Unsigned integer underflow in met data interpolation if out of met data range

[lrobion]

In Util/MetFunction.cpp:358 the function linInterpMetData declares the index of the met data cell below the target interpolation point as an unsigned int std::size_t idx_x1;.

Later we can assign it i_Z - 1; which can be negative if i_Z the nearest cell to the target point is index 0. This means that the target point is outside the met data and we should throw an error, but because idx_x1 is unsigned it wraps to whatever is the max value for that type and the subsequent check never catches the out of bounds:

if(idx_x1 < 0) { 
    throw std::range_error("Input flight altitude out of range of met. data!"); 
}

I introduced this bug when I was cleaning compiler warnings on int to std::size_t conversions for vector indexing in #24. I don't think anyone encountered this because most people pass the weather column from the surface to a bit above cruise but this is incorrect.

[lrobion]

A fix would be to rewrite the function like this, only computing idx_x1 once we know that it is valid. We could also add some input validation somewhere (upstream in the code would be better) to verify that altitude_init.size() > 1. This should not happen in a normal user input but should still be verified.

double linInterpMetData(const Vector_1D& altitude_init, const Vector_1D& metVar_init, double altitude_query){
    const std::size_t i_Z = nearestNeighbor( altitude_init, altitude_query );

    //Edge cases
    const double epsilon = 1e-3;
    if((i_Z == 0 || i_Z == altitude_init.size() - 1) && std::abs(altitude_init[i_Z] - altitude_query) < epsilon) {
        return metVar_init[i_Z];
    }

    const bool is_ascending = altitude_init[0] < altitude_init[1];
    const bool useLowerNeighbor = is_ascending ? altitude_query <= altitude_init[i_Z]
                                            : altitude_query >  altitude_init[i_Z];

    /* No level below the nearest one: the query is outside the profile. */
    if(useLowerNeighbor && i_Z == 0) {
        throw std::range_error("Input flight altitude out of range of met. data!");
    }

    const std::size_t idx_x1 = useLowerNeighbor ? i_Z - 1 : i_Z;
    const std::size_t idx_x2 = idx_x1 + 1;
    if(idx_x2 >= altitude_init.size()) {
        throw std::range_error("Input flight altitude out of range of met. data!");
    }

    return met::linearInterp(altitude_init[idx_x1], metVar_init[idx_x1],
                             altitude_init[idx_x2], metVar_init[idx_x2],
                             altitude_query);
}

[lrobion]

There's a very similar issue in nearestNeighbor in the same file, but that one only triggers if a value in the query point is infinity std::numeric_limits<double>::max() or the met vector is of size 0. Both should be cleaned up too but I am not worried about these