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Random fixes. E.g., (random) never returned negative values.

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* fns.c (Frandom): Use GET_EMACS_TIME for random seed, and add the
subseconds part to the entropy, as that's a bit more random.
Prefer signed to unsigned, since the signedness doesn't matter and
in general we prefer signed.  When given a limit, use a
denominator equal to INTMASK + 1, not to VALMASK + 1, because the
latter isn't right if USE_2_TAGS_FOR_INTS.
* sysdep.c (get_random): Return a value in the range 0..INTMASK,
not 0..VALMASK.  Don't discard "excess" bits that random () returns.
This commit is contained in:
Paul Eggert 2011-07-04 19:51:15 -07:00
parent b768cdcd31
commit d8ed26bd07
3 changed files with 26 additions and 7 deletions
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12
src/ChangeLog
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@ -1,3 +1,15 @@
2011-07-05 Paul Eggert <eggert@cs.ucla.edu>
Random fixes. E.g., (random) never returned negative values.
* fns.c (Frandom): Use GET_EMACS_TIME for random seed, and add the
subseconds part to the entropy, as that's a bit more random.
Prefer signed to unsigned, since the signedness doesn't matter and
in general we prefer signed. When given a limit, use a
denominator equal to INTMASK + 1, not to VALMASK + 1, because the
latter isn't right if USE_2_TAGS_FOR_INTS.
* sysdep.c (get_random): Return a value in the range 0..INTMASK,
not 0..VALMASK. Don't discard "excess" bits that random () returns.
2011-07-04 Stefan Monnier <monnier@iro.umontreal.ca>
* textprop.c (text_property_stickiness):

10
src/fns.c
View file

@ -79,10 +79,14 @@ Other values of LIMIT are ignored. */)
{
EMACS_INT val;
Lisp_Object lispy_val;
EMACS_UINT denominator;
if (EQ (limit, Qt))
seed_random (getpid () + time (NULL));
{
EMACS_TIME t;
EMACS_GET_TIME (t);
seed_random (getpid () ^ EMACS_SECS (t) ^ EMACS_USECS (t));
}
if (NATNUMP (limit) && XFASTINT (limit) != 0)
{
/* Try to take our random number from the higher bits of VAL,
@ -92,7 +96,7 @@ Other values of LIMIT are ignored. */)
it's possible to get a quotient larger than n; discarding
these values eliminates the bias that would otherwise appear
when using a large n. */
denominator = ((EMACS_UINT) 1 << VALBITS) / XFASTINT (limit);
EMACS_INT denominator = (INTMASK + 1) / XFASTINT (limit);
do
val = get_random () / denominator;
while (val >= XFASTINT (limit));

11
src/sysdep.c
View file

@ -1783,7 +1783,8 @@ seed_random (long int arg)
}
/*
* Build a full Emacs-sized word out of whatever we've got.
* Return a nonnegative random integer out of whatever we've got.
* It contains enough bits to make a random (signed) Emacs fixnum.
* This suffices even for a 64-bit architecture with a 15-bit rand.
*/
EMACS_INT
@ -1791,9 +1792,11 @@ get_random (void)
{
EMACS_UINT val = 0;
int i;
for (i = 0; i < (VALBITS + RAND_BITS - 1) / RAND_BITS; i++)
val = (val << RAND_BITS) ^ random ();
return val & (((EMACS_INT) 1 << VALBITS) - 1);
for (i = 0; i < (FIXNUM_BITS + RAND_BITS - 1) / RAND_BITS; i++)
val = (random () ^ (val << RAND_BITS)
^ (val >> (BITS_PER_EMACS_INT - RAND_BITS)));
val ^= val >> (BITS_PER_EMACS_INT - FIXNUM_BITS);
return val & INTMASK;
}
#ifndef HAVE_STRERROR