/* ode-initval2/rk1imp.c

 * 

 * Copyright (C) 2009, 2010 Tuomo Keskitalo

 * 

 * 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.

 */

/* Implicit Euler a.k.a backward Euler method. */

/* Reference: Ascher, U.M., Petzold, L.R., Computer methods for

   ordinary differential and differential-algebraic equations, SIAM,

   Philadelphia, 1998.

*/

#include <config.h>

#include <stdlib.h>

#include <string.h>

#include <gsl/gsl_math.h>

#include <gsl/gsl_errno.h>

#include <gsl/gsl_odeiv2.h>

#include "odeiv_util.h"

#include "rksubs.c"

#include "modnewton1.c"

/* Stage of method */

#define RK1IMP_STAGE 1

typedef struct

{

  gsl_matrix *A;                /* Runge-Kutta coefficients */

  double *y_onestep;            /* Result with one step */

  double *y_twostep;            /* Result with two half steps */

  double *ytmp;                 /* Temporary work space */

  double *y_save;               /* Backup space */

  double *YZ;                   /* Runge-Kutta points */

  double *fYZ;                  /* Derivatives at YZ */

  gsl_matrix *dfdy;             /* Jacobian matrix */

  double *dfdt;                 /* time derivative of f */

  modnewton1_state_t *esol;     /* nonlinear equation solver */

  double *errlev;               /* desired error level of y */

  const gsl_odeiv2_driver *driver;      /* pointer to driver object */

}

rk1imp_state_t;

static void *

rk1imp_alloc (size_t dim)

{

  rk1imp_state_t *state = (rk1imp_state_t *) malloc (sizeof (rk1imp_state_t));

  if (state == 0)

    {

      GSL_ERROR_NULL ("failed to allocate space for rk1imp_state",

                      GSL_ENOMEM);

    }

  state->A = gsl_matrix_alloc (RK1IMP_STAGE, RK1IMP_STAGE);

  if (state->A == 0)

    {

      free (state);

      GSL_ERROR_NULL ("failed to allocate space for A", GSL_ENOMEM);

    }

  state->y_onestep = (double *) malloc (dim * sizeof (double));

  if (state->y_onestep == 0)

    {

      gsl_matrix_free (state->A);

      free (state);

      GSL_ERROR_NULL ("failed to allocate space for y_onestep", GSL_ENOMEM);

    }

  state->y_twostep = (double *) malloc (dim * sizeof (double));

  if (state->y_twostep == 0)

    {

      free (state->y_onestep);

      gsl_matrix_free (state->A);

      free (state);

      GSL_ERROR_NULL ("failed to allocate space for y_onestep", GSL_ENOMEM);

    }

  state->ytmp = (double *) malloc (dim * sizeof (double));

  if (state->ytmp == 0)

    {

      free (state->y_twostep);

      free (state->y_onestep);

      gsl_matrix_free (state->A);

      free (state);

      GSL_ERROR_NULL ("failed to allocate space for ytmp", GSL_ENOMEM);

    }

  state->y_save = (double *) malloc (dim * sizeof (double));

  if (state->y_save == 0)

    {

      free (state->ytmp);

      free (state->y_twostep);

      free (state->y_onestep);

      gsl_matrix_free (state->A);

      free (state);

      GSL_ERROR_NULL ("failed to allocate space for y_save", GSL_ENOMEM);

    }

  state->YZ = (double *) malloc (dim * RK1IMP_STAGE * sizeof (double));

  if (state->YZ == 0)

    {

      free (state->y_save);

      free (state->ytmp);

      free (state->y_twostep);

      free (state->y_onestep);

      gsl_matrix_free (state->A);

      free (state);

      GSL_ERROR_NULL ("failed to allocate space for YZ", GSL_ENOMEM);

    }

  state->fYZ = (double *) malloc (dim * RK1IMP_STAGE * sizeof (double));

  if (state->fYZ == 0)

    {

      free (state->YZ);

      free (state->y_save);

      free (state->ytmp);

      free (state->y_twostep);

      free (state->y_onestep);

      gsl_matrix_free (state->A);

      free (state);

      GSL_ERROR_NULL ("failed to allocate space for fYZ", GSL_ENOMEM);

    }

  state->dfdt = (double *) malloc (dim * sizeof (double));

  if (state->dfdt == 0)

    {

      free (state->fYZ);

      free (state->YZ);

      free (state->y_save);

      free (state->ytmp);

      free (state->y_twostep);

      free (state->y_onestep);

      gsl_matrix_free (state->A);

      free (state);

