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glibc/stdlib/rshift.c
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/* mpn_rshift -- Shift right a low-level natural-number integer. | |
Copyright (C) 1991-2016 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 2.1 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; see the file COPYING.LIB. If not, see | |
<http://www.gnu.org/licenses/>. */ | |
#include <gmp.h> | |
#include "gmp-impl.h" | |
#include <assert.h> | |
/* Shift U (pointed to by UP and USIZE limbs long) CNT bits to the right | |
and store the USIZE least significant limbs of the result at WP. | |
The bits shifted out to the right are returned. | |
Argument constraints: | |
1. 0 < CNT < BITS_PER_MP_LIMB | |
2. If the result is to be written over the input, WP must be <= UP. | |
*/ | |
mp_limb_t | |
mpn_rshift (register mp_ptr wp, | |
register mp_srcptr up, mp_size_t usize, | |
register unsigned int cnt) | |
{ | |
register mp_limb_t high_limb, low_limb; | |
register unsigned sh_1, sh_2; | |
register mp_size_t i; | |
mp_limb_t retval; | |
assert (usize != 0 && cnt != 0); | |
sh_1 = cnt; | |
#if 0 | |
if (sh_1 == 0) | |
{ | |
if (wp != up) | |
{ | |
/* Copy from low end to high end, to allow specified input/output | |
overlapping. */ | |
for (i = 0; i < usize; i++) | |
wp[i] = up[i]; | |
} | |
return usize; | |
} | |
#endif | |
wp -= 1; | |
sh_2 = BITS_PER_MP_LIMB - sh_1; | |
high_limb = up[0]; | |
retval = high_limb << sh_2; | |
low_limb = high_limb; | |
for (i = 1; i < usize; i++) | |
{ | |
high_limb = up[i]; | |
wp[i] = (low_limb >> sh_1) | (high_limb << sh_2); | |
low_limb = high_limb; | |
} | |
wp[i] = low_limb >> sh_1; | |
return retval; | |
} |