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Initial revision

/* mpfr_pow_si -- power function x^y with y an unsigned int 

Copyright 2001, 2002 Free Software Foundation, Inc.

This file is part of the MPFR Library.

The MPFR 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 MPFR 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 MPFR 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 "mpfr.h"
#include "mpfr-impl.h"

 /* The computation of y=pow(x,z) is done by

    y=pow_ui(x,z) if z>0
  else
    y=1/pow_ui(x,z) if z<0
 */

int
mpfr_pow_si (mpfr_ptr y, mpfr_srcptr x, long int n, mp_rnd_t rnd_mode)
{
  if (n > 0)
    return mpfr_pow_ui(y, x, n, rnd_mode);
  else
    {
      int inexact = 0;

      if (MPFR_IS_NAN(x))
        {
          MPFR_SET_NAN(y);
          MPFR_RET_NAN;
        }

      MPFR_CLEAR_NAN(y);

      if (n == 0)
        return mpfr_set_ui(y, 1, GMP_RNDN);

      if (MPFR_IS_INF(x))
        {
          MPFR_SET_ZERO(y);
          if (MPFR_SIGN(x) > 0 || ((unsigned) n & 1) == 0)
            MPFR_SET_POS(y);
          else
            MPFR_SET_NEG(y);
          MPFR_RET(0);
        }

      if (MPFR_IS_ZERO(x))
        {
          MPFR_SET_INF(y);
          if (MPFR_SIGN(x) > 0 || ((unsigned) n & 1) == 0)
            MPFR_SET_POS(y);
          else
            MPFR_SET_NEG(y);
          MPFR_RET(0);
        }

      MPFR_CLEAR_INF(y);

      n = -n;

      /* General case */
      {
        /* Declaration of the intermediary variable */
        mpfr_t t, ti;

        /* Declaration of the size variable */
        mp_prec_t Nx = MPFR_PREC(x);   /* Precision of input variable */
        mp_prec_t Ny = MPFR_PREC(y);   /* Precision of input variable */

        mp_prec_t Nt;   /* Precision of the intermediary variable */
        long int err;  /* Precision of error */
                
        /* compute the precision of intermediary variable */
        Nt=MAX(Nx,Ny);
        /* the optimal number of bits : see algorithms.ps */
        Nt=Nt+3+_mpfr_ceil_log2(Nt);

        /* initialise of intermediary	variable */
        mpfr_init(t);
        mpfr_init(ti);

        do {

          /* reactualisation of the precision */
          mpfr_set_prec(t,Nt);                    
          mpfr_set_prec(ti,Nt);             

          /* compute 1/(x^n) n>0*/
          mpfr_pow_ui(ti,x,(unsigned long int)(n),GMP_RNDN);
          mpfr_ui_div(t,1,ti,GMP_RNDN);

          /* estimation of the error -- see pow function in algorithms.ps*/
          err = Nt - 3;

          /* actualisation of the precision */
          Nt += 10;

        } while (err<0 || !mpfr_can_round(t,err,GMP_RNDN,rnd_mode,Ny));
 
        inexact = mpfr_set(y,t,rnd_mode);
        mpfr_clear(t);
        mpfr_clear(ti);
      }
      return inexact;
    }
}