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/* mpz_mod_2exp -- divide a MP_INT by 2**n and produce a remainder.
Copyright (C) 1991 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 General Public License as published by
the Free Software Foundation; either version 2, 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 General Public License for more details.
You should have received a copy of the GNU General Public License
along with the GNU MP Library; see the file COPYING. If not, write to
the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA. */
#include "gmp.h"
#include "gmp-impl.h"
void
#ifdef __STDC__
mpz_mod_2exp (MP_INT *res, const MP_INT *in, unsigned long int cnt)
#else
mpz_mod_2exp (res, in, cnt)
MP_INT *res;
const MP_INT *in;
unsigned long int cnt;
#endif
{
mp_size in_size = ABS (in->size);
mp_size res_size;
mp_size limb_cnt = cnt / BITS_PER_MP_LIMB;
mp_srcptr in_ptr = in->d;
if (in_size > limb_cnt)
{
/* The input operand is (probably) greater than 2**CNT. */
mp_limb x;
x = in_ptr[limb_cnt] & (((mp_limb) 1 << cnt % BITS_PER_MP_LIMB) - 1);
if (x != 0)
{
res_size = limb_cnt + 1;
if (res->alloc < res_size)
_mpz_realloc (res, res_size);
res->d[limb_cnt] = x;
}
else
{
mp_size i;
for (i = limb_cnt - 1; i >= 0; i--)
if (in_ptr[i] != 0)
break;
res_size = i + 1;
if (res->alloc < res_size)
_mpz_realloc (res, res_size);
limb_cnt = res_size;
}
}
else
{
/* The input operand is smaller than 2**CNT. We perform a no-op,
apart from that we might need to copy IN to RES. */
res_size = in_size;
if (res->alloc < res_size)
_mpz_realloc (res, res_size);
limb_cnt = res_size;
}
if (res != in)
MPN_COPY (res->d, in->d, limb_cnt);
res->size = (in->size >= 0) ? res_size : -res_size;
}
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