# Xapian Categorisation Support

Table of contents

- Introduction
- How to make use of the categorisation functionality
  - Indexing
  - Searching
  - Restricting by category values

# Introduction

Xapian provides functionality which allows you to dynamically generate complete lists of category values which feature in matching documents. There are numerous potential uses this can be put to, but a common one is to offer the user the ability to narrow down their search by filtering it to only include documents with a particular value of a particular category. This is often referred to as `faceted search`.

In some applications, you may have many different categories (for example colour, price, width, height) but not always want to offer all of them for every search. If all the results are red, and none have width, it's not useful to offer to narrow the search by colour or width. Also, the user interface may not have room to include every category, so you may want to select the "best" few categories to show the user.

# How to make use of the categorisation functionality

## Indexing

When indexing a document, you need to add each category in a different number value slot.

## Searching

At search time, you need to pass a `Xapian::ValueCountMatchSpy` object for each category you want to look at to `Xapian::Enquire::add_matchspy()`, like so:

    Xapian::ValueCountMatchSpy spy0(0);
    Xapian::ValueCountMatchSpy spy1(1);
    Xapian::ValueCountMatchSpy spy3(3);

    Xapian::Enquire enq(db);
    enq.add_matchspy(spy0);
    enq.add_matchspy(spy1);
    enq.add_matchspy(spy3);

    enq.set_query(query);

    Xapian::MSet mset = enq.get_mset(0, 10, 10000);

The `10000` in the call to `get_mset()` tells Xapian to check at least 10000 documents, so the MatchSpies will be passed at least 10000 documents to tally category information from (unless fewer than 10000 documents match the query, in which case it will see all of them). Setting this higher will make the counts exact, but Xapian will have to do more work for most queries so searches will be slower.

The `spy` objects now contain the category information. You can find out how many documents they looked at by calling `spy0.get_total()`. (All the spies will have looked at the same number of documents.) You can read the values from, say, `spy0` like this:

    const map<string, size_t> & cat = spy0.get_values();
    map<string, size_t>::const_iterator i;
    for (i = cat.begin(); i != cat.end(); ++i) {
        cout << i->first << ": " << i->second << endl;
    }

## Restricting by category values

If you're using the categorisation to offer the user choices for narrowing down their search results, you then need to be able to apply a suitable filter.

For a single value, you could use `Xapian::Query::OP_VALUE_RANGE` with the same start and end, or `Xapian::MatchDecider`, but it's probably most efficient to also index the categories as suitably prefixed boolean terms and use those for filtering.