contrib/mpfr/get_z_exp.c

   1 /* mpfr_get_z_exp -- get a multiple-precision integer and an exponent
   2                      from a floating-point number
   3
   4 Copyright 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009 Free Software Foundation, Inc.
   5 Contributed by the Arenaire and Cacao projects, INRIA.
   6
   7 This file is part of the GNU MPFR Library.
   8
   9 The GNU MPFR Library is free software; you can redistribute it and/or modify
  10 it under the terms of the GNU Lesser General Public License as published by
  11 the Free Software Foundation; either version 2.1 of the License, or (at your
  12 option) any later version.
  13
  14 The GNU MPFR Library is distributed in the hope that it will be useful, but
  15 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  16 or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
  17 License for more details.
  18
  19 You should have received a copy of the GNU Lesser General Public License
  20 along with the GNU MPFR Library; see the file COPYING.LIB.  If not, write to
  21 the Free Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston,
  22 MA 02110-1301, USA. */
  23
  24 #include "mpfr-impl.h"
  25
  26 /* puts the mantissa of f into z, and returns 'exp' such that f = z * 2^exp
  27  *
  28  * 0 doesn't have an exponent, therefore the returned exponent in this case
  29  * isn't really important. We choose to return __gmpfr_emin because
  30  *   1) it is in the exponent range [__gmpfr_emin,__gmpfr_emax],
  31  *   2) the smaller a number is (in absolute value), the smaller its
  32  *      exponent is. In other words, the f -> exp function is monotonous
  33  *      on nonnegative numbers.
  34  * Note that this is different from the C function frexp().
  35  */
  36
  37 mp_exp_t
  38 mpfr_get_z_exp (mpz_ptr z, mpfr_srcptr f)
  39 {
  40   mp_size_t fn;
  41   int sh;
  42
  43   MPFR_ASSERTD (MPFR_IS_FP (f));
  44
  45   if (MPFR_UNLIKELY (MPFR_IS_ZERO (f)))
  46     {
  47       mpz_set_ui (z, 0);
  48       return __gmpfr_emin;
  49     }
  50
  51   fn = MPFR_LIMB_SIZE(f);
  52
  53   /* check whether allocated space for z is enough */
  54   if (MPFR_UNLIKELY (ALLOC (z) < fn))
  55     MPZ_REALLOC (z, fn);
  56
  57   MPFR_UNSIGNED_MINUS_MODULO (sh, MPFR_PREC (f));
  58   if (MPFR_LIKELY (sh))
  59     mpn_rshift (PTR (z), MPFR_MANT (f), fn, sh);
  60   else
  61     MPN_COPY (PTR (z), MPFR_MANT (f), fn);
  62
  63   SIZ(z) = MPFR_IS_NEG (f) ? -fn : fn;
  64
  65   /* Test if the result is representable. Later, we could choose
  66      to return MPFR_EXP_MIN if it isn't, or perhaps MPFR_EXP_MAX
  67      to signal an error. The mantissa would still be meaningful. */
  68   MPFR_ASSERTD ((mp_exp_unsigned_t) MPFR_GET_EXP (f) - MPFR_EXP_MIN
  69                 >= (mp_exp_unsigned_t) MPFR_PREC(f));
  70
  71   return MPFR_GET_EXP (f) - MPFR_PREC (f);
  72 }