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

diff --git a/math/mprand.c b/math/mprand.c
index eb779cb8..5805e6ed 100644
--- a/math/mprand.c
+++ b/math/mprand.c
@@ -49,17 +49,27 @@
  *		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.
+ *
+ *		The length @b@ may be zero; but %$\texttt{or} \ge 2^b$% is
+ *		not permitted.
  */
 
 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);
+  octet *v;
   unsigned m;
 
+  assert(b >= MPW_BITS || !(or >> b));
+
+  /* --- Special case --- */
+
+  if (!b) return (MP_ZERO);
+
   /* --- Fill buffer with random data --- */
 
+  v = x_alloc(a, sz);
   r->ops->fill(r, v, sz);
 
   /* --- Force into the correct range --- *
@@ -74,7 +84,11 @@ mp *mprand(mp *d, unsigned b, grand *r, mpw or)
   /* --- Mask, load and return --- */
 
   d = mp_loadb(d, v, sz);
-  d->v[0] |= or;
+  if (or) {
+    assert(d->sz);
+    if (!MP_LEN(d)) d->vl = d->v + 1;
+    d->v[0] |= or;
+  }
   memset(v, 0, sz);
   x_free(a, v);
   return (d);
@@ -91,7 +105,7 @@ mp *mprand(mp *d, unsigned b, grand *r, mpw or)
  *		interval %$[0, l)$%.
  *
  * Use:		Generates a uniformly-distributed pseudorandom number in the
- *		appropriate range.
+ *		appropriate range.  We must have %$l > 0$%.
  */
 
 mp *mprand_range(mp *d, mp *l, grand *r, mpw or)
@@ -119,6 +133,7 @@ mp *mprand_range(mp *d, mp *l, grand *r, mpw or)
    * As usual, the number of iterations expected is two.
    */
 
+  assert(MP_POSP(l));
   b = ((b - 1) & 7) + 1;
   m = (1 << b) - 1;
   do {