# Lua bindings for Xapian

These bindings require Lua version 5.1 or later, and have been tested with Lua 5.1, 5.2 and 5.3.

The Lua bindings for Xapian are packaged in the `xapian` namespace, and largely follow the C++ API, with the following differences and additions.

The `examples` subdirectory contains examples showing how to use the Lua bindings based on the simple examples from `xapian-examples`: simpleindex.lua, simplesearch.lua, simpleexpand.lua.

There's also simplematchdecider.lua which shows how to define a MatchDecider in Lua.

# Unicode Support

In Xapian 1.0.0 and later, the Xapian::Stem, Xapian::QueryParser, and Xapian::TermGenerator classes all assume text is in UTF-8. A Lua string is an arbitrary sequence of values which have at least 8 bits (octets); they map directly into the char type of the C compiler. Lua does not reserve any value, including NUL. That means that Lua can store a UTF-8 string without problems.

# Method names

Most methods are named the same as in the C++ API - the exceptions are:

`end` is a keyword in Lua, so such methods are renamed to `_end` - e.g. in Lua you'd use `mset:_end()` to get an end iterator for an MSet object called mset.

The C++ method `get_description()` is mapped to the `tostring` function in Lua, so `tostring(x)` will return a string describing object x.

# Exceptions

Exceptions thrown by Xapian are translated into Lua xapian.Error objects and raised as Lua errors, which you can catch by using `pcall` like so:

    ok,res = pcall(db.get_document, db, docid)
    if ok then
       print("Got document data: " .. res:get_data())
    else
       print("Got exception: " .. tostring(res))
    end

# Iterators

All iterators support `next` and `equals` methods to move through and test iterators (as for all language bindings). MSetIterator and ESetIterator also support `prev`. As "end" is a keyword in Lua, we rename it to "_end" that means the end of the iterator. The following shows an example of iterating the MSet to get the rank, document id, and data for each entry in the MSet:

    for m in mset:items() do
       print(m:get_rank() + 1, m:get_docid(), m:get_document():get_data())
    end

# Iterator dereferencing

C++ iterators are often dereferenced to get information, eg `(*it)`. With Lua these are all mapped to named methods, as follows:

Iterator | Dereferencing method
--- | ---
PositionIterator | `get_termpos`
PostingIterator | `get_docid`
TermIterator | `get_term`
ValueIterator | `get_value`
MSetIterator | `get_docid`
ESetIterator | `get_term()`

Other methods, such as `MSetIterator:get_document`, are available under the same names.

# MSet

MSet objects have some additional methods to simplify access (these work using the C++ array dereferencing):

Method name | Explanation
--- | ---
`get_hit(index)` | returns MSetItem at index
`get_documentPercentage(index)` | `convert_to_percent(get_hit(index))`
`get_document(index)` | `get_hit(index):get_document()`
`get_docid(index)` | `get_hit(index):get_docid()`

The C++ API contains a few non-class functions (the Database factory functions, and some functions reporting version information), which are wrapped like so for Lua:

- `Xapian::version_string()` is wrapped as `xapian.version_string()`
- `Xapian::major_version()` is wrapped as `xapian.major_version()`
- `Xapian::minor_version()` is wrapped as `xapian.minor_version()`
- `Xapian::revision()` is wrapped as `xapian.revision()`
- `Xapian::Auto::open_stub()` is wrapped as `xapian.open_stub()` (but is now deprecated)
- `Xapian::Chert::open()` is wrapped as `xapian.chert_open()` (but is now deprecated)
- `Xapian::InMemory::open()` is wrapped as `xapian.inmemory_open()` (but is now deprecated)
- `Xapian::Remote::open()` is wrapped as `xapian.remote_open()` (both the TCP and "program" versions are wrapped - the SWIG wrapper checks the parameter list to decide which to call).
- `Xapian::Remote::open_writable()` is wrapped as `xapian.remote_open_writable()` (both the TCP and "program" versions are wrapped - the SWIG wrapper checks the parameter list to decide which to call).

# Constants

For Lua, constants are wrapped as `xapian.CONSTANT_NAME` or `xapian.ClassName_CONSTANT_NAME`. So `Xapian::DB_CREATE_OR_OPEN` is available as `xapian.DB_CREATE_OR_OPEN`, `Xapian::Query::OP_OR` is available as `xapian.Query_OP_OR`, and so on.

As of 1.3.2, you can also use the form `xapian.ClassName.CONSTANT_NAME`, e.g. `xapian.Query.OP_OR`.

# Query

In C++ there's a Xapian::Query constructor which takes a query operator and start/end iterators specifying a number of terms or queries, plus an optional parameter. In Lua, it is wrapped to accept Lua tables to give the terms/queries, and you can specify a mixture of terms and queries if you wish. For example:

    subq = xapian.Query(xapian.Query_OP_AND, {"hello", "world"})
    q = xapian.Query(xapian.Query_OP_AND, {subq, "foo", xapian.Query("bar", 2)})

# MatchAll and MatchNothing

These are wrapped for Lua as `xapian.Query_MatchAll` and `xapian.Query_MatchNothing`.

As of 1.3.2, you can also use the forms `xapian.Query.MatchAll` and `xapian.Query.MatchNothing`.

# Enquire

There is an additional method `get_matching_terms` which takes an MSetIterator and returns a list of terms in the current query which match the document given by that iterator. You may find this more convenient than using the TermIterator directly.

# MatchDecider

Custom MatchDeciders can be created in Lua in the form of lua function; simply function ensures you create a subclass of xapian.MatchDecider, which calls the super-constructor, and overloads the operator method to callback the lua function that will do the work. The simplest example (which does nothing useful) would be as follows:

    function mymatchdecider(doc)
       return 1
    end