X-Git-Url: https://git.distorted.org.uk/u/mdw/catacomb/blobdiff_plain/893c6259cd5d374f540e54def522dcd251c4c503..f52f2db067dc1388b16ab00ddb53e26a381a6e3e:/mprand.c

diff --git a/mprand.c b/mprand.c
index 41c2b39..c0f9675 100644
--- a/mprand.c
+++ b/mprand.c
@@ -1,13 +1,13 @@
 /* -*-c-*-
  *
- * $Id: mprand.c,v 1.1 1999/12/10 23:23:05 mdw Exp $
+ * $Id: mprand.c,v 1.5 2004/04/08 01:36:15 mdw Exp $
  *
  * Generate a random multiprecision integer
  *
  * (c) 1999 Straylight/Edgeware
  */
 
-/*----- Licensing notice --------------------------------------------------* 
+/*----- Licensing notice --------------------------------------------------*
  *
  * This file is part of Catacomb.
  *
@@ -15,26 +15,18 @@
  * it under the terms of the GNU Library General Public License as
  * published by the Free Software Foundation; either version 2 of the
  * License, or (at your option) any later version.
- * 
+ *
  * Catacomb is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU Library General Public License for more details.
- * 
+ *
  * You should have received a copy of the GNU Library General Public
  * License along with Catacomb; if not, write to the Free
  * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
  * MA 02111-1307, USA.
  */
 
-/*----- Revision history --------------------------------------------------* 
- *
- * $Log: mprand.c,v $
- * Revision 1.1  1999/12/10 23:23:05  mdw
- * Support for generating random large integers.
- *
- */
-
 /*----- Header files ------------------------------------------------------*/
 
 #include <mLib/alloc.h>
@@ -63,8 +55,9 @@
 
 mp *mprand(mp *d, unsigned b, grand *r, mpw or)
 {
-  size_t sz = (b + 7) / 8;
-  octet *v = xmalloc(sz);
+  size_t sz = (b + 7) >> 3;
+  arena *a = (d && (d->f & MP_BURN)) ? arena_secure : arena_global;
+  octet *v = x_alloc(a, sz);
   unsigned m;
 
   /* --- Fill buffer with random data --- */
@@ -76,7 +69,7 @@ mp *mprand(mp *d, unsigned b, grand *r, mpw or)
    * This is slightly tricky.  Oh, well.
    */
 
-  b = (b - 1) % 8;
+  b = (b - 1) & 7;
   m = (1 << b);
   v[0] = (v[0] & (m - 1)) | m;
 
@@ -84,7 +77,63 @@ mp *mprand(mp *d, unsigned b, grand *r, mpw or)
 
   d = mp_loadb(d, v, sz);
   d->v[0] |= or;
-  free(v);
+  memset(v, 0, sz);
+  x_free(a, v);
+  return (d);
+}
+
+/* --- @mprand_range@ --- *
+ *
+ * Arguments:	@mp *d@ = destination integer
+ *		@mp *l@ = limit for random number
+ *		@grand *r@ = random number source
+ *		@mpw or@ = mask for low-order bits
+ *
+ * Returns:	A pseudorandom integer, unformly distributed over the
+ *		interval %$[0, l)$%.
+ *
+ * Use:		Generates a uniformly-distributed pseudorandom number in the
+ *		appropriate range.
+ */
+
+mp *mprand_range(mp *d, mp *l, grand *r, mpw or)
+{
+  size_t b = mp_bits(l);
+  size_t sz = (b + 7) >> 3;
+  arena *a = (d && (d->f & MP_BURN)) ? arena_secure : arena_global;
+  octet *v = x_alloc(a, sz);
+  unsigned m;
+
+  /* --- The algorithm --- *
+   *
+   * Rather simpler than most.  Find the number of bits in the number %$l$%
+   * (i.e., the integer %$b$% such that %$2^{b - 1} \le l < 2^b$%), and
+   * generate pseudorandom integers with %$n$% bits (but not, unlike in the
+   * function above, with the top bit forced to 1).  If the integer is
+   * greater than or equal to %$l$%, try again.
+   *
+   * This is similar to the algorithms used in @lcrand_range@ and friends,
+   * except that I've forced the `raw' range of the random numbers such that
+   * %$l$% itself is the largest multiple of %$l$% in the range (since, by
+   * the inequality above, %$2^b \le 2l$%).  This removes the need for costly
+   * division and remainder operations.
+   *
+   * As usual, the number of iterations expected is two.
+   */
+
+  b = ((b - 1) & 7) + 1;
+  m = (1 << b) - 1;
+  do {
+    r->ops->fill(r, v, sz);
+    v[0] &= m;
+    d = mp_loadb(d, v, sz);
+    d->v[0] |= or;
+  } while (MP_CMP(d, >=, l));
+
+  /* --- Done --- */
+
+  memset(v, 0, sz);
+  x_free(a, v);
   return (d);
 }