--- /dev/null
+/* mpz_lucnum_ui -- calculate Lucas number.
+
+Copyright 2001, 2003, 2005 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 <stdio.h>
+#include "gmp.h"
+#include "gmp-impl.h"
+
+
+/* change this to "#define TRACE(x) x" for diagnostics */
+#define TRACE(x)
+
+
+/* Notes:
+
+ For the +4 in L[2k+1] when k is even, all L[4m+3] == 4, 5 or 7 mod 8, so
+ there can't be an overflow applying +4 to just the low limb (since that
+ would leave 0, 1, 2 or 3 mod 8).
+
+ For the -4 in L[2k+1] when k is even, it seems (no proof) that
+ L[3*2^(b-2)-3] == -4 mod 2^b, so for instance with a 32-bit limb
+ L[0xBFFFFFFD] == 0xFFFFFFFC mod 2^32, and this implies a borrow from the
+ low limb. Obviously L[0xBFFFFFFD] is a huge number, but it's at least
+ conceivable to calculate it, so it probably should be handled.
+
+ For the -2 in L[2k] with k even, it seems (no proof) L[2^(b-1)] == -1 mod
+ 2^b, so for instance in 32-bits L[0x80000000] has a low limb of
+ 0xFFFFFFFF so there would have been a borrow. Again L[0x80000000] is
+ obviously huge, but probably should be made to work. */
+
+void
+mpz_lucnum_ui (mpz_ptr ln, unsigned long n)
+{
+ mp_size_t lalloc, xalloc, lsize, xsize;
+ mp_ptr lp, xp;
+ mp_limb_t c;
+ int zeros;
+ TMP_DECL;
+
+ TRACE (printf ("mpn_lucnum_ui n=%lu\n", n));
+
+ if (n <= FIB_TABLE_LUCNUM_LIMIT)
+ {
+ /* L[n] = F[n] + 2F[n-1] */
+ PTR(ln)[0] = FIB_TABLE(n) + 2 * FIB_TABLE ((int) n - 1);
+ SIZ(ln) = 1;
+ return;
+ }
+
+ /* +1 since L[n]=F[n]+2F[n-1] might be 1 limb bigger than F[n], further +1
+ since square or mul used below might need an extra limb over the true
+ size */
+ lalloc = MPN_FIB2_SIZE (n) + 2;
+ MPZ_REALLOC (ln, lalloc);
+ lp = PTR (ln);
+
+ TMP_MARK;
+ xalloc = lalloc;
+ xp = TMP_ALLOC_LIMBS (xalloc);
+
+ /* Strip trailing zeros from n, until either an odd number is reached
+ where the L[2k+1] formula can be used, or until n fits within the
+ FIB_TABLE data. The table is preferred of course. */
+ zeros = 0;
+ for (;;)
+ {
+ if (n & 1)
+ {
+ /* L[2k+1] = 5*F[k-1]*(2*F[k]+F[k-1]) - 4*(-1)^k */
+
+ mp_size_t yalloc, ysize;
+ mp_ptr yp;
+
+ TRACE (printf (" initial odd n=%lu\n", n));
+
+ yalloc = MPN_FIB2_SIZE (n/2);
+ yp = TMP_ALLOC_LIMBS (yalloc);
+ ASSERT (xalloc >= yalloc);
+
+ xsize = mpn_fib2_ui (xp, yp, n/2);
+
+ /* possible high zero on F[k-1] */
+ ysize = xsize;
+ ysize -= (yp[ysize-1] == 0);
+ ASSERT (yp[ysize-1] != 0);
+
+ /* xp = 2*F[k] + F[k-1] */
+#if HAVE_NATIVE_mpn_addlsh1_n
+ c = mpn_addlsh1_n (xp, yp, xp, xsize);
+#else
+ c = mpn_lshift (xp, xp, xsize, 1);
+ c += mpn_add_n (xp, xp, yp, xsize);
+#endif
+ ASSERT (xalloc >= xsize+1);
+ xp[xsize] = c;
+ xsize += (c != 0);
+ ASSERT (xp[xsize-1] != 0);
+
+ ASSERT (lalloc >= xsize + ysize);
+ c = mpn_mul (lp, xp, xsize, yp, ysize);
+ lsize = xsize + ysize;
+ lsize -= (c == 0);
+
+ /* lp = 5*lp */
+#if HAVE_NATIVE_mpn_addlshift
+ c = mpn_addlshift (lp, lp, lsize, 2);
+#else
+ c = mpn_lshift (xp, lp, lsize, 2);
+ c += mpn_add_n (lp, lp, xp, lsize);
+#endif
+ ASSERT (lalloc >= lsize+1);
+ lp[lsize] = c;
+ lsize += (c != 0);
+
+ /* lp = lp - 4*(-1)^k */
+ if (n & 2)
+ {
+ /* no overflow, see comments above */
+ ASSERT (lp[0] <= MP_LIMB_T_MAX-4);
+ lp[0] += 4;
+ }
+ else
+ {
+ /* won't go negative */
+ MPN_DECR_U (lp, lsize, CNST_LIMB(4));
+ }
+
+ TRACE (mpn_trace (" l",lp, lsize));
+ break;
+ }
+
+ MP_PTR_SWAP (xp, lp); /* balance the swaps wanted in the L[2k] below */
+ zeros++;
+ n /= 2;
+
+ if (n <= FIB_TABLE_LUCNUM_LIMIT)
+ {
+ /* L[n] = F[n] + 2F[n-1] */
+ lp[0] = FIB_TABLE (n) + 2 * FIB_TABLE ((int) n - 1);
+ lsize = 1;
+
+ TRACE (printf (" initial small n=%lu\n", n);
+ mpn_trace (" l",lp, lsize));
+ break;
+ }
+ }
+
+ for ( ; zeros != 0; zeros--)
+ {
+ /* L[2k] = L[k]^2 + 2*(-1)^k */
+
+ TRACE (printf (" zeros=%d\n", zeros));
+
+ ASSERT (xalloc >= 2*lsize);
+ mpn_sqr_n (xp, lp, lsize);
+ lsize *= 2;
+ lsize -= (xp[lsize-1] == 0);
+
+ /* First time around the loop k==n determines (-1)^k, after that k is
+ always even and we set n=0 to indicate that. */
+ if (n & 1)
+ {
+ /* L[n]^2 == 0 or 1 mod 4, like all squares, so +2 gives no carry */
+ ASSERT (xp[0] <= MP_LIMB_T_MAX-2);
+ xp[0] += 2;
+ n = 0;
+ }
+ else
+ {
+ /* won't go negative */
+ MPN_DECR_U (xp, lsize, CNST_LIMB(2));
+ }
+
+ MP_PTR_SWAP (xp, lp);
+ ASSERT (lp[lsize-1] != 0);
+ }
+
+ /* should end up in the right spot after all the xp/lp swaps */
+ ASSERT (lp == PTR(ln));
+ SIZ(ln) = lsize;
+
+ TMP_FREE;
+}
projects / msp430-gcc.git / blobdiff