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/* interpolation/steffen.c
*
* Copyright (C) 2014 Jean-François Caron
*
* 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.
*/
/* Author: J.-F. Caron
*
* This interpolation method is taken from
* M.Steffen, "A simple method for monotonic interpolation in one dimension",
* Astron. Astrophys. 239, 443-450 (1990).
*
* This interpolation method guarantees monotonic interpolation functions between
* the given data points. A consequence of this is that extremal values can only
* occur at the data points. The interpolating function and its first derivative
* are guaranteed to be continuous, but the second derivative is not.
*
* The implementation is modelled on the existing Akima interpolation method
* previously included in GSL by Gerard Jungman.
*/
#include <config.h>
#include <stdlib.h>
#include <math.h>
#include <gsl/gsl_math.h>
#include <gsl/gsl_errno.h>
#include "integ_eval.h"
#include <gsl/gsl_interp.h>
typedef struct
{
double * a; /* eqs 2-5 of paper */
double * b;
double * c;
double * d;
double * y_prime; /* eq 11 of paper */
} steffen_state_t;
static void steffen_free (void * vstate);
static double steffen_copysign(const double x, const double y);
static void *
steffen_alloc (size_t size)
{
steffen_state_t *state;
state = (steffen_state_t *) calloc (1, sizeof (steffen_state_t));
if (state == NULL)
{
GSL_ERROR_NULL("failed to allocate space for state", GSL_ENOMEM);
}
state->a = (double *) malloc (size * sizeof (double));
if (state->a == NULL)
{
steffen_free(state);
GSL_ERROR_NULL("failed to allocate space for a", GSL_ENOMEM);
}
state->b = (double *) malloc (size * sizeof (double));
if (state->b == NULL)
{
steffen_free(state);
GSL_ERROR_NULL("failed to allocate space for b", GSL_ENOMEM);
}
state->c = (double *) malloc (size * sizeof (double));
if (state->c == NULL)
{
steffen_free(state);
GSL_ERROR_NULL("failed to allocate space for c", GSL_ENOMEM);
}
state->d = (double *) malloc (size * sizeof (double));
if (state->d == NULL)
{
steffen_free(state);
GSL_ERROR_NULL("failed to allocate space for d", GSL_ENOMEM);
}
state->y_prime = (double *) malloc (size * sizeof (double));
if (state->y_prime == NULL)
{
steffen_free(state);
GSL_ERROR_NULL("failed to allocate space for y_prime", GSL_ENOMEM);
}
return state;
}
static int
steffen_init (void * vstate, const double x_array[],
const double y_array[], size_t size)
{
steffen_state_t *state = (steffen_state_t *) vstate;
size_t i;
double *a = state->a;
double *b = state->b;
double *c = state->c;
double *d = state->d;
double *y_prime = state->y_prime;
/*
* first assign the interval and slopes for the left boundary.
* We use the "simplest possibility" method described in the paper
* in section 2.2
*/
double h0 = (x_array[1] - x_array[0]);
double s0 = (y_array[1] - y_array[0]) / h0;
y_prime[0] = s0;
/* Now we calculate all the necessary s, h, p, and y' variables
from 1 to N-2 (0 to size - 2 inclusive) */
for (i = 1; i < (size - 1); i++)
{
double pi;
/* equation 6 in the paper */
double hi = (x_array[i+1] - x_array[i]);
double him1 = (x_array[i] - x_array[i - 1]);
/* equation 7 in the paper */
double si = (y_array[i+1] - y_array[i]) / hi;
double sim1 = (y_array[i] - y_array[i - 1]) / him1;
/* equation 8 in the paper */
pi = (sim1*hi + si*him1) / (him1 + hi);
/* This is a C equivalent of the FORTRAN statement below eqn 11 */
y_prime[i] = (steffen_copysign(1.0,sim1) + steffen_copysign(1.0,si)) *
GSL_MIN(fabs(sim1),
GSL_MIN(fabs(si), 0.5*fabs(pi)));
}
/*
* we also need y' for the rightmost boundary; we use the
* "simplest possibility" method described in the paper in
* section 2.2
*
* y' = s_{n-1}
*/
y_prime[size-1] = (y_array[size - 1] - y_array[size - 2]) /
(x_array[size - 1] - x_array[size - 2]);
/* Now we can calculate all the coefficients for the whole range. */
for (i = 0; i < (size - 1); i++)
{
double hi = (x_array[i+1] - x_array[i]);
double si = (y_array[i+1] - y_array[i]) / hi;
/* These are from equations 2-5 in the paper. */
a[i] = (y_prime[i] + y_prime[i+1] - 2*si) / hi / hi;
b[i] = (3*si - 2*y_prime[i] - y_prime[i+1]) / hi;
c[i] = y_prime[i];
d[i] = y_array[i];
}
return GSL_SUCCESS;
}
static void
steffen_free (void * vstate)
{
steffen_state_t *state = (steffen_state_t *) vstate;
