====================
Sorting and faceting
====================

.. note::
    The API for sorting and faceting changed in semlix 3.0.

Overview
========

Sorting and faceting search results in semlix is based on **facets**. Each
facet associates a value with each document in the search results, allowing you
to sort by the keys or use them to group the documents. semlix includes a variety
of **facet types** you can use for sorting and grouping (see below).


Sorting
=======

By default, the results of a search are sorted with the highest-scoring
documents first. You can use the ``sortedby`` keyword argument to order the
results by some other criteria instead, such as the value of a field.


Making fields sortable
----------------------

In order to sort on a field, you should create the field using the
``sortable=True`` keyword argument::

    schema = fields.Schema(title=fields.TEXT(sortable=True),
                           content=fields.TEXT,
                           modified=fields.DATETIME(sortable=True)
                           )

It's possible to sort on a field that doesn't have ``sortable=True``, but this
requires semlix to load the unique terms in the field into memory. Using
``sortable`` is much more efficient.


About column types
------------------

When you create a field using ``sortable=True``, you are telling semlix to store
per-document values for that field in a *column*. A column object specifies the
format to use to store the per-document values on disk.

The :mod:`semlix.columns` module contains several different column object
implementations. Each field type specifies a reasonable default column type (for
example, the default for text fields is :class:`semlix.columns.VarBytesColumn`,
the default for numeric fields is :class:`semlix.columns.NumericColumn`).
However, if you want maximum efficiency you may want to use a different column
type for a field.

For example, if all document values in a field are a fixed length, you can use a
:class:`semlix.columns.FixedBytesColumn`. If you have a field where many
documents share a relatively small number of possible values (an example might
be a "category" field, or "month" or other enumeration type fields), you might
want to use :class:`semlix.columns.RefBytesColumn` (which can handle both
variable and fixed-length values). There are column types for storing
per-document bit values, structs, pickled objects, and compressed byte values.

To specify a custom column object for a field, pass it as the ``sortable``
keyword argument instead of ``True``::

    from semlix import columns, fields

    category_col = columns.RefBytesColumn()
    schema = fields.Schema(title=fields.TEXT(sortable=True),
                           category=fields.KEYWORD(sortable=category_col)


Using a COLUMN field for custom sort keys
-----------------------------------------

When you add a document with a sortable field, semlix uses the value you pass
for the field as the sortable value. For example, if "title" is a sortable
field, and you add this document::

    writer.add_document(title="Mr. Palomar")

...then ``Mr. Palomar`` is stored in the field column as the sorting key for the
document.

This is usually good, but sometimes you need to "massage" the sortable key so
it's different from the value the user searches and/or sees in the interface.
For example, if you allow the user to sort by title, you might want to use
different values for the visible title and the value used for sorting::

    # Visible title
    title = "The Unbearable Lightness of Being"

    # Sortable title: converted to lowercase (to prevent different ordering
    # depending on uppercase/lowercase), with initial article moved to the end
    sort_title = "unbearable lightness of being, the"

The best way to do this is to use an additional field just for sorting. You can
use the :class:`semlix.fields.COLUMN` field type to create a field that is not
indexed or stored, it only holds per-document column values::

    schema = fields.Schema(title=fields.TEXT(stored=True),
                           sort_title=fields.COLUMN(columns.VarBytesColumn())
                           )

The single argument to the :class:`semlix.fields.COLUMN` initializer is a
:class:`semlix.columns.ColumnType` object. You can use any of the various
column types in the :mod:`semlix.columns` module.

As another example, say you are indexing documents that have a custom sorting
order associated with each document, such as a "priority" number::

    name=Big Wheel
    price=100
    priority=1

    name=Toss Across
    price=40
    priority=3

    name=Slinky
    price=25
    priority=2
    ...

You can use a column field with a numeric column object to hold the "priority"
and use it for sorting::

    schema = fields.Schema(name=fields.TEXT(stored=True),
                           price=fields.NUMERIC(stored=True),
                           priority=fields.COLUMN(columns.NumericColumn("i"),
                           )

(Note that :class:`columns.NumericColumn` takes a type code character like the
codes used by Python's ``struct`` and ``array`` modules.)


