mpfr_add_one_ulp and mpfr_sub_one_ulp changed.

git-svn-id: svn://scm.gforge.inria.fr/svn/mpfr/trunk@1570 280ebfd0-de03-0410-8827-d642c229c3f4

1 files changed, 63 insertions, 0 deletions

diff --git a/sub_one_ulp.c b/sub_one_ulp.c
new file mode 100644
index 000000000..d4ed213a1
--- /dev/null
+++ b/sub_one_ulp.c
@@ -0,0 +1,63 @@
+/* mpfr_sub_one_ulp -- subtract one unit in last place
+
+Copyright (C) 1999, 2001 Free Software Foundation, Inc.
+
+This file is part of the MPFR Library.
+
+The MPFR Library is free software; you can redistribute it and/or modify
+it under the terms of the GNU Lesser General Public License as published by
+the Free Software Foundation; either version 2.1 of the License, or (at your
+option) any later version.
+
+The MPFR Library 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 Lesser General Public
+License for more details.
+
+You should have received a copy of the GNU Lesser General Public License
+along with the MPFR Library; see the file COPYING.LIB.  If not, write to
+the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
+MA 02111-1307, USA. */
+
+#include "gmp.h"
+#include "gmp-impl.h"
+#include "mpfr.h"
+#include "mpfr-impl.h"
+
+/* sets x to x-sign(x)*ulp(x) */
+int
+mpfr_sub_one_ulp(mpfr_ptr x, mp_rnd_t rnd_mode)
+{
+  mp_size_t xn;
+  int sh;
+  mp_limb_t *xp;
+
+  if (MPFR_IS_NAN(x))
+    MPFR_RET_NAN;
+
+  if (MPFR_IS_INF(x) || MPFR_IS_ZERO(x))
+    return 0;
+
+  if (MPFR_PREC(x) == 1)
+    return mpfr_sub(x, x, x, rnd_mode);  /* to generate the correct zero */
+
+  xn = 1 + (MPFR_PREC(x) - 1) / BITS_PER_MP_LIMB;
+  sh = xn * BITS_PER_MP_LIMB - MPFR_PREC(x);
+  xp = MPFR_MANT(x);
+  mpn_sub_1 (xp, xp, xn, MP_LIMB_T_ONE << sh);
+  if (xp[xn-1] >> (BITS_PER_MP_LIMB - 1) == 0)
+    { /* was an exact power of two: not normalized any more */
+      mp_exp_t exp = MPFR_EXP(x);
+      if (exp == __mpfr_emin)
+        return mpfr_set_underflow(x, rnd_mode, MPFR_SIGN(x));
+      else
+        {
+          int i;
+          MPFR_EXP(x)--;
+          xp[0] = (sh + 1 == BITS_PER_MP_LIMB) ? 0 : MP_LIMB_T_MAX << (sh + 1);
+          for (i = 1; i < xn; i++)
+            xp[i] = MP_LIMB_T_MAX;
+        }
+    }
+  return 0;
+}