source/libs/gmp/gmp-src/mpn/generic/pow_1.c

   1 /* mpn_pow_1 -- Compute powers R = U^exp.
   2
   3    THE FUNCTIONS IN THIS FILE ARE FOR INTERNAL USE ONLY.  THEY'RE ALMOST
   4    CERTAIN TO BE SUBJECT TO INCOMPATIBLE CHANGES OR DISAPPEAR COMPLETELY IN
   5    FUTURE GNU MP RELEASES.
   6
   7 Copyright 2002, 2014 Free Software Foundation, Inc.
   8
   9 This file is part of the GNU MP Library.
  10
  11 The GNU MP Library is free software; you can redistribute it and/or modify
  12 it under the terms of either:
  13
  14   * the GNU Lesser General Public License as published by the Free
  15     Software Foundation; either version 3 of the License, or (at your
  16     option) any later version.
  17
  18 or
  19
  20   * the GNU General Public License as published by the Free Software
  21     Foundation; either version 2 of the License, or (at your option) any
  22     later version.
  23
  24 or both in parallel, as here.
  25
  26 The GNU MP Library is distributed in the hope that it will be useful, but
  27 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  28 or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  29 for more details.
  30
  31 You should have received copies of the GNU General Public License and the
  32 GNU Lesser General Public License along with the GNU MP Library.  If not,
  33 see https://www.gnu.org/licenses/.  */
  34
  35
  36 #include "gmp.h"
  37 #include "gmp-impl.h"
  38 #include "longlong.h"
  39
  40 mp_size_t
  41 mpn_pow_1 (mp_ptr rp, mp_srcptr bp, mp_size_t bn, mp_limb_t exp, mp_ptr tp)
  42 {
  43   mp_limb_t x;
  44   int cnt, i;
  45   mp_size_t rn;
  46   int par;
  47
  48   ASSERT (bn >= 1);
  49   /* FIXME: Add operand overlap criteria */
  50
  51   if (exp <= 1)
  52     {
  53       if (exp == 0)
  54         {
  55           rp[0] = 1;
  56           return 1;
  57         }
  58       else
  59         {
  60           MPN_COPY (rp, bp, bn);
  61           return bn;
  62         }
  63     }
  64
  65   /* Count number of bits in exp, and compute where to put initial square in
  66      order to magically get results in the entry rp.  Use simple code,
  67      optimized for small exp.  For large exp, the bignum operations will take
  68      so much time that the slowness of this code will be negligible.  */
  69   par = 0;
  70   cnt = GMP_LIMB_BITS;
  71   x = exp;
  72   do
  73     {
  74       par ^= x;
  75       cnt--;
  76       x >>= 1;
  77     } while (x != 0);
  78   exp <<= cnt;
  79
  80   if (bn == 1)
  81     {
  82       mp_limb_t bl = bp[0];
  83
  84       if ((cnt & 1) != 0)
  85         MP_PTR_SWAP (rp, tp);
  86
  87       mpn_sqr (rp, bp, bn);
  88       rn = 2 * bn; rn -= rp[rn - 1] == 0;
  89
  90       for (i = GMP_LIMB_BITS - cnt - 1;;)
  91         {
  92           exp <<= 1;
  93           if ((exp & GMP_LIMB_HIGHBIT) != 0)
  94             {
  95               rp[rn] = mpn_mul_1 (rp, rp, rn, bl);
  96               rn += rp[rn] != 0;
  97             }
  98
  99           if (--i == 0)
 100             break;
 101
 102           mpn_sqr (tp, rp, rn);
 103           rn = 2 * rn; rn -= tp[rn - 1] == 0;
 104           MP_PTR_SWAP (rp, tp);
 105         }
 106     }
 107   else
 108     {
 109       if (((par ^ cnt) & 1) == 0)
 110         MP_PTR_SWAP (rp, tp);
 111
 112       mpn_sqr (rp, bp, bn);
 113       rn = 2 * bn; rn -= rp[rn - 1] == 0;
 114
 115       for (i = GMP_LIMB_BITS - cnt - 1;;)
 116         {
 117           exp <<= 1;
 118           if ((exp & GMP_LIMB_HIGHBIT) != 0)
 119             {
 120               rn = rn + bn - (mpn_mul (tp, rp, rn, bp, bn) == 0);
 121               MP_PTR_SWAP (rp, tp);
 122             }
 123
 124           if (--i == 0)
 125             break;
 126
 127           mpn_sqr (tp, rp, rn);
 128           rn = 2 * rn; rn -= tp[rn - 1] == 0;
 129           MP_PTR_SWAP (rp, tp);
 130         }
 131     }
 132
 133   return rn;
 134 }