interp1Dcomp.c

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#include <stdlib.h>
#include <math.h>
#include "../ascot5.h"
#include "../math.h"
#include "interp.h"
#include "spline.h"
 
int interp1Dcomp_init_coeff(real* c, real* f, int n_x, int bc_x,
                            real x_min, real x_max) {
 
    /* Check boundary condition and calculate grid interval. Grid interval
       needed because we use normalized grid intervals. For periodic boundary
       condition, grid maximum value and the last data point are not the same.
       Take this into account in grid interval. */
    real x_grid;
    if(bc_x == PERIODICBC || bc_x == NATURALBC) {
        x_grid = (x_max - x_min) / ( n_x - 1 * (bc_x == NATURALBC) );
    }
    else {
        return 1;
    }
 
    if(c == NULL) {
        return 1;
    }
 
    /* Calculate cubic spline coefficients, i.e. second derivative. For each
       grid cell i_x, there are two coefficients: [f, fxx]. Note how we account
       for normalized grid. */
 
    /* Cubic spline along x, using f values to get fxx */
    splinecomp(f, n_x, bc_x, c);
    for(int i_x=0; i_x<n_x; i_x++) {
        /* Accounting for normalized grid. Affects fxx, but not f. */
        c[i_x*2 + 1] = c[i_x*2+1] / (x_grid*x_grid);
    }
 
    return 0;
}
 
void interp1Dcomp_init_spline(interp1D_data* str, real* c,
                              int n_x, int bc_x, real x_min, real x_max) {
 
    /* Calculate grid interval. For periodic boundary condition, grid maximum
       value and the last data point are not the same. Take this into account
       in grid interval. */
    real x_grid = (x_max - x_min) / ( n_x - 1 * (bc_x == NATURALBC) );
 
    /* Initialize the interp1D_data struct */
    str->n_x    = n_x;
    str->bc_x   = bc_x;
    str->x_min  = x_min;
    str->x_max  = x_max;
    str->x_grid = x_grid;
    str->c      = c;
}
 
a5err interp1Dcomp_eval_f(real* f, interp1D_data* str, real x) {
 
    /* Make sure periodic coordinates are within [min, max] region. */
    if(str->bc_x == PERIODICBC) {
        x = fmod(x - str->x_min, str->x_max - str->x_min) + str->x_min;
        x = x + (x < str->x_min) * (str->x_max - str->x_min);
    }
 
    /* Index for x variable. The -1 needed at exactly grid end. */
    int i_x   = (x-str->x_min) / str->x_grid - 1*(x==str->x_max);
    /* Normalized x coordinate in current cell */
    real dx   = ( x - (str->x_min + i_x*str->x_grid) ) / str->x_grid;
    /* Helper varibles */
    real dx3  =  dx * (dx*dx - 1.0);
    real dxi  = 1.0 - dx;
    real dxi3 = dxi * (dxi*dxi - 1.0);
    real xg2  = str->x_grid*str->x_grid;
 
    int n  = i_x*2; /* Index jump to cell       */
    int x1 = 2;     /* Index jump one x forward */
 
    int err = 0;
 
    /* Enforce periodic BC or check that the coordinate is within the grid. */
    if( str->bc_x == PERIODICBC && i_x == str->n_x-1 ) {
        x1 = -(str->n_x-1)*x1;
    }
    else if( str->bc_x == NATURALBC && !(x >= str->x_min && x <= str->x_max) ) {
        err = 1;
    }
 
    if(!err) {
        *f =
                      dxi *str->c[n+0]+dx *str->c[n+x1+0]
            +(xg2/6)*(dxi3*str->c[n+1]+dx3*str->c[n+x1+1]);
    }
 
    return err;
}
 
a5err interp1Dcomp_eval_df(real* f_df, interp1D_data* str, real x) {
 
    /* Make sure periodic coordinates are within [min, max] region. */
    if(str->bc_x == PERIODICBC) {
        x = fmod(x - str->x_min, str->x_max - str->x_min) + str->x_min;
        x = x + (x < str->x_min) * (str->x_max - str->x_min);
    }
 
    int i_x     = (x - str->x_min) / str->x_grid - 1*(x==str->x_max);
    real dx     = ( x - (str->x_min + i_x*str->x_grid) ) / str->x_grid;
    /* Helper varibles */
    real dx3    =  dx * (dx*dx - 1.0);
    real dx3dx  = 3*dx*dx - 1;
    real dxi    = 1.0 - dx;
    real dxi3   = dxi * (dxi*dxi - 1);
    real dxi3dx = -3*dxi*dxi + 1;
    real xg     = str->x_grid;
    real xg2    = xg*xg;
    real xgi    = 1.0 / xg;
 
    int n  = i_x*2; /* Index jump to cell */
    int x1 = 2;     /* Index jump one x forward */
 
    int err = 0;
 
    /* Enforce periodic BC or check that the coordinate is within the grid. */
    if( str->bc_x == PERIODICBC && i_x == str->n_x-1 ) {
        x1 = -(str->n_x-1)*x1;
    }
    else if( str->bc_x == NATURALBC && !(x >= str->x_min && x <= str->x_max) ) {
        err = 1;
    }
 
    if(!err) {
        /* f */
        f_df[0] =
                      dxi *str->c[n+0]+dx *str->c[n+x1+0]
            +(xg2/6)*(dxi3*str->c[n+1]+dx3*str->c[n+x1+1]);
 
        /* df/dx */
        f_df[1] =
                      xgi*(str->c[n+x1+0]-       str->c[n+0])
            +(xg/6)*(dx3dx*str->c[n+x1+1]+dxi3dx*str->c[n+1]);
 
        /* d2f/dx2 */
        f_df[2] = dxi*str->c[n+1]+dx*str->c[n+x1+1];
    }
 
    return err;
}