rand: add `shuffle` function for array

vlib/rand/rand.v

@@ -467,3 +467,12 @@ 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]
+	}
+}

vlib/rand/random_numbers_test.v

@@ -317,3 +317,10 @@ fn test_new_global_rng() {
 
 	rand.set_rng(old)
 }
+
+fn test_shuffle() {
+	mut a := get_n_random_ints(seeds[0], 10)
+	rand.shuffle(mut a)
+	assert a == [4, 0, 8, 0, 1, 2, 1, 1, 0, 9]
+}
+