Making existing fields sortable
-------------------------------

If you have an existing index from before the ``sortable`` argument was added
in semlix 3.0, or you didn't think you needed a field to be sortable but now
you find that you need to sort it, you can add "sortability" to an existing
index using the :func:`semlix.sorting.add_sortable` utility function::

    from semlix import columns, fields, index, sorting

    # Say we have an existing index with this schema
    schema = fields.Schema(title=fields.TEXT,
                           price=fields.NUMERIC)

    # To use add_sortable, first open a writer for the index
    ix = index.open_dir("indexdir")
    with ix.writer() as w:
        # Add sortable=True to the "price" field using field terms as the
        # sortable values
        sorting.add_sortable(w, "price", sorting.FieldFacet("price"))

        # Add sortable=True to the "title" field using the
        # stored field values as the sortable value
        sorting.add_sortable(w, "title", sorting.StoredFieldFacet("title"))

You can specify a custom column type when you call ``add_sortable`` using the
``column`` keyword argument::

    add_sortable(w, "chapter", sorting.FieldFacet("chapter"),
                 column=columns.RefBytesColumn())

See the documentation for :func:`~semlix.sorting.add_sortable` for more
information.


Sorting search results
----------------------

When you tell semlix to sort by a field (or fields), it uses the per-document
values in the field's column as sorting keys for the documents.

Normally search results are sorted by descending relevance score. You can tell
semlix to use a different ordering by passing the ``sortedby`` keyword argument
to the :meth:`~semlix.searching.Searcher.search` method::

    from semlix import fields, index, qparser

    schema = fields.Schema(title=fields.TEXT(stored=True),
                           price=fields.NUMERIC(sortable=True))
    ix = index.create_in("indexdir", schema)

    with ix.writer() as w:
        w.add_document(title="Big Deal", price=20)
        w.add_document(title="Mr. Big", price=10)
        w.add_document(title="Big Top", price=15)

    with ix.searcher() as s:
        qp = qparser.QueryParser("big", ix.schema)
        q = qp.parse(user_query_string)

        # Sort search results from lowest to highest price
        results = s.search(q, sortedby="price")
        for hit in results:
            print(hit["title"])

You can use any of the following objects as ``sortedby`` values:

A ``FacetType`` object
    Uses this object to sort the documents. See below for the available facet
    types.

A field name string
    Converts the field name into a ``FieldFacet`` (see below) and uses it to
    sort the documents.

A list of ``FacetType`` objects and/or field name strings
    Bundles the facets together into a ``MultiFacet`` so you can sort by
    multiple keys. Note that this shortcut does not allow you to reverse
    the sort direction of individual facets. To do that, you need to construct
    the ``MultiFacet`` object yourself.

.. note::
    You can use the ``reverse=True`` keyword argument to the
    ``Searcher.search()`` method to reverse the overall sort direction. This
    is more efficient than reversing each individual facet.


Examples
--------

Sort by the value of the size field::

    results = searcher.search(myquery, sortedby="size")

Sort by the reverse (highest-to-lowest) order of the "price" field::

    facet = sorting.FieldFacet("price", reverse=True)
    results = searcher.search(myquery, sortedby=facet)

Sort by ascending size and then descending price::

    mf = sorting.MultiFacet()
    mf.add_field("size")
    mf.add_field("price", reverse=True)
    results = searcher.search(myquery, sortedby=mf)

    # or...
    sizes = sorting.FieldFacet("size")
    prices = sorting.FieldFacet("price", reverse=True)
    results = searcher.search(myquery, sortedby=[sizes, prices])

Sort by the "category" field, then by the document's score::

    cats = sorting.FieldFacet("category")
    scores = sorting.ScoreFacet()
    results = searcher.search(myquery, sortedby=[cats, scores])


Accessing column values
-----------------------

Per-document column values are available in :class:`~semlix.searching.Hit`
objects just like stored field values::

    schema = fields.Schema(title=fields.TEXT(stored=True),
                           price=fields.NUMERIC(sortable=True))

    ...

    results = searcher.search(myquery)
    for hit in results:
        print(hit["title"], hit["price"])

ADVANCED: if you want to access abitrary per-document values quickly you can get
a column reader object::

    with ix.searcher() as s:
        reader = s.reader()

        colreader = s.reader().column_reader("price")
        for docnum in reader.all_doc_ids():
            print(colreader[docnum])


Grouping
========

It is often very useful to present "faceted" search results to the user.
Faceting is dynamic grouping of search results into categories. The
categories let users view a slice of the total results based on the categories
they're interested in.

