The R Project SVN R

/*
 *  R : A Computer Language for Statistical Data Analysis
 *  Copyright (C) 1995-1998   Robert Gentleman, Ross Ihaka and the
 *                            R Development Core Team
 *
 *  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., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 */

#ifdef HAVE_CONFIG_H
#include <Rconfig.h>
#endif

#include "Arith.h"
#include "Error.h"
#include "Applic.h"

/* Linear and Step Function Interpolation */
/* Assumes that ordinates are in ascending order */
/* The right interval is found by bisection */
/* Linear/constant interpolation then takes place on that interval*/

static double ylow;
static double yhigh;
static double f1;
static double f2;

static double approx1(double v, double *x, double *y, int n, int method)
{
    int i, j, ij;

    i = 0;
    j = n - 1;

    /* handle out-of-domain points */

    if(v < x[i]) return ylow;
    if(v > x[j]) return yhigh;

    /* find the correct interval by bisection */

    while(i < j - 1) {
    ij = (i + j)/2;
    if(v < x[ij]) j = ij;
    else i = ij;
    }

    /* interpolation */

    if(v == x[i]) return y[i];
    if(v == x[j]) return y[j];

    if(method == 1) {
    return (x[i] == x[j]) ?
        y[i] :
        y[i] + (y[j] - y[i]) * ((v - x[i])/(x[j] - x[i]));
    }
    else {
    return (x[i] == x[j]) ?
        y[i] :
        y[i] * f1 + y[j] * f2;
    }
}

    /* R Frontend for Linear and Constant Interpolation */

void approx(double *x, double *y, int *nxy, double *xout, int *nout,
        int *method, double *yleft, double *yright, double *f)
{
    int i;

    /* check interpolation method */

    switch(*method) {
    case 1:
    break;
    case 2:
    if(!R_FINITE(*f) || *f < 0 || *f > 1)
        error("approx(): invalid f value");
    f2 = *f;
    f1 = 1 - *f;
    break;
    default:
    error("approx(): invalid interpolation method");
    break;
    }

    for(i=0 ; i<*nxy ; i++)
    if(ISNA(x[i]) || ISNA(y[i]))
        error("approx(): attempted to interpolate NA values");

    ylow = *yleft;
    yhigh = *yright;

    for(i=0 ; i<*nout ; i++)
    if(!ISNA(xout[i]))
        xout[i] = approx1(xout[i], x, y, *nxy, *method);
}