shithub: libmujs

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Math.random: Use Lehmer LCG instead of borrowing the hack from musl.

For simplicity of implementation, use the minimal standard generator
described in "Random Number Generators: Good ones are hard to find"
by Park & Miller (ACM 1988, Volume 31, Number 10).

--- a/jsmath.c

+++ b/jsmath.c

@@ -2,24 +2,15 @@

 #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>

-#define JS_RAND_MAX (0x7fffffff)

-

-static unsigned int jsM_rand_temper(unsigned int x)

-{

-	x ^= x>>11;

-	x ^= x<<7 & 0x9D2C5680;

-	x ^= x<<15 & 0xEFC60000;

-	x ^= x>>18;

-	return x;

-}

-

-static int jsM_rand_r(unsigned int *seed)

-{

-	return jsM_rand_temper(*seed = *seed * 1103515245 + 12345)/2;

-}

-

 static double jsM_round(double x)

 {

 	if (isnan(x)) return x;

@@ -94,9 +85,23 @@

 static void Math_random(js_State *J)

 {

-	js_pushnumber(J, jsM_rand_r(&J->seed) / (JS_RAND_MAX + 1.0));

+	/* 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);

 }

+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;

+}

+

 static void Math_round(js_State *J)

 {

 	double x = js_tonumber(J, 1);

@@ -156,8 +161,7 @@

 void jsB_initmath(js_State *J)

 {

-	J->seed = time(NULL);

-

+	Math_init_random(J);

 	js_pushobject(J, jsV_newobject(J, JS_CMATH, J->Object_prototype));

 	{

 		jsB_propn(J, "E", 2.7182818284590452354);