For example, if you are programming a shopping website, you might want to
display categories with the search results such as the manufacturers and price
ranges.

==================== =================
Manufacturer         Price
-------------------- -----------------
Apple (5)            $0 - $100 (2)
Sanyo (1)            $101 - $500 (10)
Sony (2)             $501 - $1000 (1)
Toshiba (5)
==================== =================

You can let your users click the different facet values to only show results
in the given categories.

Another useful UI pattern is to show, say, the top 5 results for different
types of found documents, and let the user click to see more results from a
category they're interested in, similarly to how the Spotlight quick results
work on Mac OS X.


The ``groupedby`` keyword argument
----------------------------------

You can use the following objects as ``groupedby`` values:

A ``FacetType`` object
    Uses this object to group the documents. See below for the available facet
    types.

A field name string
    Converts the field name into a ``FieldFacet`` (see below) and uses it to
    sort the documents. The name of the field is used as the facet name.

A list or tuple of field name strings
    Sets up multiple field grouping criteria.

A dictionary mapping facet names to ``FacetType`` objects
    Sets up multiple grouping criteria.

A ``Facets`` object
    This object is a lot like using a dictionary, but has some convenience
    methods to make setting up multiple groupings a little easier.


Examples
--------

Group by the value of the "category" field::

    results = searcher.search(myquery, groupedby="category")

Group by the value of the "category" field and also by the value of the "tags"
field and a date range::

    cats = sorting.FieldFacet("category")
    tags = sorting.FieldFacet("tags", allow_overlap=True)
    results = searcher.search(myquery, groupedby={"category": cats, "tags": tags})

    # ...or, using a Facets object has a little less duplication
    facets = sorting.Facets()
    facets.add_field("category")
    facets.add_field("tags", allow_overlap=True)
    results = searcher.search(myquery, groupedby=facets)

To group results by the *intersected values of multiple fields*, use a
``MultiFacet`` object (see below). For example, if you have two fields named
``tag`` and ``size``, you could group the results by all combinations of the
``tag`` and ``size`` field, such as ``('tag1', 'small')``,
``('tag2', 'small')``, ``('tag1', 'medium')``, and so on::

    # Generate a grouping from the combination of the "tag" and "size" fields
    mf = MultiFacet(["tag", "size"])
    results = searcher.search(myquery, groupedby={"tag/size": mf})


Getting the faceted groups
--------------------------

The ``Results.groups("facetname")`` method  returns a dictionary mapping
category names to lists of **document IDs**::

    myfacets = sorting.Facets().add_field("size").add_field("tag")
    results = mysearcher.search(myquery, groupedby=myfacets)
    results.groups("size")
    # {"small": [8, 5, 1, 2, 4], "medium": [3, 0, 6], "large": [7, 9]}

If there is only one facet, you can just use ``Results.groups()`` with no
argument to access its groups::

    results = mysearcher.search(myquery, groupedby=myfunctionfacet)
    results.groups()

By default, the values in the dictionary returned by ``groups()`` are lists of
document numbers in the same relative order as in the results. You can use the
``Searcher`` object's ``stored_fields()`` method to take a document number and
return the document's stored fields as a dictionary::

    for category_name in categories:
        print "Top 5 documents in the %s category" % category_name
        doclist = categories[category_name]
        for docnum, score in doclist[:5]:
            print "  ", searcher.stored_fields(docnum)
        if len(doclist) > 5:
            print "  (%s more)" % (len(doclist) - 5)

If you want different information about the groups, for example just the count
of documents in each group, or you don't need the groups to be ordered, you can
specify a :class:`semlix.sorting.FacetMap` type or instance with the
``maptype`` keyword argument when creating the ``FacetType``::

    # This is the same as the default
    myfacet = FieldFacet("size", maptype=sorting.OrderedList)
    results = mysearcher.search(myquery, groupedby=myfacet)
    results.groups()
    # {"small": [8, 5, 1, 2, 4], "medium": [3, 0, 6], "large": [7, 9]}

