+func NextPermutation(a []int) (res []int, ok bool) {
+	n := len(a)
+	res = make([]int, n)
+	copy(res, a)
+
+	if n < 2 {
+		return res, false
+	}
+
+	k := n - 2
+	for a[k] >= a[k+1] {
+		k--
+		if k < 0 {
+			return res, false
+		}
+	}
+
+	l := n - 1
+	for a[k] >= a[l] {
+		l--
+	}
+
+	res[k], res[l] = res[l], res[k]
+	ReverseIntSlice(res[k+1:])
+
+	return res, true
+}