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Make int-pair-to-real conversion more portable

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And utilize more bits even if DBL_MANT_DIG > 53.
This commit is contained in:
Nobuyoshi Nakada 2020-05-04 00:00:27 +09:00
parent 5aaa75e7c1
commit 5b28f01d77
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GPG key ID: 7CD2805BFA3770C6
1 changed files with 16 additions and 5 deletions
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21
random.c
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@ -14,6 +14,7 @@
#include <errno.h> #include <errno.h>
#include <limits.h> #include <limits.h>
#include <math.h> #include <math.h>
#include <float.h>
#include <time.h> #include <time.h>
#ifdef HAVE_UNISTD_H #ifdef HAVE_UNISTD_H
@ -75,12 +76,22 @@ genrand_real(struct MT *mt)
return int_pair_to_real_exclusive(a, b); return int_pair_to_real_exclusive(a, b);
} }
static const double dbl_reduce_scale = /* 2**(-DBL_MANT_DIG) */
(1.0
/ (double)(DBL_MANT_DIG > 2*31 ? (1ul<<31) : 1.0)
/ (double)(DBL_MANT_DIG > 1*31 ? (1ul<<31) : 1.0)
/ (double)(1ul<<(DBL_MANT_DIG%31)));
static double static double
int_pair_to_real_exclusive(uint32_t a, uint32_t b) int_pair_to_real_exclusive(uint32_t a, uint32_t b)
{ {
a >>= 5; static const int a_shift = DBL_MANT_DIG < 64 ?
b >>= 6; (64-DBL_MANT_DIG)/2 : 0;
return(a*67108864.0+b)*(1.0/9007199254740992.0); static const int b_shift = DBL_MANT_DIG < 64 ?
(64-DBL_MANT_DIG)-a_shift : 0;
a >>= a_shift;
b >>= b_shift;
return (a*(double)(1ul<<(32-b_shift))+b)*dbl_reduce_scale;
} }
/* generates a random number on [0,1] with 53-bit resolution*/ /* generates a random number on [0,1] with 53-bit resolution*/
@ -148,7 +159,7 @@ static double
int_pair_to_real_inclusive(uint32_t a, uint32_t b) int_pair_to_real_inclusive(uint32_t a, uint32_t b)
{ {
double r; double r;
enum {dig = 53}; enum {dig = DBL_MANT_DIG};
enum {dig_u = dig-32, dig_r64 = 64-dig, bmask = ~(~0u<<(dig_r64))}; enum {dig_u = dig-32, dig_r64 = 64-dig, bmask = ~(~0u<<(dig_r64))};
#if defined HAVE_UINT128_T #if defined HAVE_UINT128_T
const uint128_t m = ((uint128_t)1 << dig) | 1; const uint128_t m = ((uint128_t)1 << dig) | 1;
@ -163,7 +174,7 @@ int_pair_to_real_inclusive(uint32_t a, uint32_t b)
b = (b >> dig_r64) + (((a >> dig_u) + (b & bmask)) >> dig_r64); b = (b >> dig_r64) + (((a >> dig_u) + (b & bmask)) >> dig_r64);
r = (double)a * (1 << dig_u) + b; r = (double)a * (1 << dig_u) + b;
#endif #endif
return ldexp(r, -dig); return r * dbl_reduce_scale;
} }
VALUE rb_cRandom; VALUE rb_cRandom;