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

Copyright 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009 Free Software Foundation, Inc.
Contributed by the Arenaire and Cacao projects, INRIA.

This file is part of the GNU MPFR Library.

The GNU 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 GNU 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 GNU MPFR Library; see the file COPYING.LIB.  If not, write to
the Free Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston,
MA 02110-1301, 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 <= 1))
    {
      if (n == 1)
        return mpfr_set_ui (x, y, rnd);     /* y^1 = y */
      else
        return mpfr_set_ui (x, 1, rnd);     /* y^0 = 1 for any y */
    }
  else if (MPFR_UNLIKELY (y <= 1))
    {
      if (y == 1)
        return mpfr_set_ui (x, 1, rnd);     /* 1^n = 1 for any n > 0 */
      else
        return mpfr_set_ui (x, 0, rnd);     /* 0^n = 0 for any n > 0 */
    }

  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);
}