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

/* mpfr_cot - cotangent function.

Copyright 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. */

/* the cotangent is defined by cot(x) = 1/tan(x) = cos(x)/sin(x).
   cot (NaN) = NaN.
   cot (+Inf) = csc (-Inf) = NaN.
   cot (+0) = +Inf.
   cot (-0) = -Inf.
*/

#define FUNCTION mpfr_cot
#define INVERSE  mpfr_tan
#define ACTION_NAN(y) do { MPFR_SET_NAN(y); MPFR_RET_NAN; } while (1)
#define ACTION_INF(y) do { MPFR_SET_NAN(y); MPFR_RET_NAN; } while (1)
#define ACTION_ZERO(y,x) do { MPFR_SET_SAME_SIGN(y,x); MPFR_SET_INF(y); \
                              mpfr_set_divby0 (); MPFR_RET(0); } while (1)

/* (This analysis is adapted from that for mpfr_coth.)
   Near x=0, cot(x) = 1/x - x/3 + ..., more precisely we have
   |cot(x) - 1/x| <= 0.36 for |x| <= 1. The error term has
   the opposite sign as 1/x, thus |cot(x)| <= |1/x|. Then:
   (i) either x is a power of two, then 1/x is exactly representable, and
       as long as 1/2*ulp(1/x) > 0.36, we can conclude;
   (ii) otherwise assume x has <= n bits, and y has <= n+1 bits, then
   |y - 1/x| >= 2^(-2n) ufp(y), where ufp means unit in first place.
   Since |cot(x) - 1/x| <= 0.36, if 2^(-2n) ufp(y) >= 0.72, then
   |y - cot(x)| >= 2^(-2n-1) ufp(y), and rounding 1/x gives the correct
   result. If x < 2^E, then y > 2^(-E), thus ufp(y) > 2^(-E-1).
   A sufficient condition is thus EXP(x) + 1 <= -2 MAX(PREC(x),PREC(Y)).
   The division can be inexact in case of underflow or overflow; but
   an underflow is not possible as emin = - emax. The overflow is a
   real overflow possibly except when |x| = 2^emin. */
#define ACTION_TINY(y,x,r) \
  if (MPFR_EXP(x) + 1 <= -2 * (mpfr_exp_t) MAX(MPFR_PREC(x), MPFR_PREC(y))) \
    {                                                                   \
      int two2emin;                                                     \
      int signx = MPFR_SIGN(x);                                         \
      MPFR_ASSERTN (MPFR_EMIN_MIN + MPFR_EMAX_MAX == 0);                \
      if ((two2emin = mpfr_get_exp (x) == __gmpfr_emin + 1 &&           \
           mpfr_powerof2_raw (x)))                                      \
        {                                                               \
          /* Case |x| = 2^emin. 1/x is not representable; so, compute   \
             1/(2x) instead (exact), and correct the result later. */   \
          mpfr_set_si_2exp (y, signx, __gmpfr_emax, MPFR_RNDN);         \
          inexact = 0;                                                  \
        }                                                               \
      else                                                              \
        inexact = mpfr_ui_div (y, 1, x, r);                             \
      if (inexact == 0) /* x is a power of two */                       \
        { /* result always 1/x, except when rounding to zero */         \
          if (rnd_mode == MPFR_RNDA)                                    \
            rnd_mode = (signx > 0) ? MPFR_RNDU : MPFR_RNDD;             \
          if (rnd_mode == MPFR_RNDU || (rnd_mode == MPFR_RNDZ && signx < 0)) \
            {                                                           \
              if (signx < 0)                                            \
                mpfr_nextabove (y); /* -2^k + epsilon */                \
              inexact = 1;                                              \
            }                                                           \
          else if (rnd_mode == MPFR_RNDD || rnd_mode == MPFR_RNDZ)      \
            {                                                           \
              if (signx > 0)                                            \
                mpfr_nextbelow (y); /* 2^k - epsilon */                 \
              inexact = -1;                                             \
            }                                                           \
          else /* round to nearest */                                   \
            inexact = signx;                                            \
          if (two2emin)                                                 \
            mpfr_mul_2ui (y, y, 1, r);  /* overflow in MPFR_RNDN */      \
        }                                                               \
      /* Underflow is not possible with emin = - emax, but we cannot */ \
      /* add an assert as the underflow flag could have already been */ \
      /* set before the call to mpfr_cot.                            */ \
      MPFR_SAVE_EXPO_UPDATE_FLAGS (expo, __gmpfr_flags);                \
      goto end;                                                         \
    }

#include "gen_inverse.h"