text_indexing 0.17.0

Dart library for creating an inverted index on a collection of text documents.

Create an inverted index on a collection of text documents.

THIS PACKAGE IS PRE-RELEASE AND SUBJECT TO DAILY BREAKING CHANGES.

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• Overview
• Usage
• API
• Definitions
• References
• Issues

Overview

This library provides interfaces and implementation classes that build and maintain a (positional, zoned) inverted index for a collection of documents or corpus (see definitions).

The indexer uses a tokenizer to construct three artifacts:

• the dictionary holds the vocabulary of terms and the frequency of occurrence for each term in the corpus;
• the k-gram index maps k-grams to terms in the dictionary; and
• the postings index holds a list of references to the documents for each term (postings list). The postings list includes the positions of the term in the document's zones (fields), making the postings a positional, zoned inverted index.

Refer to the references to learn more about information retrieval systems and the theory behind this library.

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Performance

A sample data set consisting of stock data for the U.S. markets was used to benchmark performance of TextIndexer and InvertedIndex implementations. The data set contains 20,770 JSON documents with basic information on each stock and the JSON data file is 22MB in size.

For the benchmarking tests we created an implementation InvertedIndex class that uses Hive as local storage and benchmarked that against InMemoryIndex. Both indexes were given the same phrase length (2), k-gram length (2) and zones ('name', 'symbol', 'ticker', 'description', 'hashTag').

Benchmarking was performed as part of unit tests in the VS Code IDE on a Windows 10 workstation with an Intel(R) Core(TM) i9-7900X CPU running at 3.30GHz and 64GB of DDR4-2666 RAM.

Indexing the corpus

The typical times taken by TextIndexer to index our sample dataset to 243,700 terms and 18,276 k-grams using InMemoryIndex vs the Hive-based index is shown below.

InvertedIndex	Elapsed time	Per document
InMemoryIndex	~15 seconds	0.68 mS
Hive-based Index	~41 minutes	112 mS

Building the index while holding all the index artifacts in memory is 165 times faster than placing them in a Hive box (a relatively fast local storage option).

If memory and the size of the corpus allows, InMemoryIndex is a clear winner. The memory required for the postings, in particular, may not make this practical for larger document collections. The AsyncCallbackIndex class provides the flexibility to access each of the three index hashmaps from a different data source, so implementing applications can, for example, hold the dictionary and k-gram indexes in memory, with the postings in local storage. Alternatively in-memory caching may also provide performance improvements for a corpus with many repeated terms.

Regardless of the InvertedIndex implementation, applications should avoid running the TextIndexer in the same thread as the user interface to avoid UI "jank".

The dictionary, k-gram index and postings are 8MB, 41MB and 362MB in size, respectively, for our sample index of 243,700 terms and 18,276 k-grams.

Querying the indexes

Having created a persisted index on our sample data set, we ran a query on a search phrase of 9 terms we know are present in the sample data. The query requires a few round trips to each of the three indexes to match the terms, calculate the inverse document term frequencies etc. The elapsed time for retrieving the data from the InMemoryIndex vs the Hive-based index is shown below.

InvertedIndex	Elapsed time
InMemoryIndex	~22 mS
Hive-based Index	~205 mS

As expected, the InMemoryIndex is quicker than the Hive-based index, but the differences are unlikely to be material in a real-world application, even for predictive text or auto-correct applications.

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Usage

In the pubspec.yaml of your flutter project, add the text_indexing dependency.

dependencies:
  text_indexing: <latest version>

In your code file add the text_indexing import.

// import the core classes
import 'package:text_indexing/text_indexing.dart'; 

// import the typedefs, if needed
import 'package:text_indexing/type_definitions.dart'; 

// import the extensions, if needed
import 'package:text_indexing/extensions.dart'; 

For small collections, instantiate a TextIndexer with a InMemoryIndex, (optionally passing empty Dictionary and Postings hashmaps).

