jacbase.c
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上传日期:2022-08-06
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- /* mpn_jacobi_base -- limb/limb Jacobi symbol with restricted arguments.
- THIS INTERFACE IS PRELIMINARY AND MIGHT DISAPPEAR OR BE SUBJECT TO
- INCOMPATIBLE CHANGES IN A FUTURE RELEASE OF GMP.
- Copyright 1999, 2000, 2001, 2002 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"
- #include "longlong.h"
- /* Use the simple loop by default. The generic count_trailing_zeros is not
- very fast, and the extra trickery of method 3 has proven to be less use
- than might have been though. */
- #ifndef JACOBI_BASE_METHOD
- #define JACOBI_BASE_METHOD 2
- #endif
- /* Use count_trailing_zeros. */
- #if JACOBI_BASE_METHOD == 1
- #define PROCESS_TWOS_ANY
- {
- mp_limb_t twos;
- count_trailing_zeros (twos, a);
- result_bit1 ^= JACOBI_TWOS_U_BIT1 (twos, b);
- a >>= twos;
- }
- #define PROCESS_TWOS_EVEN PROCESS_TWOS_ANY
- #endif
- /* Use a simple loop. A disadvantage of this is that there's a branch on a
- 50/50 chance of a 0 or 1 low bit. */
- #if JACOBI_BASE_METHOD == 2
- #define PROCESS_TWOS_EVEN
- {
- int two;
- two = JACOBI_TWO_U_BIT1 (b);
- do
- {
- a >>= 1;
- result_bit1 ^= two;
- ASSERT (a != 0);
- }
- while ((a & 1) == 0);
- }
- #define PROCESS_TWOS_ANY
- if ((a & 1) == 0)
- PROCESS_TWOS_EVEN;
- #endif
- /* Process one bit arithmetically, then a simple loop. This cuts the loop
- condition down to a 25/75 chance, which should branch predict better.
- The CPU will need a reasonable variable left shift. */
- #if JACOBI_BASE_METHOD == 3
- #define PROCESS_TWOS_EVEN
- {
- int two, mask, shift;
-
- two = JACOBI_TWO_U_BIT1 (b);
- mask = (~a & 2);
- a >>= 1;
-
- shift = (~a & 1);
- a >>= shift;
- result_bit1 ^= two ^ (two & mask);
-
- while ((a & 1) == 0)
- {
- a >>= 1;
- result_bit1 ^= two;
- ASSERT (a != 0);
- }
- }
- #define PROCESS_TWOS_ANY
- {
- int two, mask, shift;
-
- two = JACOBI_TWO_U_BIT1 (b);
- shift = (~a & 1);
- a >>= shift;
-
- mask = shift << 1;
- result_bit1 ^= (two & mask);
-
- while ((a & 1) == 0)
- {
- a >>= 1;
- result_bit1 ^= two;
- ASSERT (a != 0);
- }
- }
- #endif
- /* Calculate the value of the Jacobi symbol (a/b) of two mp_limb_t's, but
- with a restricted range of inputs accepted, namely b>1, b odd, and a<=b.
- The initial result_bit1 is taken as a parameter for the convenience of
- mpz_kronecker_ui() et al. The sign changes both here and in those
- routines accumulate nicely in bit 1, see the JACOBI macros.
- The return value here is the normal +1, 0, or -1. Note that +1 and -1
- have bit 1 in the "BIT1" sense, which could be useful if the caller is
- accumulating it into some extended calculation.
- Duplicating the loop body to avoid the MP_LIMB_T_SWAP(a,b) would be
- possible, but a couple of tests suggest it's not a significant speedup,
- and may even be a slowdown, so what's here is good enough for now.
- Future: The code doesn't demand a<=b actually, so maybe this could be
- relaxed. All the places this is used currently call with a<=b though. */
- int
- mpn_jacobi_base (mp_limb_t a, mp_limb_t b, int result_bit1)
- {
- ASSERT (b & 1); /* b odd */
- ASSERT (b != 1);
- ASSERT (a <= b);
- if (a == 0)
- return 0;
- PROCESS_TWOS_ANY;
- if (a == 1)
- goto done;
- for (;;)
- {
- result_bit1 ^= JACOBI_RECIP_UU_BIT1 (a, b);
- MP_LIMB_T_SWAP (a, b);
- do
- {
- /* working on (a/b), a,b odd, a>=b */
- ASSERT (a & 1);
- ASSERT (b & 1);
- ASSERT (a >= b);
- if ((a -= b) == 0)
- return 0;
- PROCESS_TWOS_EVEN;
- if (a == 1)
- goto done;
- }
- while (a >= b);
- }
- done:
- return JACOBI_BIT1_TO_PN (result_bit1);
- }