X-Git-Url: https://git.distorted.org.uk/u/mdw/catacomb/blobdiff_plain/ba6e6b64033b1f9de49feccb5c9cd438354481f7..0f00dc4c8eb47e67bc0f148c2dd109f73a451e0a:/math/mprand.c

diff --git a/math/mprand.c b/math/mprand.c
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+/* -*-c-*-
+ *
+ * Generate a random multiprecision integer
+ *
+ * (c) 1999 Straylight/Edgeware
+ */
+
+/*----- Licensing notice --------------------------------------------------*
+ *
+ * This file is part of Catacomb.
+ *
+ * Catacomb is free software; you can redistribute it and/or modify
+ * 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.
+ */
+
+/*----- Header files ------------------------------------------------------*/
+
+#include <mLib/alloc.h>
+
+#include "grand.h"
+#include "mp.h"
+#include "mprand.h"
+
+/*----- Main code ---------------------------------------------------------*/
+
+/* --- @mprand@ --- *
+ *
+ * Arguments:	@mp *d@ = destination integer
+ *		@unsigned b@ = number of bits
+ *		@grand *r@ = pointer to random number source
+ *		@mpw or@ = mask to OR with low-order bits
+ *
+ * Returns:	A random integer with the requested number of bits.
+ *
+ * Use:		Constructs an arbitrarily large pseudorandom integer.
+ *		Assuming that the generator @r@ is good, the result is
+ *		uniformly distributed in the interval %$[2^{b - 1}, 2^b)$%.
+ *		The result is then ORred with the given @or@ value.  This
+ *		will often be 1, to make the result odd.
+ */
+
+mp *mprand(mp *d, unsigned b, grand *r, mpw or)
+{
+  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 --- */
+
+  r->ops->fill(r, v, sz);
+
+  /* --- Force into the correct range --- *
+   *
+   * This is slightly tricky.  Oh, well.
+   */
+
+  b = (b - 1) & 7;
+  m = (1 << b);
+  v[0] = (v[0] & (m - 1)) | m;
+
+  /* --- Mask, load and return --- */
+
+  d = mp_loadb(d, v, sz);
+  d->v[0] |= or;
+  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);
+}
+
+/*----- That's all, folks -------------------------------------------------*/