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