      GSL_ERROR_NULL ("failed to allocate space for dfdt", GSL_ENOMEM);

    }

  state->dfdy = gsl_matrix_alloc (dim, dim);

  if (state->dfdy == 0)

    {

      free (state->dfdt);

      free (state->fYZ);

      free (state->YZ);

      free (state->y_save);

      free (state->ytmp);

      free (state->y_twostep);

      free (state->y_onestep);

      gsl_matrix_free (state->A);

      free (state);

      GSL_ERROR_NULL ("failed to allocate space for dfdy", GSL_ENOMEM);

    }

  state->esol = modnewton1_alloc (dim, RK1IMP_STAGE);

  if (state->esol == 0)

    {

      gsl_matrix_free (state->dfdy);

      free (state->dfdt);

      free (state->fYZ);

      free (state->YZ);

      free (state->y_save);

      free (state->ytmp);

      free (state->y_twostep);

      free (state->y_onestep);

      gsl_matrix_free (state->A);

      free (state);

      GSL_ERROR_NULL ("failed to allocate space for esol", GSL_ENOMEM);

    }

  state->errlev = (double *) malloc (dim * sizeof (double));

  if (state->errlev == 0)

    {

      modnewton1_free (state->esol);

      gsl_matrix_free (state->dfdy);

      free (state->dfdt);

      free (state->fYZ);

      free (state->YZ);

      free (state->y_save);

      free (state->ytmp);

      free (state->y_twostep);

      free (state->y_onestep);

      gsl_matrix_free (state->A);

      free (state);

      GSL_ERROR_NULL ("failed to allocate space for errlev", GSL_ENOMEM);

    }

  state->driver = NULL;

  return state;

}

static int

rk1imp_apply (void *vstate, size_t dim, double t, double h,

              double y[], double yerr[],

              const double dydt_in[], double dydt_out[],

              const gsl_odeiv2_system * sys)

{

  /* Makes an implicit Euler step with size h and estimates the local

     error of the step by step doubling.

   */

  rk1imp_state_t *state = (rk1imp_state_t *) vstate;

  double *const y_onestep = state->y_onestep;

  double *const y_twostep = state->y_twostep;

  double *const ytmp = state->ytmp;

  double *const y_save = state->y_save;

  double *const YZ = state->YZ;

  double *const fYZ = state->fYZ;

  gsl_matrix *const dfdy = state->dfdy;

  double *const dfdt = state->dfdt;

  double *const errlev = state->errlev;

  const modnewton1_state_t *esol = state->esol;

  /* Runge-Kutta coefficients */

  gsl_matrix *A = state->A;

  const double b[] = { 1.0 };

  const double c[] = { 1.0 };

  gsl_matrix_set (A, 0, 0, 1.0);

  if (esol == NULL)

    {

      GSL_ERROR ("no non-linear equation solver speficied", GSL_EINVAL);

    }

  /* Get desired error levels via gsl_odeiv2_control object through driver

     object, which is a requirement for this stepper.

   */

  if (state->driver == NULL)

    {

      return GSL_EFAULT;

    }

  else

    {

      size_t i;

      for (i = 0; i < dim; i++)

        {

          if (dydt_in != NULL)

            {

              gsl_odeiv2_control_errlevel (state->driver->c, y[i],

                                           dydt_in[i], h, i, &errlev[i]);

            }

          else

            {

              gsl_odeiv2_control_errlevel (state->driver->c, y[i],

                                           0.0, h, i, &errlev[i]);

            }

        }

    }

  /* Evaluate Jacobian for modnewton1 */

  {

    int s = GSL_ODEIV_JA_EVAL (sys, t, y, dfdy->data, dfdt);

    if (s != GSL_SUCCESS)

      {

        return s;

      }

  }

  /* Calculate a single step with size h */

  {

    int s = modnewton1_init ((void *) esol, A, h, dfdy, sys);

    if (s != GSL_SUCCESS)

      {

        return s;

      }

  }

  {

    int s = modnewton1_solve ((void *) esol, A, c, t, h, y,

                              sys, YZ, errlev);

    if (s != GSL_SUCCESS)

      {

        return s;

      }

  }

  {

    int s = GSL_ODEIV_FN_EVAL (sys, t + c[0] * h, YZ, fYZ);

    if (s != GSL_SUCCESS)

      {

        return s;

      }

  }

  {

    int s = rksubs (y_onestep, h, y, fYZ, b, RK1IMP_STAGE, dim);

    if (s != GSL_SUCCESS)

      return s;

