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approx.c
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approx.c
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/*
* 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 <config.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 R_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);
}