/*!
 * \file
 * \brief Implementation of modified Bessel functions of third kind
 * \author Tony Ottosson
 *
 * -------------------------------------------------------------------------
 *
 * IT++ - C++ library of mathematical, signal processing, speech processing,
 *        and communications classes and functions
 *
 * Copyright (C) 1995-2009  (see AUTHORS file for a list of contributors)
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program 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 General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
 *
 * -------------------------------------------------------------------------
 *
 * This is slightly modified routine from the Cephes library:
 * http://www.netlib.org/cephes/
 */

#include <itpp/base/bessel/bessel_internal.h>
#include <itpp/base/itassert.h>


/*
 * Modified Bessel function, third kind, order zero
 *
 * doule x, y, k0();
 *
 * y = k0( x );
 *
 * DESCRIPTION:
 *
 * Returns modified Bessel function of the third kind
 * of order zero of the argument.
 *
 * The range is partitioned into the two intervals [0,8] and
 * (8, infinity).  Chebyshev polynomial expansions are employed
 * in each interval.
 *
 * ACCURACY:
 *
 * Tested at 2000 random points between 0 and 8.  Peak absolute
 * error (relative when K0 > 1) was 1.46e-14; rms, 4.26e-15.
 *                      Relative error:
 * arithmetic   domain     # trials      peak         rms
 *    IEEE      0, 30       30000       1.2e-15     1.6e-16
 *
 * ERROR MESSAGES:
 *
 *   message         condition      value returned
 *  K0 domain          x <= 0          MAXNUM
 */

/*
 * Modified Bessel function, third kind, order zero, exponentially scaled
 *
 * double x, y, k0e();
 *
 * y = k0e( x );
 *
 * DESCRIPTION:
 *
 * Returns exponentially scaled modified Bessel function
 * of the third kind of order zero of the argument.
 *
 * ACCURACY:
 *
 *                      Relative error:
 * arithmetic   domain     # trials      peak         rms
 *    IEEE      0, 30       30000       1.4e-15     1.4e-16
 * See k0().
 */

/*
  Cephes Math Library Release 2.8:  June, 2000
  Copyright 1984, 1987, 2000 by Stephen L. Moshier
*/


/* Chebyshev coefficients for K0(x) + log(x/2) I0(x)
 * in the interval [0,2].  The odd order coefficients are all
 * zero; only the even order coefficients are listed.
 *
 * lim(x->0){ K0(x) + log(x/2) I0(x) } = -EUL.
 */

static double A[] = {
  1.37446543561352307156E-16,
  4.25981614279661018399E-14,
  1.03496952576338420167E-11,
  1.90451637722020886025E-9,
  2.53479107902614945675E-7,
  2.28621210311945178607E-5,
  1.26461541144692592338E-3,
  3.59799365153615016266E-2,
  3.44289899924628486886E-1,
  -5.35327393233902768720E-1
};


/* Chebyshev coefficients for exp(x) sqrt(x) K0(x)
 * in the inverted interval [2,infinity].
 *
 * lim(x->inf){ exp(x) sqrt(x) K0(x) } = sqrt(pi/2).
 */

static double B[] = {
  5.30043377268626276149E-18,
  -1.64758043015242134646E-17,
  5.21039150503902756861E-17,
  -1.67823109680541210385E-16,
  5.51205597852431940784E-16,
  -1.84859337734377901440E-15,
  6.34007647740507060557E-15,
  -2.22751332699166985548E-14,
  8.03289077536357521100E-14,
  -2.98009692317273043925E-13,
  1.14034058820847496303E-12,
  -4.51459788337394416547E-12,
  1.85594911495471785253E-11,
  -7.95748924447710747776E-11,
  3.57739728140030116597E-10,
  -1.69753450938905987466E-9,
  8.57403401741422608519E-9,
  -4.66048989768794782956E-8,
  2.76681363944501510342E-7,
  -1.83175552271911948767E-6,
  1.39498137188764993662E-5,
  -1.28495495816278026384E-4,
  1.56988388573005337491E-3,
  -3.14481013119645005427E-2,
  2.44030308206595545468E0
};


#define MAXNUM 1.79769313486231570815E308    /* 2**1024*(1-MACHEP) */

double k0(double x)
{
  double y, z;

  if (x <= 0.0) {
    it_warning("besselk:: argument domain error");
    //mtherr( "k0", DOMAIN );
    return(MAXNUM);
  }

  if (x <= 2.0) {
    y = x * x - 2.0;
    y = chbevl(y, A, 10) - log(0.5 * x) * i0(x);
    return(y);
  }
  z = 8.0 / x - 2.0;
  y = exp(-x) * chbevl(z, B, 25) / sqrt(x);
  return(y);
}


double k0e(double x)
{
  double y;

  if (x <= 0.0) {
    it_warning("besselk:: argument domain error");
    //mtherr( "k0e", DOMAIN );
    return(MAXNUM);
  }

  if (x <= 2.0) {
    y = x * x - 2.0;
    y = chbevl(y, A, 10) - log(0.5 * x) * i0(x);
    return(y * exp(x));
  }

  y = chbevl(8.0 / x - 2.0, B, 25) / sqrt(x);
  return(y);
}