    # Don't sort the groups to match the order of documents in the results
    # (faster)
    myfacet = FieldFacet("size", maptype=sorting.UnorderedList)
    results = mysearcher.search(myquery, groupedby=myfacet)
    results.groups()
    # {"small": [1, 2, 4, 5, 8], "medium": [0, 3, 6], "large": [7, 9]}

    # Only count the documents in each group
    myfacet = FieldFacet("size", maptype=sorting.Count)
    results = mysearcher.search(myquery, groupedby=myfacet)
    results.groups()
    # {"small": 5, "medium": 3, "large": 2}

    # Only remember the "best" document in each group
    myfacet = FieldFacet("size", maptype=sorting.Best)
    results = mysearcher.search(myquery, groupedby=myfacet)
    results.groups()
    # {"small": 8, "medium": 3, "large": 7}

Alternatively you can specify a ``maptype`` argument in the
``Searcher.search()`` method call which applies to all facets::

    results = mysearcher.search(myquery, groupedby=["size", "tag"],
                                maptype=sorting.Count)

(You can override this overall ``maptype`` argument on individual facets by
specifying the ``maptype`` argument for them as well.)


Facet types
===========

FieldFacet
----------

This is the most common facet type. It sorts or groups based on the
value in a certain field in each document. This generally works best
(or at all) if each document has only one term in the field (e.g. an ID
field)::

    # Sort search results by the value of the "path" field
    facet = sorting.FieldFacet("path")
    results = searcher.search(myquery, sortedby=facet)

    # Group search results by the value of the "parent" field
    facet = sorting.FieldFacet("parent")
    results = searcher.search(myquery, groupedby=facet)
    parent_groups = results.groups("parent")

By default, ``FieldFacet`` only supports **non-overlapping** grouping, where a
document cannot belong to multiple facets at the same time (each document will
be sorted into one category arbitrarily.) To get overlapping groups with
multi-valued fields, use the ``allow_overlap=True`` keyword argument::

    facet = sorting.FieldFacet(fieldname, allow_overlap=True)

This supports overlapping group membership where documents have more than one
term in a field (e.g. KEYWORD fields). If you don't need overlapping, don't
use ``allow_overlap`` because it's *much* slower and uses more memory (see
the secion on ``allow_overlap`` below).


QueryFacet
----------

You can set up categories defined by arbitrary queries. For example, you can
group names using prefix queries::

    # Use queries to define each category
    # (Here I'll assume "price" is a NUMERIC field, so I'll use
    # NumericRange)
    qdict = {}
    qdict["A-D"] = query.TermRange("name", "a", "d")
    qdict["E-H"] = query.TermRange("name", "e", "h")
    qdict["I-L"] = query.TermRange("name", "i", "l")
    # ...

    qfacet = sorting.QueryFacet(qdict)
    r = searcher.search(myquery, groupedby={"firstltr": qfacet})

By default, ``QueryFacet`` only supports **non-overlapping** grouping, where a
document cannot belong to multiple facets at the same time (each document will
be sorted into one category arbitrarily). To get overlapping groups with
multi-valued fields, use the ``allow_overlap=True`` keyword argument::

    facet = sorting.QueryFacet(querydict, allow_overlap=True)


RangeFacet
----------

The ``RangeFacet`` is for NUMERIC field types. It divides a range of possible
values into groups. For example, to group documents based on price into
buckets $100 "wide"::

    pricefacet = sorting.RangeFacet("price", 0, 1000, 100)

The first argument is the name of the field. The next two arguments are the
full range to be divided. Value outside this range (in this example, values
below 0 and above 1000) will be sorted into the "missing" (None) group. The
fourth argument is the "gap size", the size of the divisions in the range.

The "gap" can be a list instead of a single value. In that case, the values in
the list will be used to set the size of the initial divisions, with the last
value in the list being the size for all subsequent divisions. For example::

    pricefacet = sorting.RangeFacet("price", 0, 1000, [5, 10, 35, 50])

...will set up divisions of 0-5, 5-15, 15-50, 50-100, and then use 50 as the
size for all subsequent divisions (i.e. 100-150, 150-200, and so on).

