From 6ba36e4f5de082b333d4b3fffd5292a234cdf790 Mon Sep 17 00:00:00 2001
From: Tor Andersson <tor.andersson@artifex.com>
Date: Thu, 23 Jun 2022 12:54:38 +0200
Subject: [PATCH] Improve Math.random randomness.

Largest possible MLCG that only uses integer arithmetic with doubles.

Parameters from Pierre l'Ecuyers paper "Tables of linear congruential
generators of different sizes and good lattice structure".
---
 jsi.h    |  2 +-
 jsmath.c | 27 +++++++++++----------------
 2 files changed, 12 insertions(+), 17 deletions(-)

diff --git a/jsi.h b/jsi.h
index c9e17fd..e2bca8b 100644
--- a/jsi.h
+++ b/jsi.h
@@ -239,7 +239,7 @@ struct js_State
 	js_Object *TypeError_prototype;
 	js_Object *URIError_prototype;
 
-	unsigned int seed; /* Math.random seed */
+	double seed; /* Math.random seed */
 
 	int nextref; /* for js_ref use */
 	js_Object *R; /* registry of hidden values */
diff --git a/jsmath.c b/jsmath.c
index 1998c5d..631d616 100644
--- a/jsmath.c
+++ b/jsmath.c
@@ -2,13 +2,6 @@
 #include "jsvalue.h"
 #include "jsbuiltin.h"
 
-#if defined(_MSC_VER) && (_MSC_VER < 1700) /* VS2012 has stdint.h */
-typedef unsigned int uint32_t;
-typedef unsigned __int64 uint64_t;
-#else
-#include <stdint.h>
-#endif
-
 #include <time.h>
 
 static double jsM_round(double x)
@@ -85,21 +78,23 @@ static void Math_pow(js_State *J)
 
 static void Math_random(js_State *J)
 {
-	/* Lehmer generator with a=48271 and m=2^31-1 */
-	/* Park & Miller (1988). Random Number Generators: Good ones are hard to find. */
-	J->seed = (uint64_t) J->seed * 48271 % 0x7fffffff;
-	js_pushnumber(J, (double) J->seed / 0x7fffffff);
+	// A Lehmer (MLCG) RNG using doubles.
+	// Parameters from Pierre l'Ecuyer's paper:
+	// https://www.ams.org/journals/mcom/1999-68-225/S0025-5718-99-00996-5/S0025-5718-99-00996-5.pdf
+	// m = 2^35 - 31
+	J->seed = J->seed * fmod(200105.0, 34359738337.0);
+	js_pushnumber(J, J->seed / 34359738337.0);
 }
 
 static void Math_init_random(js_State *J)
 {
 	/* Pick initial seed by scrambling current time with Xorshift. */
 	/* Marsaglia (2003). Xorshift RNGs. */
-	J->seed = time(0) + 123;
-	J->seed ^= J->seed << 13;
-	J->seed ^= J->seed >> 17;
-	J->seed ^= J->seed << 5;
-	J->seed %= 0x7fffffff;
+	unsigned int seed = time(0) + 123;
+	seed ^= seed << 13;
+	seed ^= seed >> 17;
+	seed ^= seed << 5;
+	J->seed = seed;
 }
 
 static void Math_round(js_State *J)