mirror of https://github.com/macssh/macssh.git
137 lines
4.2 KiB
C
Executable File
137 lines
4.2 KiB
C
Executable File
/* 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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/*
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Copyright (C) 1999, 2000 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., 59 Temple Place - Suite 330, Boston,
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MA 02111-1307, 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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#if COUNT_TRAILING_ZEROS_TIME <= 7
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/* If count_trailing_zeros is fast, use it.
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K7 at 7 cycles and P6 at 2 are good here. K6 at 12-27 and P5 at 18-42
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are not. The default 15 in longlong.h is meant to mean not good here. */
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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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#else
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/* Use a loop instead. With "a" uniformly distributed there will usually be
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only a few trailing zeros.
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Unfortunately the branch for the while loop here will be on a 50/50
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chance of a 1 or 0, which is bad for branch prediction. */
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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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/* 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_zi_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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#if __STDC__
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mpn_jacobi_base (mp_limb_t a, mp_limb_t b, int result_bit1)
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#else
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mpn_jacobi_base (a, b, result_bit1)
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mp_limb_t a;
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mp_limb_t b;
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int result_bit1;
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#endif
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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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