Add reproducible randomized test order

src/random_order.zigadded+42-0

Changes to src/random_order.zig

		@@ -0,0 +1,42 @@
	1	+const std = ;
	2	+const compat = ;
	3	+
	4	+/// Return the seed that should control this run. Supplying a seed enables
	5	+/// shuffling even when `shuffle` is false; otherwise random order is opt-in.
	6	+pub fn resolveSeed(shuffle: bool, configured: ?u64) ?u64 {
	7	+ if (configured) |seed| return seed;
	8	+ if (!shuffle) return null;
	9	+ return @truncate(@as(u128, @bitCast(compat.nanoTimestamp())));
	10	+}
	11	+
	12	+pub const Shuffler = struct {
	13	+ prng: std.Random.DefaultPrng,
	14	+
	15	+ pub fn init(seed: u64) Shuffler {
	16	+ return .{ .prng = std.Random.DefaultPrng.init(seed) };
	17	+ }
	18	+
	19	+ pub fn shuffle(self: *Shuffler, comptime T: type, items: []T) void {
	20	+ self.prng.random().shuffleWithIndex(T, items, u64);
	21	+ }
	22	+};
	23	+
	24	+test "random order is opt-in and an explicit seed enables it" {
	25	+ try std.testing.expect(resolveSeed(false, null) == null);
	26	+ try std.testing.expect(resolveSeed(true, null) != null);
	27	+ try std.testing.expectEqual(@as(?u64, 0), resolveSeed(false, 0));
	28	+ try std.testing.expectEqual(@as(?u64, 42), resolveSeed(true, 42));
	29	+}
	30	+
	31	+test "the same seed produces the same order" {
	32	+ var first = [_]u8{ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
	33	+ var second = first;
	34	+ var first_shuffler = Shuffler.init(123456789);
	35	+ var second_shuffler = Shuffler.init(123456789);
	36	+
	37	+ first_shuffler.shuffle(u8, &first);
	38	+ second_shuffler.shuffle(u8, &second);
	39	+
	40	+ try std.testing.expectEqualSlices(u8, &first, &second);
	41	+ try std.testing.expect(!std.mem.eql(u8, &first, &[_]u8{ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 }));
	42	+}