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Random numbers broken on 64-bit platforms
From: |
Andreas Rottmann |
Subject: |
Random numbers broken on 64-bit platforms |
Date: |
Fri, 16 Jul 2010 15:54:39 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/23.2 (gnu/linux) |
Hi!
While implementing SRFI-27, I noticed that Guile's RNG is quite broken
on 64-bit platforms (where unsigned long is 64 bit). This has two
consequences:
- Crashes for some numbers between 2^32 and 2^63; for example try this:
(random (expt 2 46))
- Bignum random numbers will be seriously distorted in their
distribution, since scm_c_random() only returns 32 random bits, and
that function is used by scm_c_random_bignum(), which expects it to
return a `unsigned long' worth of random bits, AFAICT.
I believe the attached patch fixes this issue.
From: Andreas Rottmann <address@hidden>
Subject: Fix random number generator on 64-bit platforms
* libguile/random.c (scm_c_random): On platforms where `unsigned long' has 64
bit, generate up to 64 bit of randomness. This is expected by
scm_c_random_bignum(), and hence was a serious distortion of the random value
distribution for values exceeding 2^32. This change also fixes a crash when
the `m' argument is a value above 2^32.
---
libguile/random.c | 28 +++++++++++++++++++++++++++-
1 files changed, 27 insertions(+), 1 deletions(-)
diff --git a/libguile/random.c b/libguile/random.c
index 281d43a..1a9fd59 100644
--- a/libguile/random.c
+++ b/libguile/random.c
@@ -223,7 +223,8 @@ unsigned char scm_masktab[256];
unsigned long
scm_c_random (scm_t_rstate *state, unsigned long m)
{
- unsigned int r, mask;
+ unsigned long r, mask;
+#if SCM_SIZEOF_UNSIGNED_LONG == 4
mask = (m < 0x100
? scm_masktab[m]
: (m < 0x10000
@@ -232,6 +233,31 @@ scm_c_random (scm_t_rstate *state, unsigned long m)
? scm_masktab[m >> 16] << 16 | 0xffff
: scm_masktab[m >> 24] << 24 | 0xffffff)));
while ((r = scm_the_rng.random_bits (state) & mask) >= m);
+#elif SCM_SIZEOF_UNSIGNED_LONG == 8
+ mask = (m < 0x100
+ ? scm_masktab[m]
+ : (m < 0x10000
+ ? scm_masktab[m >> 8] << 8 | 0xff
+ : (m < 0x1000000
+ ? scm_masktab[m >> 16] << 16 | 0xffff
+ : (m < (1UL << 32)
+ ? scm_masktab[m >> 24] << 24 | 0xffffff
+ : (m < (1UL << 40)
+ ? ((unsigned long) scm_masktab[m >> 32] << 32
+ | 0xffffffffUL)
+ : (m < (1UL << 48)
+ ? ((unsigned long) scm_masktab[m >> 40] << 40
+ | 0xffffffffffUL)
+ : (m < (1UL << 56)
+ ? ((unsigned long) scm_masktab[m >> 48] << 48
+ | 0xffffffffffffUL)
+ : ((unsigned long) scm_masktab[m >> 56] << 56
+ | 0xffffffffffffffUL))))))));
+ while ((r = ((scm_the_rng.random_bits (state) << 32
+ | scm_the_rng.random_bits (state))) & mask) >= m);
+#else
+#error "Cannot deal with this platform's unsigned long size"
+#endif
return r;
}
--
tg: (3fdc1d0..) t/fix-random-64bit (depends on: master)
Regards, Rotty
--
Andreas Rottmann -- <http://rotty.yi.org/>
- Random numbers broken on 64-bit platforms,
Andreas Rottmann <=