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| <html> |
| <head> |
| <title>Taxonomy of Categories</title> |
| </head> |
| <body> |
| <h1>Taxonomy of Categories</h1> |
| |
| Facets are defined using a hierarchy of categories, known as a <i>Taxonomy</i>. |
| For example, the taxonomy of a book store application might have the following structure: |
| <ul> |
| <li>Author |
| <ul> |
| <li>Mark Twain</li> |
| <li>J. K. Rowling</li> |
| </ul> |
| </li> |
| </ul> |
| <ul> |
| <li>Date |
| <ul> |
| <li>2010 |
| <ul> |
| <li>March</li> |
| <li>April</li> |
| </ul> |
| </li> |
| <li>2009</li> |
| </ul> |
| </li> |
| </ul> |
| |
| The <i>Taxonomy</i> translates category-paths into interger identifiers (often termed <i>ordinals</i>) and vice versa. |
| The category <code>Author/Mark Twain</code> adds two nodes to the taxonomy: <code>Author</code> and |
| <code>Author/Mark Twain</code>, each is assigned a different ordinal. The taxonomy maintains the invariant that a |
| node always has an ordinal that is < all its children. |
| </body> |
| </html> |
