public void positivityClampedMultiTest() {
final double[] xValues = new double[] {1., 2., 3., 4., 5.};
final double[][] yValues =
new double[][] {{0., 0.1, 1., 1., 20., 5., 0.}, {-10., 0.1, 1., 1., 20., 5., 0.}};
PiecewisePolynomialInterpolator interp = new CubicSplineInterpolator();
PiecewisePolynomialResult result = interp.interpolate(xValues, yValues);
PiecewisePolynomialFunction1D function = new PiecewisePolynomialFunction1D();
PiecewisePolynomialInterpolator interpPos =
new NonnegativityPreservingCubicSplineInterpolator(interp);
PiecewisePolynomialResult resultPos = interpPos.interpolate(xValues, yValues);
assertEquals(resultPos.getDimensions(), result.getDimensions());
assertEquals(resultPos.getNumberOfIntervals(), result.getNumberOfIntervals());
assertEquals(resultPos.getOrder(), result.getOrder());
final int nPts = 101;
for (int i = 0; i < 101; ++i) {
final double key = 1. + 4. / (nPts - 1) * i;
assertTrue(function.evaluate(resultPos, key).get(0) >= 0.);
}
int dim = yValues.length;
int nData = xValues.length;
for (int j = 0; j < dim; ++j) {
for (int i = 1; i < nData - 2; ++i) {
DoubleMatrix coefMatrix = resultPos.getCoefMatrix();
double tau = Math.signum(coefMatrix.get(dim * i + j, 3));
assertTrue(
coefMatrix.get(dim * i + j, 2) * tau
>= -3. * yValues[j][i + 1] * tau / (xValues[i + 1] - xValues[i]));
assertTrue(
coefMatrix.get(dim * i + j, 2) * tau
<= 3. * yValues[j][i + 1] * tau / (xValues[i] - xValues[i - 1]));
}
}
}