1D Array resizing with Spline Functions using Apache Math - How to?

Question

I am looking for an example on resizing an 1D array using the Spline Functions with the Apache Commons - Math.

What I need is a method to expand and/or shrink the input array (double[]).

I could not find a good example trying to search online.

Answer 1

The trick here is that you need two arrays to create a spline but you only have one. Thus you need to fabricate an array. You can assume that the input array contains your y values and that the new fabricated array contains your x values so for any given x you have a corresponding y.

Disclaimer, I have not tested this code so make sure to adjust accordingly.

// To expand the array
public static double[] expand(double[] array, int newSize) {

    final int length = array.length;

    // let's calculate the new step size
    double step = (double) length / (newSize + 1);

    // fabricated array of x values
    double[] x = new double[length];
    for(int i = 0; i < length; ++i) {
        x[i] = i;
    }

    // using Linear interpolator but it can be any other interpolator
    LinearInterpolator li = new LinearInterpolator(); // or other interpolator
    PolynomialSplineFunction psf = li.interpolate(x, array);

    double[] expandedArray = new double[newSize];
    double xi = x[0];
    for (int i = 0; i < newSize - 1; ++i) {
       expandedArray[i] = psf.value(xi);
       xi += step;
    }
    expandedArray[newSize - 1] = array[length - 1];
    return expandedArray;
}

To shrink the array you can either decimate the input array i.e. just create a new smaller array and just take the values based on the new step size or use an interpolator as before:

// To shrink the array
public static double[] shrink(double[] array, int newSize) {

    final int length = array.length;

    // let's calculate the new step size
    double step = (double) length / (newSize - 1);

    // fabricated array of x values
    double[] x = new double[length];
    for(int i = 0; i < length; ++i) {
        x[i] = i;
    }

    // using Linear interpolator but it can be any other interpolator
    LinearInterpolator li = new LinearInterpolator(); // or other interpolator
    PolynomialSplineFunction psf = li.interpolate(x, array);

    double[] expandedArray = new double[newSize];
    double xi = x[0];
    for (int i = 0; i < newSize - 1; ++i) {
       expandedArray[i] = psf.value(xi);
       xi += step;
    }
    expandedArray[newSize - 1] = array[length - 1];
    return expandedArray;
}