Lucene 4.6.0 facet API

Provides faceted indexing and search capabilities.

See:
Description

Packages

org.apache.lucene.facet	faceted search
org.apache.lucene.facet.associations	Allows associating arbitrary values with a category.
org.apache.lucene.facet.collections	Various optimized Collections implementations.
org.apache.lucene.facet.complements	Allows to cache the total counts of categories, so that during search which returns a large number of results (>60% of segment size), the complement set of matching documents is counted.
org.apache.lucene.facet.encoding	Offers various encoders and decoders for category ordinals.
org.apache.lucene.facet.index	Facets indexing code.
org.apache.lucene.facet.old	Old faceted search API, kept until complements, sampling and partitions are migrated to the new API.
org.apache.lucene.facet.params	Facets indexing and search parameters.
org.apache.lucene.facet.partitions	Category Partitions
org.apache.lucene.facet.range	Code to compute facets for numeric ranges.
org.apache.lucene.facet.sampling	Facets sampling.
org.apache.lucene.facet.search	Facets search code.
org.apache.lucene.facet.sortedset	Classes to perform faceting without a separate taxonomy index, using on SortedSetDocValuesField.
org.apache.lucene.facet.taxonomy	Taxonomy of Categories
org.apache.lucene.facet.taxonomy.directory	Taxonomy index implementation using on top of a Directory.
org.apache.lucene.facet.taxonomy.writercache	Improves indexing time by caching a map of CategoryPath to their Ordinal.
org.apache.lucene.facet.taxonomy.writercache.cl2o	Category->Ordinal caching implementation using an optimized data-structures
org.apache.lucene.facet.taxonomy.writercache.lru	An LRU cache implementation for the CategoryPath to Ordinal map
org.apache.lucene.facet.util	Various utilities for faceted search

Provides faceted indexing and search capabilities. Checkout this and this blog posts for some overview on the facets module as well as source code examples here.