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".

Author: artifex-tor

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 */

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)