83 lines
2.1 KiB
C
83 lines
2.1 KiB
C
/* mpz_mod_2exp -- divide a MP_INT by 2**n and produce a remainder.
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Copyright (C) 1991 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 General Public License as published by
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the Free Software Foundation; either version 2, or (at your option)
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any later version.
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The GNU MP Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with the GNU MP Library; see the file COPYING. If not, write to
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the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA. */
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#include "gmp.h"
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#include "gmp-impl.h"
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void
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#ifdef __STDC__
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mpz_mod_2exp (MP_INT *res, const MP_INT *in, unsigned long int cnt)
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#else
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mpz_mod_2exp (res, in, cnt)
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MP_INT *res;
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const MP_INT *in;
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unsigned long int cnt;
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#endif
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{
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mp_size in_size = ABS (in->size);
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mp_size res_size;
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mp_size limb_cnt = cnt / BITS_PER_MP_LIMB;
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mp_srcptr in_ptr = in->d;
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if (in_size > limb_cnt)
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{
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/* The input operand is (probably) greater than 2**CNT. */
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mp_limb x;
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x = in_ptr[limb_cnt] & (((mp_limb) 1 << cnt % BITS_PER_MP_LIMB) - 1);
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if (x != 0)
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{
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res_size = limb_cnt + 1;
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if (res->alloc < res_size)
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_mpz_realloc (res, res_size);
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res->d[limb_cnt] = x;
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}
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else
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{
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mp_size i;
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for (i = limb_cnt - 1; i >= 0; i--)
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if (in_ptr[i] != 0)
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break;
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res_size = i + 1;
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if (res->alloc < res_size)
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_mpz_realloc (res, res_size);
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limb_cnt = res_size;
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}
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}
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else
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{
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/* The input operand is smaller than 2**CNT. We perform a no-op,
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apart from that we might need to copy IN to RES. */
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res_size = in_size;
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if (res->alloc < res_size)
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_mpz_realloc (res, res_size);
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limb_cnt = res_size;
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}
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if (res != in)
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MPN_COPY (res->d, in->d, limb_cnt);
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res->size = (in->size >= 0) ? res_size : -res_size;
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}
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