From b4b90af4e0a9b15a940d836d275ac9dc50198217 Mon Sep 17 00:00:00 2001
From: Frank Denis <124872+jedisct1@users.noreply.github.com>
Date: Thu, 8 Jul 2021 23:19:49 +0200
Subject: [PATCH] Switch rand.DefaultPrng to Xoshiro256++ (#9301)

Xoroshiro128+ is the current default non-cryptographic random
number generator.

This algorithm was designed to generate floating-point numbers, by
only using the top 53 bits. Lower bits have a significant bias, that
contradicts the documented properties for `rand.DefaultPrng`. This
also has implications on everything using `Random.fill()`, including
the way we generate random floating-point numbers.

In addition, Xoroshiro128+ has known issues. See for example:
- https://lemire.me/blog/2017/08/22/cracking-random-number-generators-xoroshiro128/
- https://www.pcg-random.org/posts/xoroshiro-fails-truncated.html

Xoshiro256++ addresses these issues, while remaining very fast.
---
 lib/std/rand.zig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/std/rand.zig b/lib/std/rand.zig
index d4235722d0..66f758cb48 100644
--- a/lib/std/rand.zig
+++ b/lib/std/rand.zig
@@ -23,7 +23,7 @@ const ziggurat = @import("rand/ziggurat.zig");
 const maxInt = std.math.maxInt;
 
 /// Fast unbiased random numbers.
-pub const DefaultPrng = Xoroshiro128;
+pub const DefaultPrng = Xoshiro256;
 
 /// Cryptographically secure random numbers.
 pub const DefaultCsprng = Gimli;