/* mpfr_pow_si -- power function x^y with y a signed int

Copyright 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. */

#define MPFR_NEED_LONGLONG_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
    {
      if (MPFR_UNLIKELY (MPFR_IS_SINGULAR (x)))
	{
	  if (MPFR_IS_NAN (x))
	    {
	      MPFR_SET_NAN (y);
	      MPFR_RET_NAN;
	    }
	  else if (MPFR_IS_INF (x))
	    {
	      MPFR_SET_ZERO (y);
	      if (MPFR_IS_POS (x) || ((unsigned) n & 1) == 0)
		MPFR_SET_POS (y);
	      else
		MPFR_SET_NEG (y);
	      MPFR_RET (0);
	    }
	  else /* x is zero */
	    {
              MPFR_ASSERTD (MPFR_IS_ZERO (x));
	      MPFR_SET_INF(y);
	      if (MPFR_IS_POS (x) || ((unsigned) n & 1) == 0)
		MPFR_SET_POS (y);
	      else
		MPFR_SET_NEG (y);
	      MPFR_RET(0);
	    }
	}
      MPFR_CLEAR_FLAGS (y);

      /* detect exact powers: x^(-n) is exact iff x is a power of 2 */
      if (mpfr_cmp_si_2exp (x, MPFR_SIGN(x), MPFR_EXP(x) - 1) == 0)
        {
          mp_exp_t expx = MPFR_EXP (x); /* warning: x and y may be the same
                                            variable */
          mpfr_set_si (y, (n % 2) ? MPFR_INT_SIGN(x) : 1, rnd_mode);
	  expx --;
	  MPFR_ASSERTD (n < 0);
	  /* Warning n*expx may overflow! 
	     Some systems abort with LONG_MIN / 1 or LONG_MIN/-1*/
	  if (n != -1 && expx > 0 && -expx < MPFR_EXP_MIN / (-n))
	    MPFR_EXP (y) = MPFR_EMIN_MIN - 1; /* Underflow */
	  else if (n != -1 && expx < 0 && -expx > MPFR_EXP_MAX / (-n))
	    MPFR_EXP (y) = MPFR_EMAX_MAX + 1; /* Overflow */
          else
	    MPFR_EXP (y) += n * expx;
          return mpfr_check_range (y, 0, rnd_mode);
        }

      n = -n;

      /* General case */
      {
        /* Declaration of the intermediary variable */
        mpfr_t t;
        /* 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 output variable */
        mp_prec_t Nt;         /* Precision of the intermediary variable */
        mp_exp_t  err;                            /* Precision of error */
        int inexact;
	MPFR_SAVE_EXPO_DECL (expo);
	MPFR_ZIV_DECL (loop);

        /* compute the precision of intermediary variable */
        Nt = MAX (Nx, Ny);
        /* the optimal number of bits : see algorithms.ps */
        Nt = Nt + 3 + MPFR_INT_CEIL_LOG2 (Nt);

	MPFR_SAVE_EXPO_MARK (expo);

        /* initialise of intermediary	variable */
        mpfr_init2 (t, Nt);
	
	MPFR_ZIV_INIT (loop, Nt);
        for (;;)
	  {
	    /* compute 1/(x^n) n>0*/
	    mpfr_pow_ui (t, x, (unsigned long int) n, GMP_RNDN);
	    inexact = MPFR_IS_ZERO (t) || MPFR_IS_INF (t);
	    mpfr_ui_div (t, 1, t, GMP_RNDN);
	    inexact = inexact || MPFR_IS_ZERO (t) || MPFR_IS_INF (t);

	    /* error estimate -- see pow function in algorithms.ps */
	    err = Nt - 3;
	    if (MPFR_LIKELY (inexact != 0
			     || mpfr_can_round (t, err, GMP_RNDN, GMP_RNDZ,
						Ny + (rnd_mode == GMP_RNDN))))
	      break;

	    /* actualisation of the precision */
	    Nt += BITS_PER_MP_LIMB;
	    mpfr_set_prec (t, Nt);
	  }
	MPFR_ZIV_FREE (loop);

        inexact = mpfr_set (y, t, rnd_mode);
        mpfr_clear (t);
	MPFR_SAVE_EXPO_FREE (expo);
        return mpfr_check_range (y, inexact, rnd_mode);
      }
    }
}