**Bug report** Extrapolating a spline from `dumux/common/spline.hh` on the right side fails. **What happened / Problem description**: Spline extrapolation failed: ``` dumux/dumux/common/splinecommon_.hh:636: Dumux::SplineCommon_<ScalarT, ImplementationT>::Scalar Dumux::SplineCommon_<ScalarT, ImplementationT>::h_(int) const [with ScalarT = double; ImplementationT = Dumux::VariableLengthSpline_<double>; Dumux::SplineCommon_<ScalarT, ImplementationT>::Scalar = double]: Assertion `x_(i) > x_(i-1)' failed. ``` When calling `spline.eval(x, /*interpolate=*/true)`, ~~the derivative is evaluated as a forward difference, which doesn't work on the right end.~~ It looks like the segment index for interpolation on the right is just 1 to high. It is `numSamples_() - 1``, but should probably be `numSamples_() - 2``. **What you expected to happen**: I expected spline extrapolation to not fail. **How to reproduce it (as minimally and precisely as possible)**: ``` #include <vector> #include <dumux/common/spline.hh> using namespace Dumux; using namespace std; int main() { vector<double> x = {0.0, 0.2, 0.4, 0.6, 0.8, 1.0}; vector<double> y = {1.0, 1.0, 1.0, 1.0, 1.0, 1.0}; Spline<double, -1> spline; spline.setXYContainers(x, y); // interpolation cout << "interpolation: " << spline.eval(0.5) << endl; // returns 1 // lower end extrapolation cout << "lower end extrapolation: " << spline.eval(-0.1, true) << endl; // returns 1 // upper end extrapolation cout << "upper end extrapolation: " << spline.eval(1.1, true) << endl; // fails } ``` **Anything else we need to know?**: There seems to be another small bug in `eval` (splinecommon_.hh:141): ``` Scalar eval(Scalar x, bool extrapolate=false) const { assert(extrapolate || applies(x)); // handle extrapolation if (extrapolate) { if (x < xMin()) { Scalar m = evalDerivative(xMin(), /*segmentIdx=*/0); Scalar y0 = y_(0); return y0 + m*(x - xMin()); } else if (x > xMax()) { Scalar m = evalDerivative(xMax(), /*segmentIdx=*/numSamples_()-1); Scalar y0 = y_(numSamples_()-1); return y0 + m*(x - xMax()); } } return eval_(x, segmentIdx_(x)); } ``` I think the calls to `evalDerivative` should be to `evalDerivative_` (underscore at the end). **Environment**: - DuMux version: master