/* Author:  J. Theiler (modifications by G. Jungman)
 * RCS:     $Id: erfc.c,v 1.18 1999/08/30 10:35:59 bjg Exp $
 */
/*
 * See Hart et al, Computer Approximations, John Wiley and Sons, New York (1968)
 * (This applies only to the erfc8 stuff, which is the part
 *  of the original code that survives. I have replaced much of
 *  the other stuff with Chebyshev fits. These are simpler and
 *  more precise than the original approximations. [GJ])
 */
#include <config.h>
#include <gsl_math.h>
#include <gsl_errno.h>
#include "gsl_sf_chebyshev.h"
#include "gsl_sf_erf.h"

#define LogRootPi_  0.57236494292470008706


static double erfc8_sum(double x)
{
  /* estimates erfc(x) valid for 8 < x < 100 */
  /* This is based on index 5725 in Hart et al */

  static double P[] = {
      2.97886562639399288862,
      7.409740605964741794425,
      6.1602098531096305440906,
      5.019049726784267463450058,
      1.275366644729965952479585264,
      0.5641895835477550741253201704
  };
  static double Q[] = {
      3.3690752069827527677,
      9.608965327192787870698,
      17.08144074746600431571095,
      12.0489519278551290360340491,
      9.396034016235054150430579648,
      2.260528520767326969591866945,
      1.0
  };
  double num=0.0, den=0.0;
  int i;

  num = P[5];
  for (i=4; i>=0; --i) {
      num = x*num + P[i];
  }
  den = Q[6];
  for (i=5; i>=0; --i) {
      den = x*den + Q[i];
  }

  return num/den;
}

inline
static double erfc8(double x)
{
  double e;
  e = erfc8_sum(x);
  e *= exp(-x*x);
  return e;
}

inline
static double log_erfc8(double x)
{
  double e;
  e = erfc8_sum(x);
  e = log(e) - x*x;
  return e;
}

#if 0
/* Abramowitz+Stegun, 7.2.14 */
static double erfcasympsum(double x)
{
  int i;
  double e = 1.;
  double coef = 1.;
  for (i=1; i<5; ++i) {
    /* coef *= -(2*i-1)/(2*x*x); ??? [GJ] */
    coef *= -(2*i+1)/(i*(4*x*x*x*x));
    e += coef;
    /*
    if (fabs(coef) < 1.0e-15) break;
    if (fabs(coef) > 1.0e10) break;
    
    [GJ]: These tests are not useful. This function is only
    used below. Took them out; they gum up the pipeline.
    */
  }
  return e;
}
#endif /* 0 */


/* Abramowitz+Stegun, 7.1.5 */
static int erfseries(double x, gsl_sf_result * result)
{
  double coef = x;
  double e    = coef;
  double del;
  int k;
  for (k=1; k<30; ++k) {
    coef *= -x*x/k;
    del   = coef/(2.0*k+1.0);
    e += del;
  }
  result->val = 2.0 / M_SQRTPI * e;
  result->err = 2.0 / M_SQRTPI * (fabs(del) + GSL_DBL_EPSILON);
  return GSL_SUCCESS;
}


/* Chebyshev fit for erfc((t+1)/2), -1 < t < 1
 */
static double erfc_xlt1_data[20] = {
  1.06073416421769980345174155056,
 -0.42582445804381043569204735291,
  0.04955262679620434040357683080,
  0.00449293488768382749558001242,
 -0.00129194104658496953494224761,
 -0.00001836389292149396270416979,
  0.00002211114704099526291538556,
 -5.23337485234257134673693179020e-7,
 -2.78184788833537885382530989578e-7,
  1.41158092748813114560316684249e-8,
  2.72571296330561699984539141865e-9,
 -2.06343904872070629406401492476e-10,
 -2.14273991996785367924201401812e-11,
  2.22990255539358204580285098119e-12,
  1.36250074650698280575807934155e-13,
 -1.95144010922293091898995913038e-14,
 -6.85627169231704599442806370690e-16,
  1.44506492869699938239521607493e-16,
  2.45935306460536488037576200030e-18,
 -9.29599561220523396007359328540e-19
};
static gsl_sf_cheb_series erfc_xlt1_cs = {
  erfc_xlt1_data,
  19,
  -1, 1,
  (double *) 0,
  (double *) 0,
  12
};

