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

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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).
This commit is contained in:
Tor Andersson
2021-10-24 13:12:07 +02:00
parent fe8cac61e3
commit 3bd234c685
1 changed files with 23 additions and 19 deletions
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42
jsmath.c
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@@ -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;
@@ -94,7 +85,21 @@ static void Math_pow(js_State *J)
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)
@@ -156,8 +161,7 @@ static void Math_min(js_State *J)
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);