rand: move functions from rand.util to the main module (#13840)

vlib/rand/random_numbers_test.v

@@ -326,7 +326,6 @@ fn test_shuffle() {
 		a := get_n_random_ints(seed, 10)
 		arrays << a
 	}
-	//
 	mut digits := []map[int]int{len: 10}
 	for digit in 0 .. 10 {
 		digits[digit] = {}
@@ -337,7 +336,7 @@ fn test_shuffle() {
 	for mut a in arrays {
 		o := a.clone()
 		for _ in 0 .. 100 {
-			rand.shuffle(mut a)
+			rand.shuffle(mut a) or { panic('shuffle failed') }
 			assert *a != o
 			for idx in 0 .. 10 {
 				digits[idx][a[idx]]++
@@ -355,12 +354,25 @@ fn test_shuffle() {
 	}
 }
 
+fn test_shuffle_partial() ? {
+	mut a := [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+	mut b := a.clone()
+
+	rand.shuffle(mut a, start: 4) ?
+	assert a[..4] == b[..4]
+
+	a = b.clone()
+	rand.shuffle(mut a, start: 3, end: 7) ?
+	assert a[..3] == b[..3]
+	assert a[7..] == b[7..]
+}
+
 fn test_shuffle_clone() {
 	original := [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
 	mut a := original.clone()
 	mut results := [][]int{}
 	for _ in 0 .. 10 {
-		results << rand.shuffle_clone(a)
+		results << rand.shuffle_clone(a) or { panic('shuffle failed') }
 	}
 	assert original == a
 	for idx in 1 .. 10 {
@@ -369,3 +381,33 @@ fn test_shuffle_clone() {
 		assert results[idx] != original
 	}
 }
+
+fn test_choose() ? {
+	lengths := [1, 3, 4, 5, 6, 7]
+	a := ['one', 'two', 'three', 'four', 'five', 'six', 'seven']
+	for length in lengths {
+		b := rand.choose(a, length) ?
+		assert b.len == length
+		for element in b {
+			assert element in a
+			// make sure every element occurs once
+			mut count := 0
+			for e in b {
+				if e == element {
+					count++
+				}
+			}
+			assert count == 1
+		}
+	}
+}
+
+fn test_sample() {
+	k := 20
+	a := ['heads', 'tails']
+	b := rand.sample(a, k)
+	assert b.len == k
+	for element in b {
+		assert element in a
+	}
+}