/* mpfr_atan2 -- arc-tan 2 of a floating-point number

Copyright 2005, 2006, 2007, 2008, 2009, 2010, 2011 Free Software Foundation, Inc.
Contributed by the Arenaire and Cacao projects, INRIA.

This file is part of the GNU MPFR Library, and was contributed by Mathieu Dutour.

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"

static int
pi_div_2ui (mpfr_ptr dest, int i, int neg, mpfr_rnd_t rnd_mode)
{
  int inexact;
  MPFR_SAVE_EXPO_DECL (expo);

  MPFR_SAVE_EXPO_MARK (expo);
  if (neg) /* -PI/2^i */
    {
      inexact = - mpfr_const_pi (dest, MPFR_INVERT_RND (rnd_mode));
      MPFR_CHANGE_SIGN (dest);
    }
  else /* PI/2^i */
    {
      inexact = mpfr_const_pi (dest, rnd_mode);
    }
  mpfr_div_2ui (dest, dest, i, rnd_mode);  /* exact */
  MPFR_SAVE_EXPO_FREE (expo);
  return mpfr_check_range (dest, inexact, rnd_mode);
}

int
mpfr_atan2 (mpfr_ptr dest, mpfr_srcptr y, mpfr_srcptr x, mpfr_rnd_t rnd_mode)
{
  mpfr_t tmp, pi;
  int inexact;
  mpfr_prec_t prec;
  mpfr_exp_t e;
  MPFR_SAVE_EXPO_DECL (expo);
  MPFR_ZIV_DECL (loop);

  MPFR_LOG_FUNC (("y[%#R]=%R x[%#R]=%R rnd=%d", y, y, x, x, rnd_mode),
                 ("atan[%#R]=%R inexact=%d", dest, dest, inexact));

  /* Special cases */
  if (MPFR_ARE_SINGULAR (x, y))
    {
      /* atan2(0, 0) does not raise the "invalid" floating-point
         exception, nor does atan2(y, 0) raise the "divide-by-zero"
         floating-point exception.
         -- atan2(±0, -0) returns ±pi.313)
         -- atan2(±0, +0) returns ±0.
         -- atan2(±0, x) returns ±pi, for x < 0.
         -- atan2(±0, x) returns ±0, for x > 0.
         -- atan2(y, ±0) returns -pi/2 for y < 0.
         -- atan2(y, ±0) returns pi/2 for y > 0.
         -- atan2(±oo, -oo) returns ±3pi/4.
         -- atan2(±oo, +oo) returns ±pi/4.
         -- atan2(±oo, x) returns ±pi/2, for finite x.
         -- atan2(±y, -oo) returns ±pi, for finite y > 0.
         -- atan2(±y, +oo) returns ±0, for finite y > 0.
      */
      if (MPFR_IS_NAN (x) || MPFR_IS_NAN (y))
        {
          MPFR_SET_NAN (dest);
          MPFR_RET_NAN;
        }
      if (MPFR_IS_ZERO (y))
        {
          if (MPFR_IS_NEG (x)) /* +/- PI */
            {
            set_pi:
              if (MPFR_IS_NEG (y))
                {
                  inexact =  mpfr_const_pi (dest, MPFR_INVERT_RND (rnd_mode));
                  MPFR_CHANGE_SIGN (dest);
                  return -inexact;
                }
              else
                return mpfr_const_pi (dest, rnd_mode);
            }
          else /* +/- 0 */
            {
            set_zero:
              MPFR_SET_ZERO (dest);
              MPFR_SET_SAME_SIGN (dest, y);
              return 0;
            }
        }
      if (MPFR_IS_ZERO (x))
        {
          return pi_div_2ui (dest, 1, MPFR_IS_NEG (y), rnd_mode);
        }
      if (MPFR_IS_INF (y))
        {
          if (!MPFR_IS_INF (x)) /* +/- PI/2 */
            return pi_div_2ui (dest, 1, MPFR_IS_NEG (y), rnd_mode);
          else if (MPFR_IS_POS (x)) /* +/- PI/4 */
            return pi_div_2ui (dest, 2, MPFR_IS_NEG (y), rnd_mode);
          else /* +/- 3*PI/4: Ugly since we have to round properly */
            {
              mpfr_t tmp2;
              MPFR_ZIV_DECL (loop2);
              mpfr_prec_t prec2 = MPFR_PREC (dest) + 10;

              MPFR_SAVE_EXPO_MARK (expo);
              mpfr_init2 (tmp2, prec2);
              MPFR_ZIV_INIT (loop2, prec2);
              for (;;)
                {
                  mpfr_const_pi (tmp2, MPFR_RNDN);
                  mpfr_mul_ui (tmp2, tmp2, 3, MPFR_RNDN); /* Error <= 2  */
                  mpfr_div_2ui (tmp2, tmp2, 2, MPFR_RNDN);
                  if (mpfr_round_p (MPFR_MANT (tmp2), MPFR_LIMB_SIZE (tmp2),
                                    MPFR_PREC (tmp2) - 2,
                                    MPFR_PREC (dest) + (rnd_mode == MPFR_RNDN)))
                    break;
                  MPFR_ZIV_NEXT (loop2, prec2);
                  mpfr_set_prec (tmp2, prec2);
                }
              MPFR_ZIV_FREE (loop2);
              if (MPFR_IS_NEG (y))
                MPFR_CHANGE_SIGN (tmp2);
              inexact = mpfr_set (dest, tmp2, rnd_mode);
              mpfr_clear (tmp2);
              MPFR_SAVE_EXPO_FREE (expo);
              return mpfr_check_range (dest, inexact, rnd_mode);
            }
        }
      MPFR_ASSERTD (MPFR_IS_INF (x));
      if (MPFR_IS_NEG (x))
        goto set_pi;
      else
        goto set_zero;
    }

