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

/* mpfr_hypot -- Euclidean distance

Copyright 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"

/* The computation of hypot of x and y is done by  *
 *    hypot(x,y)= sqrt(x^2+y^2) = z                */

int
mpfr_hypot (mpfr_ptr z, mpfr_srcptr x, mpfr_srcptr y, mpfr_rnd_t rnd_mode)
{
  int inexact, exact;
  mpfr_t t, te, ti; /* auxiliary variables */
  mpfr_prec_t N, Nz; /* size variables */
  mpfr_prec_t Nt;   /* precision of the intermediary variable */
  mpfr_prec_t threshold;
  mpfr_exp_t Ex, sh;
  mpfr_uexp_t diff_exp;

  MPFR_SAVE_EXPO_DECL (expo);
  MPFR_ZIV_DECL (loop);
  MPFR_BLOCK_DECL (flags);

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

  /* particular cases */
  if (MPFR_ARE_SINGULAR (x, y))
    {
      if (MPFR_IS_INF (x) || MPFR_IS_INF (y))
        {
          /* Return +inf, even when the other number is NaN. */
          MPFR_SET_INF (z);
          MPFR_SET_POS (z);
          MPFR_RET (0);
        }
      else if (MPFR_IS_NAN (x) || MPFR_IS_NAN (y))
        {
          MPFR_SET_NAN (z);
          MPFR_RET_NAN;
        }
      else if (MPFR_IS_ZERO (x))
        return mpfr_abs (z, y, rnd_mode);
      else /* y is necessarily 0 */
        return mpfr_abs (z, x, rnd_mode);
    }

  if (mpfr_cmpabs (x, y) < 0)
    {
      mpfr_srcptr u;
      u = x;
      x = y;
      y = u;
    }

  /* now |x| >= |y| */

  Ex = MPFR_GET_EXP (x);
  diff_exp = (mpfr_uexp_t) Ex - MPFR_GET_EXP (y);

  N = MPFR_PREC (x);   /* Precision of input variable */
  Nz = MPFR_PREC (z);   /* Precision of output variable */
  threshold = (MAX (N, Nz) + (rnd_mode == MPFR_RNDN ? 1 : 0)) << 1;
  if (rnd_mode == MPFR_RNDA)
    rnd_mode = MPFR_RNDU; /* since the result is positive, RNDA = RNDU */

  /* Is |x| a suitable approximation to the precision Nz ?
     (see algorithms.tex for explanations) */
  if (diff_exp > threshold)
    /* result is |x| or |x|+ulp(|x|,Nz) */
    {
      if (MPFR_UNLIKELY (rnd_mode == MPFR_RNDU))
        {
          /* If z > abs(x), then it was already rounded up; otherwise
             z = abs(x), and we need to add one ulp due to y. */
          if (mpfr_abs (z, x, rnd_mode) == 0)
            mpfr_nexttoinf (z);
          MPFR_RET (1);
        }
      else /* MPFR_RNDZ, MPFR_RNDD, MPFR_RNDN */
        {
          if (MPFR_LIKELY (Nz >= N))
            {
              mpfr_abs (z, x, rnd_mode);  /* exact */
              MPFR_RET (-1);
            }
          else
            {
              MPFR_SET_EXP (z, Ex);
              MPFR_SET_SIGN (z, 1);
              MPFR_RNDRAW_GEN (inexact, z, MPFR_MANT (x), N, rnd_mode, 1,
                               goto addoneulp,
                               if (MPFR_UNLIKELY (++ MPFR_EXP (z) >
                                                  __gmpfr_emax))
                                 return mpfr_overflow (z, rnd_mode, 1);
                               );

              if (MPFR_UNLIKELY (inexact == 0))
                inexact = -1;
              MPFR_RET (inexact);
            }
        }
    }

  /* General case */

  N = MAX (MPFR_PREC (x), MPFR_PREC (y));

  /* working precision */
  Nt = Nz + MPFR_INT_CEIL_LOG2 (Nz) + 4;

  mpfr_init2 (t, Nt);
  mpfr_init2 (te, Nt);
  mpfr_init2 (ti, Nt);

  MPFR_SAVE_EXPO_MARK (expo);

  /* Scale x and y to avoid overflow/underflow in x^2 and overflow in y^2
     (as |x| >= |y|). The scaling of y can underflow only when the target
     precision is huge, otherwise the case would already have been handled
     by the diff_exp > threshold code. */
  sh = mpfr_get_emax () / 2 - Ex - 1;

  MPFR_ZIV_INIT (loop, Nt);
  for (;;)
    {
      mpfr_prec_t err;

      exact = mpfr_mul_2si (te, x, sh, MPFR_RNDZ);
      exact |= mpfr_mul_2si (ti, y, sh, MPFR_RNDZ);
      exact |= mpfr_sqr (te, te, MPFR_RNDZ);
      /* Use fma in order to avoid underflow when diff_exp<=MPFR_EMAX_MAX-2 */
      exact |= mpfr_fma (t, ti, ti, te, MPFR_RNDZ);
      exact |= mpfr_sqrt (t, t, MPFR_RNDZ);

      err = Nt < N ? 4 : 2;
      if (MPFR_LIKELY (exact == 0
                       || MPFR_CAN_ROUND (t, Nt-err, Nz, rnd_mode)))
        break;

      MPFR_ZIV_NEXT (loop, Nt);
      mpfr_set_prec (t, Nt);
      mpfr_set_prec (te, Nt);
      mpfr_set_prec (ti, Nt);
    }
  MPFR_ZIV_FREE (loop);

  MPFR_BLOCK (flags, inexact = mpfr_div_2si (z, t, sh, rnd_mode));
  MPFR_ASSERTD (exact == 0 || inexact != 0);

  mpfr_clear (t);
  mpfr_clear (ti);
  mpfr_clear (te);

  /*
    exact  inexact
    0         0         result is exact, ternary flag is 0
    0       non zero    t is exact, ternary flag given by inexact
    1         0         impossible (see above)
    1       non zero    ternary flag given by inexact
  */

  MPFR_SAVE_EXPO_FREE (expo);

  if (MPFR_OVERFLOW (flags))
    mpfr_set_overflow ();
  /* hypot(x,y) >= |x|, thus underflow is not possible. */

  return mpfr_check_range (z, inexact, rnd_mode);
}