In this chapter, we walk the reader through the different components of the SpatialDomains Directory. We begin with a discussion of the mathematical fundamentals, for which we use the book by Karniadakis and Sherwin  as our principle reference. We then provide the reader with an overview of the primary data structures introduced within the SpatialDomains Directory (often done through C++ objects), and then present the major algorithms – expressed as either object methods or functions – employed over these data structures.
The SpatialDomains Directory and its corresponding class definitions serve two principal purposes:
When designing Nektar++, developing a class hierarchy for StdRegions (those fundamental domains over which we define integration and differentiation) and LocalRegions (i.e. elements in world-space) was fairly straightforward following . For instance, a triangle in world-space is-a standard triangle. The first question that arose was where to store geometric information, as information within the LocalRegions element or as information encapsulated from the element so that multiple Expansions could all point to the same geometric information. The decision we made was to store geometric information – that is, the vertex information in world-space that defines an element and the edge and face curvature information – in its own data structure that could be shared by multiple Expansions (functions) over the same domain (element) in world-space. Hence SpatialDomains started as the directory containing Geometry and GeomFactors class definitions to meet the first item listed above. A LocalRegion is-a StdRegion and has-a SpatialDomain (i.e. Geometry and GeomFactors).
We then realized that in order to jump-start the process of constructing elements and combining them together into MultiRegions (collections of elements that represent a (sub)-domain of interest), we needed devise a light-weight data structure into which we could load geometric information from our geometry file and from which we could then construct Expansions (with their mappings, etc.). The light-weight data structure we devised was MeshGraph, and it was meant to meet the second item listed above.