xref: /dflybsd-src/contrib/mpfr/src/pow_ui.c (revision 2786097444a0124b5d33763854de247e230c6629)

/* mpfr_pow_ui-- compute the power of a floating-point
                                  by a machine integer

Copyright 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Free Software Foundation, Inc.
Contributed by the AriC and Caramel 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 3 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.LESSER.  If not, see
http://www.gnu.org/licenses/ or write to the Free Software Foundation, Inc.,
51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA. */

#define MPFR_NEED_LONGLONG_H
#include "mpfr-impl.h"

/* sets y to x^n, and return 0 if exact, non-zero otherwise */
int
mpfr_pow_ui (mpfr_ptr y, mpfr_srcptr x, unsigned long int n, mpfr_rnd_t rnd)
{
  unsigned long m;
  mpfr_t res;
  mpfr_prec_t prec, err;
  int inexact;
  mpfr_rnd_t rnd1;
  MPFR_SAVE_EXPO_DECL (expo);
  MPFR_ZIV_DECL (loop);
  MPFR_BLOCK_DECL (flags);

  MPFR_LOG_FUNC
    (("x[%Pu]=%.*Rg n=%lu rnd=%d",
      mpfr_get_prec (x), mpfr_log_prec, x, n, rnd),
     ("y[%Pu]=%.*Rg inexact=%d",
      mpfr_get_prec (y), mpfr_log_prec, y, inexact));

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

  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))
        {
          /* Inf^n = Inf, (-Inf)^n = Inf for n even, -Inf for n odd */
          if (MPFR_IS_NEG (x) && (n & 1) == 1)
            MPFR_SET_NEG (y);
          else
            MPFR_SET_POS (y);
          MPFR_SET_INF (y);
          MPFR_RET (0);
        }
      else /* x is zero */
        {
          MPFR_ASSERTD (MPFR_IS_ZERO (x));
          /* 0^n = 0 for any n */
          MPFR_SET_ZERO (y);
          if (MPFR_IS_POS (x) || (n & 1) == 0)
            MPFR_SET_POS (y);
          else
            MPFR_SET_NEG (y);
          MPFR_RET (0);
        }
    }
  else if (MPFR_UNLIKELY (n <= 2))
    {
      if (n < 2)
        /* x^1 = x */
        return mpfr_set (y, x, rnd);
      else
        /* x^2 = sqr(x) */
        return mpfr_sqr (y, x, rnd);
    }

  /* Augment exponent range */
  MPFR_SAVE_EXPO_MARK (expo);

  /* setup initial precision */
  prec = MPFR_PREC (y) + 3 + GMP_NUMB_BITS
    + MPFR_INT_CEIL_LOG2 (MPFR_PREC (y));
  mpfr_init2 (res, prec);

  rnd1 = MPFR_IS_POS (x) ? MPFR_RNDU : MPFR_RNDD; /* away */

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

      for (m = n, i = 0; m; i++, m >>= 1)
        ;
      /* now 2^(i-1) <= n < 2^i */
      MPFR_ASSERTD (prec > (mpfr_prec_t) i);
      err = prec - 1 - (mpfr_prec_t) i;
      /* First step: compute square from x */
      MPFR_BLOCK (flags,
                  inexact = mpfr_mul (res, x, x, MPFR_RNDU);
                  MPFR_ASSERTD (i >= 2);
                  if (n & (1UL << (i-2)))
                    inexact |= mpfr_mul (res, res, x, rnd1);
                  for (i -= 3; i >= 0 && !MPFR_BLOCK_EXCEP; i--)
                    {
                      inexact |= mpfr_mul (res, res, res, MPFR_RNDU);
                      if (n & (1UL << i))
                        inexact |= mpfr_mul (res, res, x, rnd1);
                    });
      /* let r(n) be the number of roundings: we have r(2)=1, r(3)=2,
         and r(2n)=2r(n)+1, r(2n+1)=2r(n)+2, thus r(n)=n-1.
         Using Higham's method, to each rounding corresponds a factor
         (1-theta) with 0 <= theta <= 2^(1-p), thus at the end the
         absolute error is bounded by (n-1)*2^(1-p)*res <= 2*(n-1)*ulp(res)
         since 2^(-p)*x <= ulp(x). Since n < 2^i, this gives a maximal
         error of 2^(1+i)*ulp(res).
      */
      if (MPFR_LIKELY (inexact == 0
                       || MPFR_OVERFLOW (flags) || MPFR_UNDERFLOW (flags)
                       || MPFR_CAN_ROUND (res, err, MPFR_PREC (y), rnd)))
        break;
      /* Actualisation of the precision */
      MPFR_ZIV_NEXT (loop, prec);
      mpfr_set_prec (res, prec);
    }
  MPFR_ZIV_FREE (loop);

  if (MPFR_UNLIKELY (MPFR_OVERFLOW (flags) || MPFR_UNDERFLOW (flags)))
    {
      mpz_t z;

      /* Internal overflow or underflow. However the approximation error has
       * not been taken into account. So, let's solve this problem by using
       * mpfr_pow_z, which can handle it. This case could be improved in the
       * future, without having to use mpfr_pow_z.
       */
      MPFR_LOG_MSG (("Internal overflow or underflow,"
                     " let's use mpfr_pow_z.\n", 0));
      mpfr_clear (res);
      MPFR_SAVE_EXPO_FREE (expo);
      mpz_init (z);
      mpz_set_ui (z, n);
      inexact = mpfr_pow_z (y, x, z, rnd);
      mpz_clear (z);
      return inexact;
    }

  inexact = mpfr_set (y, res, rnd);
  mpfr_clear (res);

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