/* Chebyshev fit for erfc(x) exp(x^2), 1 < x < 5, x = 2t + 3, -1 < t < 1
 */
static double erfc_x15_data[25] = {
  0.44045832024338111077637466616,
 -0.143958836762168335790826895326,
  0.044786499817939267247056666937,
 -0.013343124200271211203618353102,
  0.003824682739750469767692372556,
 -0.001058699227195126547306482530,
  0.000283859419210073742736310108,
 -0.000073906170662206760483959432,
  0.000018725312521489179015872934,
 -4.62530981164919445131297264430e-6,
  1.11558657244432857487884006422e-6,
 -2.63098662650834130067808832725e-7,
  6.07462122724551777372119408710e-8,
 -1.37460865539865444777251011793e-8,
  3.05157051905475145520096717210e-9,
 -6.65174789720310713757307724790e-10,
  1.42483346273207784489792999706e-10,
 -3.00141127395323902092018744545e-11,
  6.22171792645348091472914001250e-12,
 -1.26994639225668496876152836555e-12,
  2.55385883033257575402681845385e-13,
 -5.06258237507038698392265499770e-14,
  9.89705409478327321641264227110e-15,
 -1.90685978789192181051961024995e-15,
  3.50826648032737849245113757340e-16
};
static gsl_sf_cheb_series erfc_x15_cs = {
  erfc_x15_data,
  24,
  -1, 1,
  (double *) 0,
  (double *) 0,
  16
};

/* Chebyshev fit for erfc(x) x exp(x^2), 5 < x < 10, x = (5t + 15)/2, -1 < t < 1
 */
static double erfc_x510_data[20] = {
  1.11684990123545698684297865808,
  0.003736240359381998520654927536,
 -0.000916623948045470238763619870,
  0.000199094325044940833965078819,
 -0.000040276384918650072591781859,
  7.76515264697061049477127605790e-6,
 -1.44464794206689070402099225301e-6,
  2.61311930343463958393485241947e-7,
 -4.61833026634844152345304095560e-8,
  8.00253111512943601598732144340e-9,
 -1.36291114862793031395712122089e-9,
  2.28570483090160869607683087722e-10,
 -3.78022521563251805044056974560e-11,
  6.17253683874528285729910462130e-12,
 -9.96019290955316888445830597430e-13,
  1.58953143706980770269506726000e-13,
 -2.51045971047162509999527428316e-14,
  3.92607828989125810013581287560e-15,
 -6.07970619384160374392535453420e-16,
  9.12600607264794717315507477670e-17
};
static gsl_sf_cheb_series erfc_x510_cs = {
  erfc_x510_data,
  19,
  -1, 1,
  (double *) 0,
  (double *) 0,
  12
};

#if 0
inline
static double
erfc_asymptotic(double x)
{
  return exp(-x*x)/x * erfcasympsum(x) / M_SQRTPI;
}
inline
static double
log_erfc_asymptotic(double x)
{
  return log(erfcasympsum(x)/x) - x*x - LogRootPi_;
}
#endif /* 0 */


/*-*-*-*-*-*-*-*-*-*-*-* (semi)Private Implementations *-*-*-*-*-*-*-*-*-*-*-*/

int gsl_sf_erfc_impl(double x, gsl_sf_result * result)
{
  const double ax = fabs(x);
  double e_val, e_err;

  if(result == 0) {
    return GSL_EFAULT;
  }
  else if(ax <= 1.0) {
    double t = 2.0*ax - 1.0;
    gsl_sf_result c;
    gsl_sf_cheb_eval_impl(&erfc_xlt1_cs, t, &c);
    e_val = c.val;
    e_err = c.err;
  }
  else if(ax <= 5.0) {
    double ex2 = exp(-x*x);
    double t = 0.5*(ax-3.0);
    gsl_sf_result c;
    gsl_sf_cheb_eval_impl(&erfc_x15_cs, t, &c);
    e_val = ex2 * c.val;
    e_err = ex2 * (c.err + 2.0*fabs(x)*GSL_DBL_EPSILON);
  }
  else if(ax < 10.0) {
    double exterm = exp(-x*x) / x;
    double t = (2.0*x - 15.0)/5.0;
    gsl_sf_result c;
    gsl_sf_cheb_eval_impl(&erfc_x510_cs, t, &c);
    e_val = exterm * c.val;
    e_err = exterm * (c.err + 2.0*fabs(x)*GSL_DBL_EPSILON + GSL_DBL_EPSILON);
  }
  else {
    e_val = erfc8(ax);
    e_err = (x*x + 1.0) * GSL_DBL_EPSILON * fabs(e_val);
  }