The ``hardend`` keyword argument controls whether the last division is clamped
to the end of the range or allowed to go past the end of the range. For
example, this::

    facet = sorting.RangeFacet("num", 0, 10, 4, hardend=False)

...gives divisions 0-4, 4-8, and 8-12, while this::

    facet = sorting.RangeFacet("num", 0, 10, 4, hardend=True)

...gives divisions 0-4, 4-8, and 8-10. (The default is ``hardend=False``.)

.. note::
    The ranges/buckets are always **inclusive** at the start and **exclusive**
    at the end.


DateRangeFacet
--------------

This is like ``RangeFacet`` but for DATETIME fields. The start and end values
must be ``datetime.datetime`` objects, and the gap(s) is/are
``datetime.timedelta`` objects.

For example::

    from datetime import datetime, timedelta

    start = datetime(2000, 1, 1)
    end = datetime.now()
    gap = timedelta(days=365)
    bdayfacet = sorting.DateRangeFacet("birthday", start, end, gap)

As with ``RangeFacet``, you can use a list of gaps and the ``hardend`` keyword
argument.


ScoreFacet
----------

This facet is sometimes useful for sorting.

For example, to sort by the "category" field, then for documents with the same
category, sort by the document's score::

    cats = sorting.FieldFacet("category")
    scores = sorting.ScoreFacet()
    results = searcher.search(myquery, sortedby=[cats, scores])

The ``ScoreFacet`` always sorts higher scores before lower scores.

.. note::
    While using ``sortedby=ScoreFacet()`` should give the same results as using
    the default scored ordering (``sortedby=None``), using the facet will be
    slower because semlix automatically turns off many optimizations when
    sorting.


FunctionFacet
-------------

This facet lets you pass a custom function to compute the sorting/grouping key
for documents. (Using this facet type may be easier than subclassing FacetType
and Categorizer to set up some custom behavior.)

The function will be called with the index searcher and index document ID as
arguments. For example, if you have an index with term vectors::

    schema = fields.Schema(id=fields.STORED,
                           text=fields.TEXT(stored=True, vector=True))
    ix = RamStorage().create_index(schema)

...you could use a function to sort documents higher the closer they are to
having equal occurances of two terms::

    def fn(searcher, docnum):
        v = dict(searcher.vector_as("frequency", docnum, "text"))
        # Sort documents that have equal number of "alfa" and "bravo" first
        return 0 - (1.0 / (abs(v.get("alfa", 0) - v.get("bravo", 0)) + 1.0))

    facet = sorting.FunctionFacet(fn)
    results = searcher.search(myquery, sortedby=facet)


StoredFieldFacet
----------------

This facet lets you use stored field values as the sorting/grouping key for
documents. This is usually slower than using an indexed field, but when using
``allow_overlap`` it can actually be faster for large indexes just because it
avoids the overhead of reading posting lists.

:class:`~semlix.sorting.StoredFieldFacet` supports ``allow_overlap`` by
splitting the stored value into separate keys. By default it calls the value's
``split()`` method (since most stored values are strings), but you can supply
a custom split function. See the section on ``allow_overlap`` below.


MultiFacet
==========

This facet type returns a composite of the keys returned by two or more
sub-facets, allowing you to sort/group by the intersected values of multiple
facets.

``MultiFacet`` has methods for adding facets::

    myfacet = sorting.RangeFacet(0, 1000, 10)

    mf = sorting.MultiFacet()
    mf.add_field("category")
    mf.add_field("price", reverse=True)
    mf.add_facet(myfacet)
    mf.add_score()

You can also pass a list of field names and/or ``FacetType`` objects to the
initializer::

    prices = sorting.FieldFacet("price", reverse=True)
    scores = sorting.ScoreFacet()
    mf = sorting.MultiFacet(["category", prices, myfacet, scores])


Missing values
==============

* When sorting, documents without any terms in a given field, or whatever else
  constitutes "missing" for different facet types, will always sort to the end.

* When grouping, "missing" documents will appear in a group with the
  key ``None``.


Using overlapping groups
========================

The common supported workflow for grouping and sorting is where the given field
has *one value for document*, for example a ``path`` field containing the file
path of the original document. By default, facets are set up to support this
single-value approach.

