/* mpfr_atanh -- Inverse Hyperbolic Tangente of Unsigned Integer Number

Copyright 2001, 2002 Free Software Foundation.

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.  If not, write to
the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
MA 02111-1307, USA. */

#include "gmp.h"
#include "gmp-impl.h"
#include "mpfr.h"
#include "mpfr-impl.h"

 /* The computation of acosh is done by

    atanh= 1/2*ln(x+1)-1/2*ln(1-x)
 */

int
mpfr_atanh (mpfr_ptr y, mpfr_srcptr xt , mp_rnd_t rnd_mode) 
{
  int inexact =0;
  mpfr_t x;
  int flag_neg=0;

  mp_prec_t Nx=MPFR_PREC(xt);   /* Precision of input variable */
  mpfr_init2(x,Nx);
  mpfr_set(x,xt,GMP_RNDN); 
 
  if (MPFR_SIGN(x) < 0)
    {
      MPFR_CHANGE_SIGN(x);
      flag_neg=1;
    }

  if (MPFR_IS_NAN(x)) 
    {  
      MPFR_SET_NAN(y); 
      mpfr_clear(x);
      MPFR_RET_NAN;
    }
  MPFR_CLEAR_NAN(y);
  

  if (MPFR_IS_INF(x))
    { 
      MPFR_SET_INF(y);
      MPFR_SET_SAME_SIGN(y,x);
      if(flag_neg)
	MPFR_CHANGE_SIGN(y);
      mpfr_clear(x);
      MPFR_RET(0);
    }

  MPFR_CLEAR_INF(y);

  if (MPFR_IS_ZERO(x))
    {
      MPFR_SET_ZERO(y);   /* atanh(0) = 0 */
      MPFR_SET_SAME_SIGN(y,x);
      if(flag_neg)
	MPFR_CHANGE_SIGN(y);
      mpfr_clear(x); 
      MPFR_RET(0);
    }

  /* General case */
  {
    /* Declaration of the intermediary variable */
    mpfr_t t, te,ti;       
    
    /* 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 input variable */
    
    mp_prec_t Nt;   /* Precision of the intermediary variable */
    long int err;  /* Precision of error */
                
    /* compute the precision of intermediary variable */
    Nt=MAX(Nx,Ny);
    /* the optimal number of bits : see algorithms.ps */
    Nt=Nt+4+_mpfr_ceil_log2(Nt);

    /* initialise of intermediary	variable */
    mpfr_init(t);             
    mpfr_init(te);             
    mpfr_init(ti);                    

    /* First computation of cosh */
    do {

      /* reactualisation of the precision */
      mpfr_set_prec(t,Nt);             
      mpfr_set_prec(te,Nt);             
      mpfr_set_prec(ti,Nt);             

      /* compute atanh */
      mpfr_ui_sub(te,1,x,GMP_RNDU);   /* (1-xt)*/
      mpfr_add_ui(ti,x,1,GMP_RNDD);   /* (xt+1)*/
      mpfr_div(te,ti,te,GMP_RNDN);    /* (1+xt)/(1-xt)*/
      mpfr_log(te,te,GMP_RNDN);       /* ln((1+xt)/(1-xt))*/
      mpfr_div_2ui(t,te,1,GMP_RNDN);  /* (1/2)*ln((1+xt)/(1-xt))*/
	
      /* estimation of the error see- algorithms.ps*/
      /* err=Nt-_mpfr_ceil_log2(1+5*pow(2,1-MPFR_EXP(t)));*/
      err=Nt-(MAX(4-MPFR_EXP(t),0)+1);

      /* actualisation of the precision */
      Nt += 10;

    } while ((err < 0) || (!mpfr_can_round(t,err,GMP_RNDN,rnd_mode,Ny) || (MPFR_IS_ZERO(t))));

    if(flag_neg)
      MPFR_CHANGE_SIGN(t);

    inexact = mpfr_set(y,t,rnd_mode);

    mpfr_clear(t);
    mpfr_clear(ti);
    mpfr_clear(te);
  }
  mpfr_clear(x);
  return inexact;
}