get_ui.c
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上传日期:2022-08-06
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- /* 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;
- }