diff --git a/util_test.go b/util_test.go
new file mode 100644
index 0000000..a309202
--- /dev/null
+++ b/util_test.go
@@ -0,0 +1,40 @@
+package lottery
+
+import "testing"
+
+func TestDrawUnique(t *testing.T) {
+	for _, tc := range []struct {
+		name   string
+		count  int
+		maxNum int
+	}{
+		{"small", 3, 10},
+		{"exact", 5, 5},
+		{"larger", 10, 50},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			for range 100 {
+				nums := drawUnique(tc.count, tc.maxNum)
+				if len(nums) != tc.count {
+					t.Fatalf("got len=%d, want %d", len(nums), tc.count)
+				}
+
+				seen := make(map[int]struct{}, len(nums))
+				prev := 0
+				for _, n := range nums {
+					if n < 1 || n > tc.maxNum {
+						t.Fatalf("value %d out of range [1,%d]", n, tc.maxNum)
+					}
+					if n < prev {
+						t.Fatalf("values not sorted: %v", nums)
+					}
+					if _, ok := seen[n]; ok {
+						t.Fatalf("duplicate value %d in %v", n, nums)
+					}
+					prev = n
+					seen[n] = struct{}{}
+				}
+			}
+		})
+	}
+}