[src/digamma.c] fixed bug in mpfr_digamma_positive when t-u is zero

[tests/tdigamma.c] added corresponding non-regression test


git-svn-id: https://scm.gforge.inria.fr/anonscm/svn/mpfr/trunk@14399 280ebfd0-de03-0410-8827-d642c229c3f4

1 files changed, 18 insertions, 15 deletions

diff --git a/src/digamma.c b/src/digamma.c
index 6dba748e6..119c7430f 100644
--- a/src/digamma.c
+++ b/src/digamma.c
@@ -296,21 +296,24 @@ mpfr_digamma_positive (mpfr_ptr y, mpfr_srcptr x, mpfr_rnd_t rnd_mode)
       errt = mpfr_digamma_approx (t, x_plus_j);
       expt = MPFR_GET_EXP (t);
       mpfr_sub (t, t, u, MPFR_RNDN);
-      if (MPFR_GET_EXP (t) < expt)
-        errt += expt - MPFR_EXP(t);
-      /* Warning: if u is zero (which happens when x_plus_j >= min at the
-         beginning of the while loop above), EXP(u) is not defined.
-         In this case we have no error from u. */
-      if (MPFR_NOTZERO(u) && MPFR_GET_EXP (t) < MPFR_GET_EXP (u))
-        erru += MPFR_EXP(u) - MPFR_EXP(t);
-      if (errt > erru)
-        errt = errt + 1;
-      else if (errt == erru)
-        errt = errt + 2;
-      else
-        errt = erru + 1;
-      if (MPFR_CAN_ROUND (t, p - errt, MPFR_PREC(y), rnd_mode))
-        break;
+      if (!MPFR_IS_ZERO(t))
+        {
+          if (MPFR_GET_EXP (t) < expt)
+            errt += expt - MPFR_EXP(t);
+          /* Warning: if u is zero (which happens when x_plus_j >= min at the
+             beginning of the while loop above), EXP(u) is not defined.
+             In this case we have no error from u. */
+          if (MPFR_NOTZERO(u) && MPFR_GET_EXP (t) < MPFR_GET_EXP (u))
+            erru += MPFR_EXP(u) - MPFR_EXP(t);
+          if (errt > erru)
+            errt = errt + 1;
+          else if (errt == erru)
+            errt = errt + 2;
+          else
+            errt = erru + 1;
+          if (MPFR_CAN_ROUND (t, p - errt, MPFR_PREC(y), rnd_mode))
+            break;
+        }
       MPFR_ZIV_NEXT (loop, p);
       mpfr_set_prec (t, p);
       mpfr_set_prec (u, p);