# Value Ranges

Table of contents

- Introduction
- RangeProcessor
- DateRangeProcessor
- NumberRangeProcessor
- Custom subclasses
- Using Several RangeProcessors

# Introduction

The `Xapian::RangeProcessor` class was introduced in Xapian 1.3.6, and provides a powerful and flexible way to parse range queries in the users' query string. It's a replacement for the older `Xapian::ValueRangeProcessor` class (which dates back to Xapian 1.0.0).

This document describes the `Xapian::RangeProcessor` class and its standard subclasses, how to create your own subclasses, and how these classes are used with `Xapian::QueryParser`.

`Xapian::RangeProcessor` itself supports parsing string ranges, optionally only recognising ranges with a specified prefix or suffix. There are standard subclasses supporting ranges of dates (`Xapian::DateRangeProcessor`) and of numbers (`Xapian::NumberRangeProcessor`). User subclasses can support custom range types.

`Xapian::QueryParser` maintains a list of `Xapian::RangeProcessor` objects which it tries in order for each range specified in the query until one accepts it, or all have been tried (in which case an error is reported).

So you can support multiple filters distinguished by a prefix or suffix. For example, if you want to support range filters on price and weight, you can do that like this:

    Xapian::QueryParser qp;
    Xapian::NumberRangeProcessor price_proc(0, "$");
    Xapian::NumberRangeProcessor weight_proc(1, "kg", Xapian::RP_SUFFIX);
    qp.add_rangeprocessor(&price_proc);
    qp.add_rangeprocessor(&weight_proc);

Then the user can enter queries like:

    laptop $300..800 ..1.5kg

A common way to use this feature is with a prefix string which is a "field name" followed by a colon, for example:

    created:1/1/1999..1/1/2003

Each `Xapian::RangeProcessor` is passed the start and end of the range. If it doesn't understand the range, it should `Xapian::Query(Xapian::Query::OP_INVALID)`. If it does understand the range, it should return a query object matching the range (which will often use query operator `Xapian::Query::OP_VALUE_RANGE` but can be any query).

In Xapian 1.2.1 and later, `Xapian::QueryParser` supports open-ended ranges - if the start of the range is empty, that means any value less than the end, and similarly if the end is empty, that means any value greater than the start. The start and end can't both be empty.

# RangeProcessor

This understands any range passed which has the specified prefix or suffix. If no prefix or suffix is specified it will match any range (so it's not useful to specify further `RangeProcessor` objects after such an object as they can't match).

For example, suppose you have stored author names in value number 4, and want the user to be able to filter queries by specifying ranges of values such as:

    mars asimov..bradbury

To do this, you can use a `RangeProcessor` like so:

    Xapian::QueryParser qp;
    Xapian::RangeProcessor author_proc(4);
    qp.add_rangeprocessor(&author_proc);

The parsed query will use `OP_VALUE_RANGE`, so `query.get_description()` would report:

    Xapian::Query(mars:(pos=1) FILTER (VALUE_RANGE 4 asimov bradbury)

The `VALUE_RANGE` subquery will only match documents where value 4 is >= asimov and <= bradbury (using a string comparison).

# DateRangeProcessor

This class allows you to implement date range searches. As well as the value number to search, you can tell it whether to prefer US-style month/day/year or European-style day/month/year (by using the `Xapian::RP_DATE_PREFER_MDY` flag), and specify the epoch year to use for interpreting 2 digit years (the default is day/month/year with an epoch of 1970). The best choice of settings depends on the expectations of your users. As these settings are only applied at search time, you can also easily offer different versions of your search front-end with different settings if that is useful.

For example, if your users are American and the dates present in your database can extend a decade or so into the future, you might use something like this which specifies to prefer US-style dates and that the epoch year is 1930 (so 02/01/29 is February 1st 2029 while 02/01/30 is February 1st 1930):

    Xapian::QueryParser qp;
    Xapian::DateRangeProcessor date_proc(0, Xapian::RP_DATE_PREFER_MDY, 1930);
    qp.add_rangeprocessor(&date_proc);

The dates are converted to the format YYYYMMDD, so the values you index also need to also be in this format - for example, if `doc_time` is a `time_t`:

    char buf[9];
    if (strftime(buf, sizeof(buf), "%Y%m%d", gmtime(&doc_time))) {
        doc.add_value(0, buf);
    }

# NumberRangeProcessor

This class allows you to implement numeric range searches. The numbers used may be any number which is representable as a double, but requires that the stored values which the range is being applied have been converted to strings at index time using the `Xapian::sortable_serialise()` method:

    Xapian::Document doc;
    doc.add_value(0, Xapian::sortable_serialise(price));

This method produces strings which will sort in numeric order, so you can use it if you want to be able to sort based on the value in numeric order, too.

# Custom subclasses

You can easily create your own subclasses of `Xapian::RangeProcessor`. Your subclass needs to implement a method `Xapian::Query operator()(const std::string &begin, const std::string &end)` so for example you could implement a better version of the author range described above which only matches ranges with a prefix (e.g. `author:asimov..bradbury`) and lower-cases the names:

    struct AuthorRangeProcessor : public Xapian::RangeProcessor {
        AuthorRangeProcessor() : RangeProcessor(4, "author:") { }

        Xapian::valueno operator()(const std::string& b, const std::string& e) override {
            // Let the base class do the prefix check.
            return RangeProcessor::operator()(Xapian::Unicode::tolower(b),
                                              Xapian::Unicode::tolower(e));
        }
    };

If you want to support open-ended ranges, you need to handle begin or end being empty suitably. `Xapian::QueryParser` won't call your subclass with both begin and end being empty.

# Using Several RangeProcessors

If you want to allow the user to specify different types of ranges, you can specify multiple `RangeProcessor` objects to use. Just add them in the order you want them to be checked:

    Xapian::QueryParser qp;
    AuthorRangeProcessor author_proc();
    qp.add_rangeprocessor(&author_proc);
    Xapian::DateRangeProcessor date_proc(0, 0, 1930);
    qp.add_rangeprocessor(&date_proc);

And then you can parse queries such as `mars author:Asimov..Bradbury 01/01/1960..31/12/1969` successfully.