--- /dev/null
+/* 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 (C) 1999, 2000 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 2.1 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; see the file COPYING.LIB. If not, write to
+the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
+MA 02111-1307, USA. */
+
+#include "gmp.h"
+#include "gmp-impl.h"
+#include "longlong.h"
+
+
+#if COUNT_TRAILING_ZEROS_TIME <= 7
+/* If count_trailing_zeros is fast, use it.
+ K7 at 7 cycles and P6 at 2 are good here. K6 at 12-27 and P5 at 18-42
+ are not. The default 15 in longlong.h is meant to mean not good here. */
+
+#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
+
+#else
+/* Use a loop instead. With "a" uniformly distributed there will usually be
+ only a few trailing zeros.
+
+ Unfortunately the branch for the while loop here will be on a 50/50
+ chance of a 1 or 0, which is bad for branch prediction. */
+
+#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
+
+
+/* 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_zi_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
+#if __STDC__
+mpn_jacobi_base (mp_limb_t a, mp_limb_t b, int result_bit1)
+#else
+mpn_jacobi_base (a, b, result_bit1)
+ mp_limb_t a;
+ mp_limb_t b;
+ int result_bit1;
+#endif
+{
+ 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);
+}
projects / ghc-hetmet.git / blobdiff