95 lines
3.1 KiB
C
95 lines
3.1 KiB
C
/* mpf_get_ui -- mpf to ulong conversion
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Copyright 2001, 2002, 2004 Free Software Foundation, Inc.
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This file is part of the GNU MP Library.
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The GNU MP Library is free software; you can redistribute it and/or modify
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it under the terms of the GNU Lesser General Public License as published by
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the Free Software Foundation; either version 2.1 of the License, or (at your
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option) any later version.
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The GNU MP Library is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
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or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
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License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with the GNU MP Library; see the file COPYING.LIB. If not, write to
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the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
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MA 02110-1301, USA.
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*/
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#include "gmp.h"
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#include "gmp-impl.h"
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/* Any fraction bits are truncated, meaning simply discarded.
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For values bigger than a ulong, the low bits are returned (the low
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absolute value bits actually), like mpz_get_ui, but this isn't
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documented.
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Notice this is equivalent to mpz_set_f + mpz_get_ui.
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Implementation:
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The limb just above the radix point for us to extract is ptr[size-exp].
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We need to check that the size-exp index falls in our available data
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range, 0 to size-1 inclusive. We test this without risk of an overflow
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involving exp by requiring size>=exp (giving size-exp >= 0) and exp>0
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(giving size-exp <= size-1).
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Notice if size==0 there's no fetch, since of course size>=exp and exp>0
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can only be true if size>0. So there's no special handling for size==0,
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it comes out as 0 the same as any other time we have no data at our
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target index.
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For nails, the second limb above the radix point is also required, this
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is ptr[size-exp+1].
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Again we need to check that size-exp+1 falls in our data range, 0 to
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size-1 inclusive. We test without risk of overflow by requiring
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size+1>=exp (giving size-exp+1 >= 0) and exp>1 (giving size-exp+1 <=
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size-1).
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And again if size==0 these second fetch conditions are not satisfied
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either since size+1>=exp and exp>1 are only true if size>0.
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The code is arranged with exp>0 wrapping the exp>1 test since exp>1 is
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mis-compiled by alpha gcc prior to version 3.4. It re-writes it as
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exp-1>0, which is incorrect when exp==MP_EXP_T_MIN. By having exp>0
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tested first we ensure MP_EXP_T_MIN doesn't reach exp>1. */
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unsigned long
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mpf_get_ui (mpf_srcptr f)
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{
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mp_size_t size;
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mp_exp_t exp;
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mp_srcptr fp;
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mp_limb_t fl;
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exp = EXP (f);
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size = SIZ (f);
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fp = PTR (f);
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fl = 0;
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if (exp > 0)
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{
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/* there are some limbs above the radix point */
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size = ABS (size);
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if (size >= exp)
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fl = fp[size-exp];
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#if BITS_PER_ULONG > GMP_NUMB_BITS
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if (exp > 1 && size+1 >= exp)
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fl += (fp[size-exp+1] << GMP_NUMB_BITS);
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#endif
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}
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return (unsigned long) fl;
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}
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