duktape-1.5.2/src-separate/duk_util_tinyrandom.c

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/*
 *  A tiny random number generator.
 *
 *  Currently used for Math.random().
 *
 *  http://www.woodmann.com/forum/archive/index.php/t-3100.html
 */
 
#include "duk_internal.h"
 
#define DUK__UPDATE_RND(rnd) do { \
        (rnd) += ((rnd) * (rnd)) | 0x05; \
        (rnd) = ((rnd) & 0xffffffffU);       /* if duk_uint32_t is exactly 32 bits, this is a NOP */ \
    } while (0)
 
#define DUK__RND_BIT(rnd)  ((rnd) >> 31)  /* only use the highest bit */
 
DUK_INTERNAL duk_uint32_t duk_util_tinyrandom_get_bits(duk_hthread *thr, duk_small_int_t n) {
    duk_small_int_t i;
    duk_uint32_t res = 0;
    duk_uint32_t rnd;
 
    rnd = thr->heap->rnd_state;
 
    for (i = 0; i < n; i++) {
        DUK__UPDATE_RND(rnd);
        res <<= 1;
        res += DUK__RND_BIT(rnd);
    }
 
    thr->heap->rnd_state = rnd;
 
    return res;
}
 
DUK_INTERNAL duk_double_t duk_util_tinyrandom_get_double(duk_hthread *thr) {
    duk_double_t t;
    duk_small_int_t n;
    duk_uint32_t rnd;
 
    /*
     *  XXX: could make this a lot faster if we create the double memory
     *  representation directly.  Feasible easily (must be uniform random).
     */
 
    rnd = thr->heap->rnd_state;
 
    n = 53;  /* enough to cover the whole mantissa */
    t = 0.0;
 
    do {
        DUK__UPDATE_RND(rnd);
        t += DUK__RND_BIT(rnd);
        t /= 2.0;
    } while (--n);
 
    thr->heap->rnd_state = rnd;
 
    DUK_ASSERT(t >= (duk_double_t) 0.0);
    DUK_ASSERT(t < (duk_double_t) 1.0);
 
    return t;
}