New module 'trunc'.

[gnulib.git] / lib / trunc.c

diff --git a/lib/trunc.c b/lib/trunc.c
new file mode 100644 (file)
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--- /dev/null
+++ b/lib/trunc.c
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+/* Round towards zero.
+   Copyright (C) 2007 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 2, or (at your option)
+   any later version.
+
+   This program 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 General Public License for more details.
+
+   You should have received a copy of the GNU General Public License along
+   with this program; if not, write to the Free Software Foundation,
+   Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */
+
+/* Written by Bruno Haible <bruno@clisp.org>, 2007.  */
+
+#include <config.h>
+
+/* Specification.  */
+#include <math.h>
+
+#include <float.h>
+
+/* 2^(DBL_MANT_DIG-1).  */
+static const double TWO_MANT_DIG =
+  /* Assume DBL_MANT_DIG <= 4 * 31.
+     Use the identity
+       n = floor(n/4) + floor((n+1)/4) + floor((n+2)/4) + floor((n+3)/4).  */
+  (double) (1U << ((DBL_MANT_DIG - 1) / 4))
+  * (double) (1U << ((DBL_MANT_DIG - 1 + 1) / 4))
+  * (double) (1U << ((DBL_MANT_DIG - 1 + 2) / 4))
+  * (double) (1U << ((DBL_MANT_DIG - 1 + 3) / 4));
+
+double
+trunc (double x)
+{
+  /* The use of 'volatile' guarantees that excess precision bits are dropped
+     at each addition step and before the following comparison at the caller's
+     site.  It is necessary on x86 systems where double-floats are not IEEE
+     compliant by default, to avoid that the results become platform and compiler
+     option dependent.  'volatile' is a portable alternative to gcc's
+     -ffloat-store option.  */
+  volatile double y = x;
+  volatile double z = y;
+
+  if (z > 0)
+    {
+      /* Round to the next integer (nearest or up or down, doesn't matter).  */
+      z += TWO_MANT_DIG;
+      z -= TWO_MANT_DIG;
+      /* Enforce rounding down.  */
+      if (z > y)
+       z -= 1.0;
+    }
+  else if (z < 0)
+    {
+      /* Round to the next integer (nearest or up or down, doesn't matter).  */
+      z -= TWO_MANT_DIG;
+      z += TWO_MANT_DIG;
+      /* Enforce rounding up.  */
+      if (z < y)
+       z += 1.0;
+    }
+  return z;
+}