Probing data at a given physical location

[drummerdoc]

Damian Coveney asks: What's the easiest way to extract the value of a Multifab variable at a global (x,y) co-ordinate point?

Weiqun responds:

Real value = std::numeric_limits<Real>::lowest();
for (int ilev = finestLevel(); ilev >= 0; ilev) {
    const Geometry& geom = Geom(ilev);
    IntVect iv{AMREX_D_DECL(static_cast<int>(std::floor((x-geom.ProbLo(0))*geom.InvCellSize(0)))+geom.Domain().smallEnd(0),
                            static_cast<int>(std::floor((y-geom.ProbLo(1))*geom.InvCellSize(1)))+geom.Domain().smallEnd(1),
                            static_cast<int>(std::floor((z-geom.ProbLo(2))*geom.InvCellSize(2)))+geom.Domain().smallEnd(2))};
    if (boxArray(ilev).contains(iv)) {
        const MultiFab& mf = ....; // data on this level                                                                                                                                                  
        for (MFIter mfi(mf); mfi.isValid(); ++mfi) {
            if (mfi.validbox().contains(iv)) {
                value = mf[mfi](iv,icomp); // icomp : component index                                                                                                                                      
            }
        }
        ParallelDescriptor::ReduceRealMax(value);
        break;
    }
}

@drummerdoc thought this would be a nice post here... :)