Of course, there are situations where you want documents to be sorted into
multiple groups based on a field with multiple terms per document. The most
common example would be a ``tags`` field. The ``allow_overlap`` keyword
argument to the :class:`~semlix.sorting.FieldFacet`,
:class:`~semlix.sorting.QueryFacet`, and
:class:`~semlix.sorting.StoredFieldFacet` allows this multi-value approach.

However, there is an important caveat: using ``allow_overlap=True`` is slower
than the default, potentially *much* slower for very large result sets. This is
because semlix must read every posting of every term in the field to
create a temporary "forward index" mapping documents to terms.

If a field is indexed with *term vectors*, ``FieldFacet`` will use them to
speed up ``allow_overlap`` faceting for small result sets, but for large result
sets, where semlix has to open the vector list for every matched document, this
can still be very slow.

For very large indexes and result sets, if a field is stored, you can get
faster overlapped faceting using :class:`~semlix.sorting.StoredFieldFacet`
instead of ``FieldFacet``. While reading stored values is usually slower than
using the index, in this case avoiding the overhead of opening large numbers of
posting readers can make it worthwhile.

``StoredFieldFacet`` supports ``allow_overlap`` by loading the stored value for
the given field and splitting it into multiple values. The default is to call
the value's ``split()`` method.

For example, if you've stored the ``tags`` field as a string like
``"tag1 tag2 tag3"``::

    schema = fields.Schema(name=fields.TEXT(stored=True),
                           tags=fields.KEYWORD(stored=True))
    ix = index.create_in("indexdir")
    with ix.writer() as w:
        w.add_document(name="A Midsummer Night's Dream", tags="comedy fairies")
        w.add_document(name="Hamlet", tags="tragedy denmark")
        # etc.

...Then you can use a ``StoredFieldFacet`` like this::

    ix = index.open_dir("indexdir")
    with ix.searcher() as s:
        sff = sorting.StoredFieldFacet("tags", allow_overlap=True)
        results = s.search(myquery, groupedby={"tags": sff})

For stored Python objects other than strings, you can supply a split function
(using the ``split_fn`` keyword argument to ``StoredFieldFacet``). The function
should accept a single argument (the stored value) and return a list or tuple
of grouping keys.


Using a custom sort order
=========================

It is sometimes useful to have a custom sort order per-search. For example,
different languages use different sort orders. If you have a function to return
the sorting order you want for a given field value, such as an implementation of
the Unicode Collation Algorithm (UCA), you can customize the sort order
for the user's language.

The :class:`semlix.sorting.TranslateFacet` lets you apply a function to the
value of another facet. This lets you "translate" a field value into an
arbitrary sort key, such as with UCA::

    from pyuca import Collator

    # The Collator object has a sort_key() method which takes a unicode
    # string and returns a sort key
    c = Collator("allkeys.txt")

    # Make a facet object for the field you want to sort on
    nf = sorting.FieldFacet("name")

    # Wrap the facet in a TranslateFacet with the translation function
    # (the Collator object's sort_key method)
    tf = sorting.TranslateFacet(facet, c.sort_key)

    # Use the facet to sort the search results
    results = searcher.search(myquery, sortedby=tf)

(You can pass multiple "wrapped" facets to the ``TranslateFacet``, and it will
call the function with the values of the facets as multiple arguments.)

The TranslateFacet can also be very useful with numeric fields to sort on the
output of some formula::

    # Sort based on the average of two numeric fields
    def average(a, b):
        return (a + b) / 2.0

    # Create two facets for the fields and pass them with the function to
    # TranslateFacet
    af = sorting.FieldFacet("age")
    wf = sorting.FieldFacet("weight")
    facet = sorting.TranslateFacet(average, af, wf)

    results = searcher.search(myquery. sortedby=facet)

Remember that you can still sort by multiple facets. For example, you could sort
by a numeric value transformed by a quantizing function first, and then if that
is equal sort by the value of another field::

    # Sort by a quantized size first, then by name
    tf = sorting.TranslateFacet(quantize, sorting.FieldFacet("size"))
    results = searcher.search(myquery, sortedby=[tf, "name"])


Expert: writing your own facet
==============================

TBD.


