Home | History | Annotate | Line # | Download | only in src 
      1 /* mpfr_get_z_2exp -- get a multiple-precision integer and an exponent
      2                       from a floating-point number
      3 
      4 Copyright 2000-2023 Free Software Foundation, Inc.
      5 Contributed by the AriC and Caramba 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 3 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.LESSER.  If not, see
     21 https://www.gnu.org/licenses/ or write to the Free Software Foundation, Inc.,
     22 51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA. */
     23 
     24 #include "mpfr-impl.h"
     25 
     26 /* puts the significand 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 non-negative numbers. --> This is WRONG since the returned
     34  *      exponent is not necessarily in the exponent range!
     35  * Note that this is different from the C function frexp().
     36  *
     37  * For NaN and infinities, we choose to set z = 0 (neutral value).
     38  * The exponent doesn't really matter, so let's keep __gmpfr_emin
     39  * for consistency. The erange flag is set.
     40  */
     41 
     42 /* MPFR_LARGE_EXP can be defined when mpfr_exp_t is guaranteed to have
     43    at least 64 bits (in a portable way). */
     44 #if GMP_NUMB_BITS >= 64
     45 /* Now, we know that the constant below is supported by the compiler. */
     46 # if _MPFR_EXP_FORMAT >= 3 && LONG_MAX >= 9223372036854775807
     47 #  define MPFR_LARGE_EXP 1
     48 # endif
     49 #endif
     50 
     51 mpfr_exp_t
     52 mpfr_get_z_2exp (mpz_ptr z, mpfr_srcptr f)
     53 {
     54   mp_size_t fn;
     55   int sh;
     56 
     57   if (MPFR_UNLIKELY (MPFR_IS_SINGULAR (f)))
     58     {
     59       if (MPFR_UNLIKELY (MPFR_NOTZERO (f)))
     60         MPFR_SET_ERANGEFLAG ();
     61       mpz_set_ui (z, 0);
     62       return __gmpfr_emin;
     63     }
     64 
     65   fn = MPFR_LIMB_SIZE(f);
     66 
     67   /* FIXME: temporary assert for security. Too large values should
     68      probably be handled like infinities. */
     69   MPFR_ASSERTN (fn <= INT_MAX);  /* due to SIZ(z) being an int */
     70 
     71   /* check whether allocated space for z is enough */
     72   mpz_realloc2 (z, (mp_bitcnt_t) fn * GMP_NUMB_BITS);
     73 
     74   MPFR_UNSIGNED_MINUS_MODULO (sh, MPFR_PREC (f));
     75   if (MPFR_LIKELY (sh))
     76     mpn_rshift (PTR (z), MPFR_MANT (f), fn, sh);
     77   else
     78     MPN_COPY (PTR (z), MPFR_MANT (f), fn);
     79 
     80   SIZ(z) = MPFR_IS_NEG (f) ? -fn : fn;
     81 
     82 #ifndef MPFR_LARGE_EXP
     83   /* If mpfr_exp_t has 64 bits, then MPFR_GET_EXP(f) >= MPFR_EMIN_MIN = 1-2^62
     84      and MPFR_EXP_MIN <= 1-2^63, thus the following implies PREC(f) > 2^62,
     85      which is impossible due to memory constraints. */
     86   if (MPFR_UNLIKELY ((mpfr_uexp_t) MPFR_GET_EXP (f) - MPFR_EXP_MIN
     87                      < (mpfr_uexp_t) MPFR_PREC (f)))
     88     {
     89       /* The exponent isn't representable in an mpfr_exp_t. */
     90       MPFR_SET_ERANGEFLAG ();
     91       return MPFR_EXP_MIN;
     92     }
     93 #endif
     94 
     95   return MPFR_GET_EXP (f) - MPFR_PREC (f);
     96 }
     97