Calculated terms during a query

Greetings, I would like to search for items based on 'calculated' terms. Specifically, say I am using Lucene to search a collection of tasks, with fields "start_date" and "end_date", among others. The question to solve is: "Find all tasks that took longer than 100 days". So the easy answer

Number of documents in an index with filter

Greetings, I would like to add the number of possible hits in my queries, for example, "found 18 hits out of a possible 245,000 documents". I am assuming that IndexReader.numDocs() is the best way to get this value. However, I would like to use a filter as part of the query. What is the most e