  if(x < 0.0) {
    result->val  = 2.0 - e_val;
    result->err  = e_err;
    result->err += 2.0 * GSL_DBL_EPSILON * fabs(result->val);
  }
  else {
    result->val  = e_val;
    result->err  = e_err;
    result->err += 2.0 * GSL_DBL_EPSILON * fabs(result->val);
  }

  return GSL_SUCCESS;
}


int gsl_sf_log_erfc_impl(double x, gsl_sf_result * result)
{
  if(result == 0) {
    return GSL_EFAULT;
  }
  else if(x > 8.0) {
    result->val = log_erfc8(x);
    result->err = 2.0 * GSL_DBL_EPSILON * fabs(result->val);
    return GSL_SUCCESS;
  }
  else {
    gsl_sf_result result_erfc;
    gsl_sf_erfc_impl(x, &result_erfc);
    result->val  = log(result_erfc.val);
    result->err  = fabs(result_erfc.err / result_erfc.val);
    result->err += 2.0 * GSL_DBL_EPSILON * fabs(result->val);
    return GSL_SUCCESS;
  }
}


int gsl_sf_erf_impl(double x, gsl_sf_result * result)
{
  if(result == 0) {
    return GSL_EFAULT;
  }
  else if(fabs(x) < 1.0) {
    return erfseries(x, result);
  }
  else {
    gsl_sf_result result_erfc;
    gsl_sf_erfc_impl(x, &result_erfc);
    result->val  = 1.0 - result_erfc.val;
    result->err  = result_erfc.err;
    result->err += 2.0 * GSL_DBL_EPSILON * fabs(result->val);
    return GSL_SUCCESS;
  }
}


int gsl_sf_erf_Z_impl(double x, gsl_sf_result * result)
{
  if(result == 0) {
    return GSL_EFAULT;
  }
  else {
    const double ex2 = exp(-x*x/2.0);
    result->val  = ex2 / (M_SQRT2 * M_SQRTPI);
    result->err  = fabs(x * result->val) * GSL_DBL_EPSILON;
    result->err += 2.0 * GSL_DBL_EPSILON * fabs(result->val);
    if(result->val == 0.0) {
      return GSL_EUNDRFLW;
    }
    else {
      return GSL_SUCCESS;
    }
  }
}


int gsl_sf_erf_Q_impl(double x, gsl_sf_result * result)
{
  if(result == 0) {
    return GSL_EFAULT;
  }
  else {
    gsl_sf_result result_erfc;
    int stat = gsl_sf_erfc_impl(x/M_SQRT2, &result_erfc);
    result->val  = 0.5 * result_erfc.val;
    result->err  = 0.5 * result_erfc.err;
    result->err += 2.0 * GSL_DBL_EPSILON * fabs(result->val);
    return stat;
  }
}


/*-*-*-*-*-*-*-*-*-*-*-* Functions w/ Error Handling *-*-*-*-*-*-*-*-*-*-*-*/

int gsl_sf_erfc_e(double x, gsl_sf_result * result)
{
  int status = gsl_sf_erfc_impl(x, result);
  if(status != GSL_SUCCESS) {
    GSL_ERROR("gsl_sf_erfc_e", status);
  }
  return status;
}


int gsl_sf_log_erfc_e(double x, gsl_sf_result * result)
{
  int status = gsl_sf_log_erfc_impl(x, result);
  if(status != GSL_SUCCESS) {
    GSL_ERROR("gsl_sf_log_erfc_e", status);
  }
  return status;
}


int gsl_sf_erf_e(double x, gsl_sf_result * result)
{
  int status = gsl_sf_erf_impl(x, result);
  if(status != GSL_SUCCESS) {
    GSL_ERROR("gsl_sf_erf_e", status);
  }
  return status;
}


int gsl_sf_erf_Z_e(double x, gsl_sf_result * result)
{
  int status = gsl_sf_erf_Z_impl(x, result);
  if(status != GSL_SUCCESS) {
    GSL_ERROR("gsl_sf_erf_Z_e", status);
  }
  return status;
}


int gsl_sf_erf_Q_e(double x, gsl_sf_result * result)
{
  int status = gsl_sf_erf_Q_impl(x, result);
  if(status != GSL_SUCCESS) {
    GSL_ERROR("gsl_sf_erf_Q_e", status);
  }
  return status;
}