/*  mpfr_ui_pow_ui -- compute the power beetween two machine integer

Copyright 1999, 2000, 2001, 2002, 2003, 2004, 2005 
  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 "mpfr-impl.h"

int
mpfr_ui_pow_ui (mpfr_ptr x, unsigned long int y, unsigned long int n,
                mp_rnd_t rnd)
{
  mp_exp_t err;
  unsigned long m;
  mpfr_t res;
  mp_prec_t prec;
  int size_n;
  int inexact;
  MPFR_ZIV_DECL (loop);
  MPFR_SAVE_EXPO_DECL (expo);

  if (MPFR_UNLIKELY (n == 0))
    /* y^0 = 1 for any y */
    return mpfr_set_ui (x, 1, rnd);

  if (MPFR_UNLIKELY (y == 0))
    /* 0^n = 0 for any n > 0 */
    return mpfr_set_ui (x, 0, rnd);

  for (size_n = 0, m = n; m; size_n++, m >>= 1);

  MPFR_SAVE_EXPO_MARK (expo);
  prec = MPFR_PREC (x) + 3 + size_n;
  mpfr_init2 (res, prec);

  MPFR_ZIV_INIT (loop, prec);
  for (;;)
    {
      int i = size_n;

      inexact = mpfr_set_ui (res, y, GMP_RNDU);
      err = 1;
      /* now 2^(i-1) <= n < 2^i: i=1+floor(log2(n)) */
      for (i -= 2; i >= 0; i--)
	{
	  inexact |= mpfr_mul (res, res, res, GMP_RNDU);
	  err++;
	  if (n & (1UL << i))
	    inexact |= mpfr_mul_ui (res, res, y, GMP_RNDU);
	}
      /* since the loop is executed floor(log2(n)) times,
         we have err = 1+floor(log2(n)).
         Since prec >= MPFR_PREC(x) + 4 + floor(log2(n)), prec > err */
      err = prec - err;
      
      if (MPFR_LIKELY (inexact == 0
		       || MPFR_CAN_ROUND (res, err, MPFR_PREC (x), rnd)))
	break;
      
      /* Actualisation of the precision */
      MPFR_ZIV_NEXT (loop, prec);
      mpfr_set_prec (res, prec);
    }
  MPFR_ZIV_FREE (loop);

  inexact = mpfr_set (x, res, rnd);

  mpfr_clear (res);

  MPFR_SAVE_EXPO_FREE (expo);
  return mpfr_check_range (x, inexact, rnd);
}