  /* When x=1, atan2(y,x) = atan(y). FIXME: more generally, if x is a power
     of two, we could call directly atan(y/x) since y/x is exact. */
  if (mpfr_cmp_ui (x, 1) == 0)
    return mpfr_atan (dest, y, rnd_mode);

  MPFR_SAVE_EXPO_MARK (expo);

  /* Set up initial prec */
  prec = MPFR_PREC (dest) + 3 + MPFR_INT_CEIL_LOG2 (MPFR_PREC (dest));
  mpfr_init2 (tmp, prec);

  MPFR_ZIV_INIT (loop, prec);
  if (MPFR_IS_POS (x))
    /* use atan2(y,x) = atan(y/x) */
    for (;;)
      {
        int div_inex;
        MPFR_BLOCK_DECL (flags);

        MPFR_BLOCK (flags, div_inex = mpfr_div (tmp, y, x, MPFR_RNDN));
        if (div_inex == 0)
          {
            /* Result is exact. */
            inexact = mpfr_atan (dest, tmp, rnd_mode);
            goto end;
          }

        /* Error <= ulp (tmp) except in case of underflow or overflow. */

        /* If the division underflowed, since |atan(z)/z| < 1, we have
           an underflow. */
        if (MPFR_UNDERFLOW (flags))
          {
            int sign;

            /* In the case MPFR_RNDN with 2^(emin-2) < |y/x| < 2^(emin-1):
               The smallest significand value S > 1 of |y/x| is:
                 * 1 / (1 - 2^(-px))                        if py <= px,
                 * (1 - 2^(-px) + 2^(-py)) / (1 - 2^(-px))  if py >= px.
               Therefore S - 1 > 2^(-pz), where pz = max(px,py). We have:
               atan(|y/x|) > atan(z), where z = 2^(emin-2) * (1 + 2^(-pz)).
                           > z - z^3 / 3.
                           > 2^(emin-2) * (1 + 2^(-pz) - 2^(2 emin - 5))
               Assuming pz <= -2 emin + 5, we can round away from zero
               (this is what mpfr_underflow always does on MPFR_RNDN).
               In the case MPFR_RNDN with |y/x| <= 2^(emin-2), we round
               toward zero, as |atan(z)/z| < 1. */
            MPFR_ASSERTN (MPFR_PREC_MAX <=
                          2 * (mpfr_uexp_t) - MPFR_EMIN_MIN + 5);
            if (rnd_mode == MPFR_RNDN && MPFR_IS_ZERO (tmp))
              rnd_mode = MPFR_RNDZ;
            sign = MPFR_SIGN (tmp);
            mpfr_clear (tmp);
            MPFR_SAVE_EXPO_FREE (expo);
            return mpfr_underflow (dest, rnd_mode, sign);
          }

        mpfr_atan (tmp, tmp, MPFR_RNDN);   /* Error <= 2*ulp (tmp) since
                                             abs(D(arctan)) <= 1 */
        /* TODO: check that the error bound is correct in case of overflow. */
        /* FIXME: Error <= ulp(tmp) ? */
        if (MPFR_LIKELY (MPFR_CAN_ROUND (tmp, prec - 2, MPFR_PREC (dest),
                                         rnd_mode)))
          break;
        MPFR_ZIV_NEXT (loop, prec);
        mpfr_set_prec (tmp, prec);
      }
  else /* x < 0 */
    /*  Use sign(y)*(PI - atan (|y/x|)) */
    {
      mpfr_init2 (pi, prec);
      for (;;)
        {
          mpfr_div (tmp, y, x, MPFR_RNDN);   /* Error <= ulp (tmp) */
          /* If tmp is 0, we have |y/x| <= 2^(-emin-2), thus
             atan|y/x| < 2^(-emin-2). */
          MPFR_SET_POS (tmp);               /* no error */
          mpfr_atan (tmp, tmp, MPFR_RNDN);   /* Error <= 2*ulp (tmp) since
                                               abs(D(arctan)) <= 1 */
          mpfr_const_pi (pi, MPFR_RNDN);     /* Error <= ulp(pi) /2 */
          e = MPFR_NOTZERO(tmp) ? MPFR_GET_EXP (tmp) : __gmpfr_emin - 1;
          mpfr_sub (tmp, pi, tmp, MPFR_RNDN);          /* see above */
          if (MPFR_IS_NEG (y))
            MPFR_CHANGE_SIGN (tmp);
          /* Error(tmp) <= (1/2+2^(EXP(pi)-EXP(tmp)-1)+2^(e-EXP(tmp)+1))*ulp
                        <= 2^(MAX (MAX (EXP(PI)-EXP(tmp)-1, e-EXP(tmp)+1),
                                        -1)+2)*ulp(tmp) */
          e = MAX (MAX (MPFR_GET_EXP (pi)-MPFR_GET_EXP (tmp) - 1,
                        e - MPFR_GET_EXP (tmp) + 1), -1) + 2;
          if (MPFR_LIKELY (MPFR_CAN_ROUND (tmp, prec - e, MPFR_PREC (dest),
                                           rnd_mode)))
            break;
          MPFR_ZIV_NEXT (loop, prec);
          mpfr_set_prec (tmp, prec);
          mpfr_set_prec (pi, prec);
        }
      mpfr_clear (pi);
    }
  inexact = mpfr_set (dest, tmp, rnd_mode);

 end:
  MPFR_ZIV_FREE (loop);
  mpfr_clear (tmp);
  MPFR_SAVE_EXPO_FREE (expo);
  return mpfr_check_range (dest, inexact, rnd_mode);
}