Your question is broad, and has a lot of potential answers... 1) Lucene has a very configurable Scoring, that allows a lot of customiztaion -- much of the scoring formula can be tweaked just by changing the "Similarity" class used, other more complex things can be achieved by writing your own Query classes
2) Solr allows for a *lot* of customization using "plugins" where just about any class you can imagine (including Similarity, custom RequestHandlers, and new Query clases) can be loaded from a JAR you provide at runtime... http://wiki.apache.org/solr/SolrPlugins 3) Solr has a special type of query called a FunctionQuery which makes writing special Query Scoring based on numeric Document Fields really easy ... some very complicated things can be done right out of the box using the Function Parsing supported by the SolrQueryParser... http://lucene.apache.org/solr/api/org/apache/solr/search/QueryParsing.html#parseFunction(java.lang.String,%20org.apache.solr.schema.IndexSchema) ...but more complicated things (like distance searching) would require you to write a simple ValueSource definining your equation, and using that ValueSource in a FunctionQuery you constructi na custom RequestHandler. using FunctionQuery has been discussed on several Lucene lists in the past, there have even been some fairly in depth discussion about using it for Geo based scoring... http://www.nabble.com/forum/Search.jtp?query=FunctionQuery+distance&local=y&forum=44 -Hoss
