SplineFitter.java

package ij.measure;

public class SplineFitter {
    private double[] y2;

    public SplineFitter(int[] x, int[] y, int n) {
        initSpline(x, y, n);
    }

    void initSpline(int[] x, int[] y, int n) {
        int i,k;
        double p,qn,sig,un;
        y2 = new double[n];  // cached
        double[] u  = new double[n];
        for (i=1; i<n-1; i++) {
            // 888 chk for div by 0?
            sig = ((double) x[i]-x[i-1]) / ((double) x[i+1] - x[i-1]);
            p = sig * y2[i-1] + 2.0;
            y2[i] = (sig-1.0) / p;
            u[i] = (((double) y[i+1]-y[i]) / (x[i+1]-x[i])) -
                   (((double) y[i]-y[i-1]) / (x[i]-x[i-1]));
            u[i] = (6.0 * u[i]/ (x[i+1]-x[i-1]) - sig*u[i-1]) / p;
        }
        qn = un = 0.0;
        y2[n-1] = (un-qn*u[n-2]) / (qn*y2[n-2]+1.0);
        for (k=n-2; k>=0; k--)
            y2[k] = y2[k]*y2[k+1]+u[k];
    }

    public double evalSpline(int x[], int y[], int n, double xp) {
        int klo,khi,k;
        double h,b,a;
        klo = 0;
        khi = n-1;
        while (khi-klo > 1) {
            k = (khi+klo) >> 1;
            if (x[k] > xp) khi = k;
            else klo = k;
        }
        h = x[khi] - x[klo];
        /* orig code */
        /* if (h==0.0) FatalError("bad xvalues in splint\n"); */
        if (h==0.0) return (0.0);  /* arbitr ret for now */
        a = (x[khi]-xp)/h;
        b = (xp-x[klo])/h;

        // should have better err checking
        if(y2==null) return (0.0);

        return (a*y[klo] + b*y[khi] + ((a*a*a-a)*y2[klo] +(b*b*b-b)*y2[khi])
                * (h*h) / 6.0);
    }
}