package eu.crushedpixel.replaymod.interpolation;

import java.lang.reflect.Field;
import java.util.Collection;
import java.util.List;

public abstract class BasicSpline {
    public void calcNaturalCubic(List valueCollection, Field val, Collection<Cubic> cubicCollection) throws IllegalArgumentException, IllegalAccessException {
        int num = valueCollection.size() - 1;

        double[] gamma = new double[num + 1];
        double[] delta = new double[num + 1];
        double[] D = new double[num + 1];

        int i;
        /*
               We solve the equation
	          [2 1       ] [D[0]]   [3(x[1] - x[0])  ]
	          |1 4 1     | |D[1]|   |3(x[2] - x[0])  |
	          |  1 4 1   | | .  | = |      .         |
	          |    ..... | | .  |   |      .         |
	          |     1 4 1| | .  |   |3(x[n] - x[n-2])|
	          [       1 2] [D[n]]   [3(x[n] - x[n-1])]

	          by using row operations to convert the matrix to upper triangular
	          and then back sustitution.  The D[i] are the derivatives at the knots.
		 */
        gamma[0] = 1.0f / 2.0f;
        for(i = 1; i < num; i++) {
            gamma[i] = 1.0f / (4.0f - gamma[i - 1]);
        }
        gamma[num] = 1.0f / (2.0f - gamma[num - 1]);

        Double p0 = val.getDouble(valueCollection.get(0));
        Double p1 = val.getDouble(valueCollection.get(1));

        delta[0] = 3.0f * (p1 - p0) * gamma[0];
        for(i = 1; i < num; i++) {
            p0 = val.getDouble(valueCollection.get(i - 1));
            p1 = val.getDouble(valueCollection.get(i + 1));
            delta[i] = (3.0f * (p1 - p0) - delta[i - 1]) * gamma[i];
        }

        p0 = val.getDouble(valueCollection.get(num - 1));
        p1 = val.getDouble(valueCollection.get(num));

        delta[num] = (3.0f * (p1 - p0) - delta[num - 1]) * gamma[num];

        D[num] = delta[num];
        for(i = num - 1; i >= 0; i--) {
            D[i] = delta[i] - gamma[i] * D[i + 1];
        }

        //now compute the coefficients of the cubics
        cubicCollection.clear();

        for(i = 0; i < num; i++) {
            p0 = val.getDouble(valueCollection.get(i));
            p1 = val.getDouble(valueCollection.get(i + 1));

            cubicCollection.add(new Cubic(
                            p0,
                            D[i],
                            3 * (p1 - p0) - 2 * D[i] - D[i + 1],
                            2 * (p0 - p1) + D[i] + D[i + 1]
                    )
            );
        }
    }
}