rand: add pub fn shuffle<T>(mut a []T) { function + tests (#13811)

vlib/rand/rand.v

@@ -467,3 +467,20 @@ pub fn hex(len int) string {
 pub fn ascii(len int) string {
 	return string_from_set(rand.ascii_chars, len)
 }
+
+// shuffle randomly permutates the elements in `a`.
+pub fn shuffle<T>(mut a []T) {
+	len := a.len
+	for i in 0 .. len {
+		si := i + intn(len - i) or { len }
+		a[si], a[i] = a[i], a[si]
+	}
+}
+
+// shuffle_clone returns a random permutation of the elements in `a`.
+// The permutation is done on a fresh clone of `a`, so `a` remains unchanged.
+pub fn shuffle_clone<T>(a []T) []T {
+	mut res := a.clone()
+	shuffle(mut res)
+	return res
+}