dsplint.c

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//
// Copyright (C) 2009 Alan W. Irwin
//
// This file is part of PLplot.
//
// PLplot is free software; you can redistribute it and/or modify
// it under the terms of the GNU Library General Public License as published
// by the Free Software Foundation; either version 2 of the License, or
// (at your option) any later version.
//
// PLplot 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 Library General Public License for more details.
//
// You should have received a copy of the GNU Library General Public License
// along with PLplot; if not, write to the Free Software
// Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
//
// Provenance: This code was originally developed under the GPL as part of
// the FreeEOS project (revision 121).  This code has been converted from
// Fortran to C with the aid of f2c and relicensed for PLplot under the LGPL
// with the permission of the FreeEOS copyright holder (Alan W. Irwin).
//

#include "dsplint.h"

# define MAX( a, b )    ( ( ( a ) > ( b ) ) ? ( a ) : ( b ) )
# define MIN( a, b )    ( ( ( a ) < ( b ) ) ? ( a ) : ( b ) )

//int dsplint(double *xa, double *ya, double *y2a,
//          int n, double x, double *y, double *dy, double *d2y)
int dsplint( double *xa, double *ya, double *y2a,
             int n, double x, double *y )
{
    // Initialized data

    static int nsave = 0, khi, klo;

    int        i__1, i__2, k;
    double     a, b, h__;

//      evaluate spline = y and its derivatives dy and d2y at x given
//      xa, ya, y2a from dspline.
    // Parameter adjustments
    --y2a;
    --ya;
    --xa;

    // Function Body
    if ( n != nsave )
    {
//        if call with different n value, then redo range
        nsave = n;
        klo   = 1;
        khi   = n;
        if ( xa[klo] > x )
        {
            return 1;
        }
        if ( xa[khi] < x )
        {
            return 2;
        }
    }
    else
    {
//        optimize range assuming continuous (ascending or
//        descending x calls.
        if ( xa[klo] > x )
        {
//          x is descending so try next range.
            khi = MAX( 2, klo );
            klo = khi - 1;
//          if x smaller than next range try lower limit.
            if ( xa[klo] > x )
            {
                klo = 1;
            }
            if ( xa[klo] > x )
            {
                return 1;
            }
        }
        else if ( xa[khi] <= x )
        {
//          x is ascending so try next range.
// Computing MIN
            i__1 = khi, i__2 = n - 1;
            klo  = MIN( i__1, i__2 );
            khi  = klo + 1;
//          if x larger than next range try upper limit.
            if ( xa[khi] <= x )
            {
                khi = n;
            }
            if ( xa[khi] < x )
            {
                return 2;
            }
        }
    }
    while ( khi - klo > 1 )
    {
        k = ( khi + klo ) / 2;
        if ( xa[k] > x )
        {
            khi = k;
        }
        else
        {
            klo = k;
        }
    }
    h__ = xa[khi] - xa[klo];
    if ( h__ <= 0. )
    {
        return 3;
    }
    a  = ( xa[khi] - x ) / h__;
    b  = ( x - xa[klo] ) / h__;
    *y = a * ya[klo] + b * ya[khi] + ( a * ( a * a - 1. ) * y2a[klo] + b * ( b *
                                                                             b - 1. ) * y2a[khi] ) * ( h__ * h__ ) / 6.;
//    *dy = (-ya[klo] + ya[khi] + (-(a * 3. * a - 1.) * y2a[klo] + (b * 3. * b
//          - 1.) * y2a[khi]) * (h__ * h__) / 6.) / h__;
//d2y = a * y2a[klo] + b * y2a[khi];
    return 0;
}