Imported Upstream version 1.3.14

[debian/gzip] / lib / frexp.c

diff --git a/lib/frexp.c b/lib/frexp.c
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+/* Split a double into fraction and mantissa.
+   Copyright (C) 2007-2008 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 3 of the License, 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, see <http://www.gnu.org/licenses/>.  */
+
+/* Written by Paolo Bonzini <bonzini@gnu.org>, 2003, and
+   Bruno Haible <bruno@clisp.org>, 2007.  */
+
+#include <config.h>
+
+/* Specification.  */
+#include <math.h>
+
+#include <float.h>
+#ifdef USE_LONG_DOUBLE
+# include "isnanl-nolibm.h"
+# include "fpucw.h"
+#else
+# include "isnand-nolibm.h"
+#endif
+
+/* This file assumes FLT_RADIX = 2.  If FLT_RADIX is a power of 2 greater
+   than 2, or not even a power of 2, some rounding errors can occur, so that
+   then the returned mantissa is only guaranteed to be <= 1.0, not < 1.0.  */
+
+#ifdef USE_LONG_DOUBLE
+# define FUNC frexpl
+# define DOUBLE long double
+# define ISNAN isnanl
+# define DECL_ROUNDING DECL_LONG_DOUBLE_ROUNDING
+# define BEGIN_ROUNDING() BEGIN_LONG_DOUBLE_ROUNDING ()
+# define END_ROUNDING() END_LONG_DOUBLE_ROUNDING ()
+# define L_(literal) literal##L
+#else
+# define FUNC frexp
+# define DOUBLE double
+# define ISNAN isnand
+# define DECL_ROUNDING
+# define BEGIN_ROUNDING()
+# define END_ROUNDING()
+# define L_(literal) literal
+#endif
+
+DOUBLE
+FUNC (DOUBLE x, int *expptr)
+{
+  int sign;
+  int exponent;
+  DECL_ROUNDING
+
+  /* Test for NaN, infinity, and zero.  */
+  if (ISNAN (x) || x + x == x)
+    {
+      *expptr = 0;
+      return x;
+    }
+
+  sign = 0;
+  if (x < 0)
+    {
+      x = - x;
+      sign = -1;
+    }
+
+  BEGIN_ROUNDING ();
+
+  {
+    /* Since the exponent is an 'int', it fits in 64 bits.  Therefore the
+       loops are executed no more than 64 times.  */
+    DOUBLE pow2[64]; /* pow2[i] = 2^2^i */
+    DOUBLE powh[64]; /* powh[i] = 2^-2^i */
+    int i;
+
+    exponent = 0;
+    if (x >= L_(1.0))
+      {
+       /* A positive exponent.  */
+       DOUBLE pow2_i; /* = pow2[i] */
+       DOUBLE powh_i; /* = powh[i] */
+
+       /* Invariants: pow2_i = 2^2^i, powh_i = 2^-2^i,
+          x * 2^exponent = argument, x >= 1.0.  */
+       for (i = 0, pow2_i = L_(2.0), powh_i = L_(0.5);
+            ;
+            i++, pow2_i = pow2_i * pow2_i, powh_i = powh_i * powh_i)
+         {
+           if (x >= pow2_i)
+             {
+               exponent += (1 << i);
+               x *= powh_i;
+             }
+           else
+             break;
+
+           pow2[i] = pow2_i;
+           powh[i] = powh_i;
+         }
+       /* Avoid making x too small, as it could become a denormalized
+          number and thus lose precision.  */
+       while (i > 0 && x < pow2[i - 1])
+         {
+           i--;
+           powh_i = powh[i];
+         }
+       exponent += (1 << i);
+       x *= powh_i;
+       /* Here 2^-2^i <= x < 1.0.  */
+      }
+    else
+      {
+       /* A negative or zero exponent.  */
+       DOUBLE pow2_i; /* = pow2[i] */
+       DOUBLE powh_i; /* = powh[i] */
+
+       /* Invariants: pow2_i = 2^2^i, powh_i = 2^-2^i,
+          x * 2^exponent = argument, x < 1.0.  */
+       for (i = 0, pow2_i = L_(2.0), powh_i = L_(0.5);
+            ;
+            i++, pow2_i = pow2_i * pow2_i, powh_i = powh_i * powh_i)
+         {
+           if (x < powh_i)
+             {
+               exponent -= (1 << i);
+               x *= pow2_i;
+             }
+           else
+             break;
+
+           pow2[i] = pow2_i;
+           powh[i] = powh_i;
+         }
+       /* Here 2^-2^i <= x < 1.0.  */
+      }
+
+    /* Invariants: x * 2^exponent = argument, and 2^-2^i <= x < 1.0.  */
+    while (i > 0)
+      {
+       i--;
+       if (x < powh[i])
+         {
+           exponent -= (1 << i);
+           x *= pow2[i];
+         }
+      }
+    /* Here 0.5 <= x < 1.0.  */
+  }
+
+  if (sign < 0)
+    x = - x;
+
+  END_ROUNDING ();
+
+  *expptr = exponent;
+  return x;
+}