[src/urandom.c] Fixed bug reported by Trevor Spiteri:

  <https://sympa.inria.fr/sympa/arc/mpfr/2017-01/msg00020.html>
[tests/turandom.c] Added non-regression test.
(merged changesets r11218-11219,11221 from the trunk)

git-svn-id: svn://scm.gforge.inria.fr/svn/mpfr/branches/3.1@11648 280ebfd0-de03-0410-8827-d642c229c3f4

2 files changed, 24 insertions, 4 deletions

diff --git a/src/urandom.c b/src/urandom.c
index 8ca5f043e..0145be52c 100644
--- a/src/urandom.c
+++ b/src/urandom.c
@@ -87,19 +87,20 @@ mpfr_urandom (mpfr_ptr rop, gmp_randstate_t rstate, mpfr_rnd_t rnd_mode)
           count_leading_zeros (cnt, rp[0]);
           cnt -= GMP_NUMB_BITS - DRAW_BITS;
         }
+      exp -= cnt;  /* no integer overflow */
 
-      if (MPFR_UNLIKELY (exp < emin + cnt))
+      if (MPFR_UNLIKELY (exp < emin))
         {
           /* To get here, we have been drawing more than -emin zeros
              in a row, then return 0 or the smallest representable
              positive number.
 
              The rounding to nearest mode is subtle:
-             If exp - cnt == emin - 1, the rounding bit is set, except
+             If exp == emin - 1, the rounding bit is set, except
              if cnt == DRAW_BITS in which case the rounding bit is
              outside rp[0] and must be generated. */
           if (rnd_mode == MPFR_RNDU || rnd_mode == MPFR_RNDA
-              || (rnd_mode == MPFR_RNDN && cnt == exp - emin - 1
+              || (rnd_mode == MPFR_RNDN && exp == emin - 1
                   && (cnt != DRAW_BITS || random_rounding_bit (rstate))))
             {
               mpfr_set_ui_2exp (rop, 1, emin - 1, rnd_mode);
@@ -111,7 +112,6 @@ mpfr_urandom (mpfr_ptr rop, gmp_randstate_t rstate, mpfr_rnd_t rnd_mode)
               return -1;
             }
         }
-      exp -= cnt;
     }
   MPFR_EXP (rop) = exp; /* Warning: may be outside the current
                            exponent range */
diff --git a/tests/turandom.c b/tests/turandom.c
index 53bb7ee47..ec5ff8ff9 100644
--- a/tests/turandom.c
+++ b/tests/turandom.c
@@ -191,6 +191,24 @@ bug20100914 (void)
   gmp_randclear (s);
 }
 
+/* non-regression test for bug reported by Trevor Spiteri
+   https://sympa.inria.fr/sympa/arc/mpfr/2017-01/msg00020.html */
+static void
+bug20170123 (void)
+{
+  mpfr_t x;
+  mpfr_exp_t emin;
+
+  emin = mpfr_get_emin ();
+  mpfr_set_emin (-7);
+  mpfr_init2 (x, 53);
+  gmp_randseed_ui (mpfr_rands, 398);
+  mpfr_urandom (x, mpfr_rands, MPFR_RNDN);
+  MPFR_ASSERTN(mpfr_cmp_ui_2exp (x, 1, -8) == 0);
+  mpfr_clear (x);
+  mpfr_set_emin (emin);
+}
+
 int
 main (int argc, char *argv[])
 {
@@ -244,6 +262,8 @@ main (int argc, char *argv[])
 
   bug20100914 ();
 
+  bug20170123 ();
+
   tests_end_mpfr ();
   return 0;
 }