HeaderSize describes the general offset in a segment until the data starts, it is composed of 2 bytes for level, 2 bytes for version, 2 bytes for secondary index count, 2 bytes for strategy, 8 bytes for the pointer to the index part.

DiskTree is a read-only wrapper around a marshalled index search tree, which can be used for reading, but cannot change the underlying structure.

Key is a helper struct that can be used to build the key nodes for the segment index.