/* integration/jacobi.c
 * 
 * Copyright (C) 2017 Konrad Griessinger, Patrick Alken
 * 
 * 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 3 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 Street, Fifth Floor, Boston, MA 02110-1301, USA.
 */

/*
 * The code in this module is based on IQPACK, specifically the LGPL
 * implementation found in HERMITE_RULE:
 * https://people.sc.fsu.edu/~jburkardt/c_src/hermite_rule/hermite_rule.html
 */

#include <stdio.h>
#include <gsl/gsl_errno.h>
#include <gsl/gsl_math.h>
#include <gsl/gsl_integration.h>
#include <gsl/gsl_sf_gamma.h>

static int
jacobi_check(const size_t n, const gsl_integration_fixed_params * params)
{
  (void) n;

  if (fabs(params->b - params->a) <= GSL_DBL_EPSILON)
    {
      GSL_ERROR("|b - a| too small", GSL_EDOM);
    }
  else if (params->a >= params->b)
    {
      GSL_ERROR("lower integration limit must be smaller than upper limit", GSL_EDOM);
    }
  else if (params->alpha <= -1.0 || params->beta <= -1.0)
    {
      GSL_ERROR("alpha and beta must be > -1", GSL_EDOM);
    }
  else
    {
      return GSL_SUCCESS;
    }
}

static int
jacobi_init(const size_t n, double * diag, double * subdiag, gsl_integration_fixed_params * params)
{
  const double absum = params->beta + params->alpha;
  const double abdiff = params->beta - params->alpha;
  const double a2b2 = absum * abdiff; /* beta^2 - alpha^2 */
  size_t i;

  /* construct the diagonal and subdiagonal elements of Jacobi matrix */
  diag[0] = abdiff/(absum + 2.0);
  subdiag[0] = 2.0*sqrt((params->alpha + 1.0)*(params->beta + 1.0)/(absum + 3.0))/(absum + 2.0);
  for (i = 1; i < n; i++)
    {
      diag[i] = a2b2 / ( (absum + 2.0*i) * (absum + 2.0*i + 2.0) );
      subdiag[i] = sqrt ( 4.0*(i + 1.0) * (params->alpha + i + 1.0) * (params->beta + i + 1.0) * (absum + i + 1.0) / ( pow((absum + 2.0*i + 2.0), 2.0) - 1.0 ) ) / ( absum + 2.0*i + 2.0 );
    }

  params->zemu = pow(2.0, absum + 1.0) * gsl_sf_gamma(params->alpha + 1.0) * gsl_sf_gamma(params->beta + 1.0) / gsl_sf_gamma(absum + 2.0);
  params->shft = 0.5*(params->b + params->a);
  params->slp = 0.5*(params->b - params->a);
  params->al = params->alpha;
  params->be = params->beta;

  return GSL_SUCCESS;
}

static const gsl_integration_fixed_type jacobi_type =
{
  jacobi_check,
  jacobi_init
};

const gsl_integration_fixed_type *gsl_integration_fixed_jacobi = &jacobi_type;