gcc/gmp/mpf/get_ui.c

/* mpf_get_ui -- mpf to ulong conversion

Copyright 2001, 2002, 2004 Free Software Foundation, Inc.

This file is part of the GNU MP Library.

The GNU MP 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 3 of the License, or (at your
option) any later version.

The GNU MP 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 GNU MP Library.  If not, see http://www.gnu.org/licenses/.  */

#include "gmp.h"
#include "gmp-impl.h"


/* Any fraction bits are truncated, meaning simply discarded.

   For values bigger than a ulong, the low bits are returned (the low
   absolute value bits actually), like mpz_get_ui, but this isn't
   documented.

   Notice this is equivalent to mpz_set_f + mpz_get_ui.


   Implementation:

   The limb just above the radix point for us to extract is ptr[size-exp].

   We need to check that the size-exp index falls in our available data
   range, 0 to size-1 inclusive.  We test this without risk of an overflow
   involving exp by requiring size>=exp (giving size-exp >= 0) and exp>0
   (giving size-exp <= size-1).

   Notice if size==0 there's no fetch, since of course size>=exp and exp>0
   can only be true if size>0.  So there's no special handling for size==0,
   it comes out as 0 the same as any other time we have no data at our
   target index.

   For nails, the second limb above the radix point is also required, this
   is ptr[size-exp+1].

   Again we need to check that size-exp+1 falls in our data range, 0 to
   size-1 inclusive.  We test without risk of overflow by requiring
   size+1>=exp (giving size-exp+1 >= 0) and exp>1 (giving size-exp+1 <=
   size-1).

   And again if size==0 these second fetch conditions are not satisfied
   either since size+1>=exp and exp>1 are only true if size>0.

   The code is arranged with exp>0 wrapping the exp>1 test since exp>1 is
   mis-compiled by alpha gcc prior to version 3.4.  It re-writes it as
   exp-1>0, which is incorrect when exp==MP_EXP_T_MIN.  By having exp>0
   tested first we ensure MP_EXP_T_MIN doesn't reach exp>1.  */

unsigned long
mpf_get_ui (mpf_srcptr f)
{
  mp_size_t size;
  mp_exp_t exp;
  mp_srcptr fp;
  mp_limb_t fl;

  exp = EXP (f);
  size = SIZ (f);
  fp = PTR (f);

  fl = 0;
  if (exp > 0)
    {
      /* there are some limbs above the radix point */

      size = ABS (size);
      if (size >= exp)
        fl = fp[size-exp];

#if BITS_PER_ULONG > GMP_NUMB_BITS
      if (exp > 1 && size+1 >= exp)
        fl += (fp[size-exp+1] << GMP_NUMB_BITS);
#endif
    }

  return (unsigned long) fl;
}