RETURN_IF_NULL(state);
if (state->a)
free (state->a);
if (state->b)
free (state->b);
if (state->c)
free (state->c);
if (state->d)
free (state->d);
if (state->y_prime)
free (state->y_prime);
free (state);
}
static int
steffen_eval (const void * vstate,
const double x_array[], const double y_array[], size_t size,
double x, gsl_interp_accel * a, double *y)
{
const steffen_state_t *state = (const steffen_state_t *) vstate;
size_t index;
if (a != 0)
{
index = gsl_interp_accel_find (a, x_array, size, x);
}
else
{
index = gsl_interp_bsearch (x_array, x, 0, size - 1);
}
/* evaluate */
{
const double x_lo = x_array[index];
const double delx = x - x_lo;
const double a = state->a[index];
const double b = state->b[index];
const double c = state->c[index];
const double d = state->d[index];
/* Use Horner's scheme for efficient evaluation of polynomials. */
/* *y = a*delx*delx*delx + b*delx*delx + c*delx + d; */
*y = d + delx*(c + delx*(b + delx*a));
return GSL_SUCCESS;
}
}
static int
steffen_eval_deriv (const void * vstate,
const double x_array[], const double y_array[], size_t size,
double x, gsl_interp_accel * a, double *dydx)
{
const steffen_state_t *state = (const steffen_state_t *) vstate;
size_t index;
/* DISCARD_POINTER(y_array); /\* prevent warning about unused parameter *\/ */
if (a != 0)
{
index = gsl_interp_accel_find (a, x_array, size, x);
}
else
{
index = gsl_interp_bsearch (x_array, x, 0, size - 1);
}
/* evaluate */
{
double x_lo = x_array[index];
double delx = x - x_lo;
double a = state->a[index];
double b = state->b[index];
double c = state->c[index];
/*double d = state->d[index];*/
/* *dydx = 3*a*delx*delx*delx + 2*b*delx + c; */
*dydx = c + delx*(2*b + delx*3*a);
return GSL_SUCCESS;
}
}
static int
steffen_eval_deriv2 (const void * vstate,
const double x_array[], const double y_array[], size_t size,
double x, gsl_interp_accel * a, double *y_pp)
{
const steffen_state_t *state = (const steffen_state_t *) vstate;
size_t index;
/* DISCARD_POINTER(y_array); /\* prevent warning about unused parameter *\/ */
if (a != 0)
{
index = gsl_interp_accel_find (a, x_array, size, x);
}
else
{
index = gsl_interp_bsearch (x_array, x, 0, size - 1);
}
/* evaluate */
{
const double x_lo = x_array[index];
const double delx = x - x_lo;
const double a = state->a[index];
const double b = state->b[index];
*y_pp = 6*a*delx + 2*b;
return GSL_SUCCESS;
}
}
static int
steffen_eval_integ (const void * vstate,
const double x_array[], const double y_array[], size_t size,
gsl_interp_accel * acc, double a, double b,
double * result)
{
/* a and b are the boundaries of the integration. */
const steffen_state_t *state = (const steffen_state_t *) vstate;
size_t i, index_a, index_b;
/* Find the data points in the x_array that are nearest to the desired */
/* a and b integration boundaries. */
if (acc != 0)
{
index_a = gsl_interp_accel_find (acc, x_array, size, a);
index_b = gsl_interp_accel_find (acc, x_array, size, b);
}
else
{
index_a = gsl_interp_bsearch (x_array, a, 0, size - 1);
index_b = gsl_interp_bsearch (x_array, b, 0, size - 1);
}
*result = 0.0;
/* Iterate over all the segments between data points and sum the */
/* contributions into result. */
for(i=index_a; i<=index_b; i++)
{
const double x_hi = x_array[i + 1];
const double x_lo = x_array[i];
const double dx = x_hi - x_lo;
if(dx != 0.0)
{
/*
* check if we are at a boundary point, so take the
* a and b parameters instead of the data points.
*/
double x1 = (i == index_a) ? a-x_lo : 0.0;
double x2 = (i == index_b) ? b-x_lo : x_hi-x_lo;
*result += (1.0/4.0)*state->a[i]*(x2*x2*x2*x2 - x1*x1*x1*x1)
+(1.0/3.0)*state->b[i]*(x2*x2*x2 - x1*x1*x1)
+(1.0/2.0)*state->c[i]*(x2*x2 - x1*x1)
+state->d[i]*(x2-x1);
}
else /* if the interval was zero, i.e. consecutive x values in data. */
{
*result = 0.0;
return GSL_EINVAL;
}
}
return GSL_SUCCESS;
}
static double
steffen_copysign(const double x, const double y)
{
if ((x < 0 && y > 0) || (x > 0 && y < 0))
return -x;
return x;
}
static const gsl_interp_type steffen_type =
{
"steffen",
3,
&steffen_alloc,
&steffen_init,
&steffen_eval,
&steffen_eval_deriv,
&steffen_eval_deriv2,
&steffen_eval_integ,
&steffen_free
};
const gsl_interp_type * gsl_interp_steffen = &steffen_type;
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