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See:
Description
Interface Summary | |
CompressionFlags | This interface provides constants to be used as compression flags for the
constructors of the IndexReader and IndexWriter classes. |
IndexIterator | An iterator over an inverted list. |
TermMap | A map from term to term indices. |
Class Summary | |
AbstractTermMap | An abstract implementation of a map from term to term indices. |
Index | An abstract representation of an index. |
IndexProperties | This class provides symbolic names for properties of an index. |
IndexReader | Provides facilities to read directly an inverted index. |
IndexWriter | Provides facilities to write an inverted index. |
SkipIndex | An abstract representation of an index with skips. |
SkipIndexProperties | This class provides symbolic names for properties of a SkipIndex . |
SkipIndexReader | Provides facilities to read a skip inverted index from an InputBitStream . |
SkipIndexWriter | Provides facilities to write skip inverted indices, that is, inverted indices with an additional skip structure. |
SkipIndexWriter.TowerData | A structure maintaining statistical data about tower construction. |
Index generation and access.
This package contains the classes that handle index generation and
access. The interval iterators defined in it.unimi.dsi.mg4j.search
build upon the classes of this package to provide answer to queries using interval semantics,
but it is also possible to access an index directly.
You can easily build indices using the tools in it.unimi.dsi.mg4j.tool
. Once an index
has been built, it can be opened using an Index
object, which
gathers metadata that is necessary to access the index. You do not create an Index
with a constructor: rather, you use the static factory Index.getInstance(CharSequence)
(or one of its variants) to create an instance.
This is necessary so that different kind of indices can be treated transparently: for example, the factory
may return a SkipIndex
if the index was created with skipping information,
but you do not need to know that.
From an Index
,
you can easily obtain either an IndexReader
, which allows to
scan sequentially or randomly the index. In turn from an IndexReader
you can obtain a IndexIterator
returning the documents containing a certain term and the position of the term within the document.
But there is more: an IndexIterator
is a kind of DocumentIterator
, and
DocumentIterator
s can be combined in several ways
using the classes of the package it.unimi.dsi.mg4j.search
: for instance, you can combine
document iterators using AND/OR. Note that you can combine document iterators on different
indices, but of course the operation is meaningful only if the two indices contain different information
about the same document collection (e.g., title and main text).
More importantly, if the index is full text (the default) for each document containing the term you can get interval iterators that return intervals representing extents of text satisfying the query: for instance, in case of an AND of two terms, the intervals will contain both terms.
An inverted index is made by a sequence of inverted lists (one inverted list for each term). Inverted lists are made by document records: each document record contains information about the occurrences of the term within a certain document.
More precisely, each inverted list starts with a suitably encoded integer, called the frequency, which is the number of document records that will follow (i.e., the number of documents in which the term appears). After that, there are exactly as many document records as the frequency.
Each document record is made by two parts:
As a basic and fundamental implementation, the classes of this package provide methods that write and read document data in a default form. In this default structure, each document data is a suitable coding of a (strictly increasing) sequence of integers, that correspond to the positions where the term occurs within the document. The length of the sequence (i.e., the number of positions in at which the term appears) is called the count (it is also common to call it "frequency", but we find this usage confusing).
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