lucnum2_ui.c revision 1.1.1.2 
1/* mpz_lucnum2_ui -- calculate Lucas numbers.
2
3Copyright 2001, 2003, 2005, 2012 Free Software Foundation, Inc.
4
5This file is part of the GNU MP Library.
6
7The GNU MP Library is free software; you can redistribute it and/or modify
8it under the terms of the GNU Lesser General Public License as published by
9the Free Software Foundation; either version 3 of the License, or (at your
10option) any later version.
11
12The GNU MP Library is distributed in the hope that it will be useful, but
13WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
14or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
15License for more details.
16
17You should have received a copy of the GNU Lesser General Public License
18along with the GNU MP Library.  If not, see http://www.gnu.org/licenses/.  */
19
20#include <stdio.h>
21#include "gmp.h"
22#include "gmp-impl.h"
23
24
25void
26mpz_lucnum2_ui (mpz_ptr ln, mpz_ptr lnsub1, unsigned long n)
27{
28  mp_ptr     lp, l1p, f1p;
29  mp_size_t  size;
30  mp_limb_t  c;
31  TMP_DECL;
32
33  ASSERT (ln != lnsub1);
34
35  /* handle small n quickly, and hide the special case for L[-1]=-1 */
36  if (n <= FIB_TABLE_LUCNUM_LIMIT)
37    {
38      mp_limb_t  f  = FIB_TABLE (n);
39      mp_limb_t  f1 = FIB_TABLE ((int) n - 1);
40
41      /* L[n] = F[n] + 2F[n-1] */
42      PTR(ln)[0] = f + 2*f1;
43      SIZ(ln) = 1;
44
45      /* L[n-1] = 2F[n] - F[n-1], but allow for L[-1]=-1 */
46      PTR(lnsub1)[0] = (n == 0 ? 1 : 2*f - f1);
47      SIZ(lnsub1) = (n == 0 ? -1 : 1);
48
49      return;
50    }
51
52  TMP_MARK;
53  size = MPN_FIB2_SIZE (n);
54  f1p = TMP_ALLOC_LIMBS (size);
55
56  lp  = MPZ_REALLOC (ln,     size+1);
57  l1p = MPZ_REALLOC (lnsub1, size+1);
58
59  size = mpn_fib2_ui (l1p, f1p, n);
60
61  /* L[n] = F[n] + 2F[n-1] */
62#if HAVE_NATIVE_mpn_addlsh1_n
63  c = mpn_addlsh1_n (lp, l1p, f1p, size);
64#else
65  c = mpn_lshift (lp, f1p, size, 1);
66  c += mpn_add_n (lp, lp, l1p, size);
67#endif
68  lp[size] = c;
69  SIZ(ln) = size + (c != 0);
70
71  /* L[n-1] = 2F[n] - F[n-1] */
72  c = mpn_lshift (l1p, l1p, size, 1);
73  c -= mpn_sub_n (l1p, l1p, f1p, size);
74  ASSERT ((mp_limb_signed_t) c >= 0);
75  l1p[size] = c;
76  SIZ(lnsub1) = size + (c != 0);
77
78  TMP_FREE;
79}
80