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-impl.h"
      37  #include "longlong.h"
      38  
      39  mp_size_t
      40  mpn_pow_1 (mp_ptr rp, mp_srcptr bp, mp_size_t bn, mp_limb_t exp, mp_ptr tp)
      41  {
      42    mp_limb_t x;
      43    int cnt, i;
      44    mp_size_t rn;
      45    int par;
      46  
      47    ASSERT (bn >= 1);
      48    /* FIXME: Add operand overlap criteria */
      49  
      50    if (exp <= 1)
      51      {
      52        if (exp == 0)
      53  	{
      54  	  rp[0] = 1;
      55  	  return 1;
      56  	}
      57        else
      58  	{
      59  	  MPN_COPY (rp, bp, bn);
      60  	  return bn;
      61  	}
      62      }
      63  
      64    /* Count number of bits in exp, and compute where to put initial square in
      65       order to magically get results in the entry rp.  Use simple code,
      66       optimized for small exp.  For large exp, the bignum operations will take
      67       so much time that the slowness of this code will be negligible.  */
      68    par = 0;
      69    cnt = GMP_LIMB_BITS;
      70    x = exp;
      71    do
      72      {
      73        par ^= x;
      74        cnt--;
      75        x >>= 1;
      76      } while (x != 0);
      77    exp <<= cnt;
      78  
      79    if (bn == 1)
      80      {
      81        mp_limb_t rl, rh, bl = bp[0];
      82  
      83        if ((cnt & 1) != 0)
      84  	MP_PTR_SWAP (rp, tp);
      85  
      86        umul_ppmm (rh, rl, bl, bl << GMP_NAIL_BITS);
      87        rp[0] = rl >> GMP_NAIL_BITS;
      88        rp[1] = rh;
      89        rn = 1 + (rh != 0);
      90  
      91        for (i = GMP_LIMB_BITS - cnt - 1;;)
      92  	{
      93  	  exp <<= 1;
      94  	  if ((exp & GMP_LIMB_HIGHBIT) != 0)
      95  	    {
      96  	      rp[rn] = rh = mpn_mul_1 (rp, rp, rn, bl);
      97  	      rn += rh != 0;
      98  	    }
      99  
     100  	  if (--i == 0)
     101  	    break;
     102  
     103  	  mpn_sqr (tp, rp, rn);
     104  	  rn = 2 * rn; rn -= tp[rn - 1] == 0;
     105  	  MP_PTR_SWAP (rp, tp);
     106  	}
     107      }
     108    else
     109      {
     110        if (((par ^ cnt) & 1) == 0)
     111  	MP_PTR_SWAP (rp, tp);
     112  
     113        mpn_sqr (rp, bp, bn);
     114        rn = 2 * bn; rn -= rp[rn - 1] == 0;
     115  
     116        for (i = GMP_LIMB_BITS - cnt - 1;;)
     117  	{
     118  	  exp <<= 1;
     119  	  if ((exp & GMP_LIMB_HIGHBIT) != 0)
     120  	    {
     121  	      rn = rn + bn - (mpn_mul (tp, rp, rn, bp, bn) == 0);
     122  	      MP_PTR_SWAP (rp, tp);
     123  	    }
     124  
     125  	  if (--i == 0)
     126  	    break;
     127  
     128  	  mpn_sqr (tp, rp, rn);
     129  	  rn = 2 * rn; rn -= tp[rn - 1] == 0;
     130  	  MP_PTR_SWAP (rp, tp);
     131  	}
     132      }
     133  
     134    return rn;
     135  }