Taxonomy of Categories
Facets are defined using a hierarchy of categories, known as a Taxonomy.
For example, the taxonomy of a book store application might have the following structure:
The Taxonomy translates category-paths into interger identifiers (often termed ordinals) and vice versa.
The category Author/Mark Twain
adds two nodes to the taxonomy: Author
and
Author/Mark Twain
, each is assigned a different ordinal. The taxonomy maintains the invariant that a
node always has an ordinal that is < all its children.