  }

  /* Error estimation by step doubling */

  {

    int s = modnewton1_init ((void *) esol, A, h / 2.0, dfdy, sys);

    if (s != GSL_SUCCESS)

      {

        return s;

      }

  }

  /* 1st half step */

  {

    int s = modnewton1_solve ((void *) esol, A, c, t, h / 2.0, y,

                              sys, YZ, errlev);

    if (s != GSL_SUCCESS)

      {

        return s;

      }

  }

  {

    int s = GSL_ODEIV_FN_EVAL (sys, t + c[0] * h / 2.0, YZ, fYZ);

    if (s != GSL_SUCCESS)

      {

        return s;

      }

  }

  {

    int s = rksubs (ytmp, h / 2.0, y, fYZ, b, RK1IMP_STAGE, dim);

    if (s != GSL_SUCCESS)

      return s;

  }

  /* Save original y values in case of error */

  DBL_MEMCPY (y_save, y, dim);

  /* 2nd half step */

  {

    int s = modnewton1_solve ((void *) esol, A, c, t + h / 2.0, h / 2.0,

                              ytmp, sys, YZ, errlev);

    if (s != GSL_SUCCESS)

      {

        return s;

      }

  }

  {

    int s = GSL_ODEIV_FN_EVAL (sys, t + h / 2.0 + c[0] * h / 2.0, YZ, fYZ);

    if (s != GSL_SUCCESS)

      {

        return s;

      }

  }

  {

    /* Note: rk1imp returns y using the results from two half steps

       instead of the single step since the results are freely

       available and more precise.

     */

    int s = rksubs (y_twostep, h / 2.0, ytmp, fYZ, b, RK1IMP_STAGE, dim);

    if (s != GSL_SUCCESS)

      {

        DBL_MEMCPY (y, y_save, dim);

        return s;

      }

  }

  DBL_MEMCPY (y, y_twostep, dim);

  /* Error estimation */

  {

    size_t i;

    for (i = 0; i < dim; i++)

      {

        yerr[i] = ODEIV_ERR_SAFETY * 0.5 * fabs (y_twostep[i] - y_onestep[i]);

      }

  }

  /* Derivatives at output */

  if (dydt_out != NULL)

    {

      int s = GSL_ODEIV_FN_EVAL (sys, t + h, y, dydt_out);

      if (s != GSL_SUCCESS)

        {

          /* Restore original values */

          DBL_MEMCPY (y, y_save, dim);

          return s;

        }

    }

  return GSL_SUCCESS;

}

static int

rk1imp_set_driver (void *vstate, const gsl_odeiv2_driver * d)

{

  rk1imp_state_t *state = (rk1imp_state_t *) vstate;

  state->driver = d;

  return GSL_SUCCESS;

}

static int

rk1imp_reset (void *vstate, size_t dim)

{

  rk1imp_state_t *state = (rk1imp_state_t *) vstate;

  DBL_ZERO_MEMSET (state->y_onestep, dim);

  DBL_ZERO_MEMSET (state->y_twostep, dim);

  DBL_ZERO_MEMSET (state->ytmp, dim);

  DBL_ZERO_MEMSET (state->y_save, dim);

  DBL_ZERO_MEMSET (state->YZ, dim);

  DBL_ZERO_MEMSET (state->fYZ, dim);

  return GSL_SUCCESS;

}

static unsigned int

rk1imp_order (void *vstate)

{

  rk1imp_state_t *state = (rk1imp_state_t *) vstate;

  state = 0;                    /* prevent warnings about unused parameters */

  return 1;

}

static void

rk1imp_free (void *vstate)

{

  rk1imp_state_t *state = (rk1imp_state_t *) vstate;

  free (state->errlev);

  modnewton1_free (state->esol);

  gsl_matrix_free (state->dfdy);

  free (state->dfdt);

  free (state->fYZ);

  free (state->YZ);

  free (state->y_save);

  free (state->ytmp);

  free (state->y_twostep);

  free (state->y_onestep);

  gsl_matrix_free (state->A);

  free (state);

}

static const gsl_odeiv2_step_type rk1imp_type = {

  "rk1imp",                     /* name */

  1,                            /* can use dydt_in? */

  1,                            /* gives exact dydt_out? */

  &rk1imp_alloc,

  &rk1imp_apply,

  &rk1imp_set_driver,

  &rk1imp_reset,

  &rk1imp_order,

  &rk1imp_free

};

const gsl_odeiv2_step_type *gsl_odeiv2_step_rk1imp = &rk1imp_type;