Call the TextIndexer.indexCollection method to a a collection of documents to the index.

    // initialize an in=memory index for a JSON collection with two indexed fields
    final myIndex = InMemoryIndex(zones: {'name': 1.0, 'description': 0.5}, phraseLength: 2);

    // - initialize a `TextIndexer`, passing in the index
    final indexer =TextIndexer(index: myIndex);

    // - index the json collection `documents`
    await indexer.indexCollection(documents);
  

The examples demonstrate the use of the TextIndexer with a InMemoryIndex and AsyncCallbackIndex.

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API

The API exposes the TextIndexer interface that builds and maintains an InvertedIndex for a collection of documents.

To maximise performance of the indexers the API performs lookups in nested hashmaps of DART core types.

The API contains a fair amount of boiler-plate, but we aim to make the code as readable, extendable and re-usable as possible:

• We use an interface > implementation mixin > base-class > implementation class pattern:
  • the interface is an abstract class that exposes fields and methods but contains no implementation code. The interface may expose a factory constructor that returns an implementation class instance;
  • the implementation mixin implements the interface class methods, but not the input fields;
  • the base-class is an abstract class with the implementation mixin and exposes a default, unnamed generative const constructor for sub-classes. The intention is that implementation classes extend the base class, overriding the interface input fields with final properties passed in via a const generative constructor; and
  • the class naming convention for this pattern is "Interface" > "InterfaceMixin" > "InterfaceBase".
• To improve code legibility the API makes use of type aliases, callback function definitions and extensions. The typedefs and extensions are not exported by the text_indexing library, but can be found in the type_definitions and extensions mini-libraries. Import these libraries seperately if needed.

InvertedIndex

The InvertedIndex interface exposes properties and methods for working with Dictionary, KGramIndex and Postings hashmaps.

A mixin class implements the getTfIndex, getFtdPostings and getIdFtIndex methods.

An abstract base class mixies in [InvertedIndexMixin] https://pub.dev/documentation/text_indexing/latest/text_indexing/InvertedIndexMixin-class.html) and provides a default unnamed generative const constructor for sub-classes.

Two implementation classes are provided:

• the InMemoryIndex class is intended for fast indexing of a smaller corpus using in-memory dictionary, k-gram and postings hashmaps; and
• the AsyncCallbackIndex is intended for working with a larger corpus. It uses asynchronous callbacks to perform read and write operations on dictionary, k-gram and postings repositories.

Phrase length

InvertedIndex.phraseLength is the maximum length of phrases in the index vocabulary.

The minimum phraseLength is 1. If phrase length is greater than 1, the index also contains phrases up to phraseLength words long, concatenated from consecutive terms. The index size is increased by a factor of phraseLength. The phraseLength default is 1 for InMemoryIndex and AsyncCallbackIndex.

Zones

InvertedIndex.zones is a hashmap of zone names to their relative weight in the index.

If zones is empty, all the text fields of the collection will be indexed, which may increase the size of the index significantly.

K-gram length (k)

InvertedIndex.k is the length of k-gram entries in the k-gram index.

The preferred k-gram length is 3, or a tri-gram. This results in a good compromise between the length of the k-gram index and search efficiency.

TextIndexer

TextIndexer is an interface for classes that construct and maintain a InvertedIndex.

Text or documents can be indexed by calling the following methods:

• indexText indexes text;
• indexJson indexes the fields in a JSON document; and
• indexCollection indexes the fields of all the documents in a JSON document collection.

Use the unnamed factory constructor to instantiate a TextIndexer with the index of your choice, or extend TextIndexerBase.

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Definitions

The following definitions are used throughout the documentation:

• corpus- the collection of documents for which an index is maintained.
• character filter - filters characters from text in preparation of tokenization.
• dictionary - is a hash of terms (vocabulary) to the frequency of occurence in the corpus documents.
• document - a record in the corpus, that has a unique identifier (docId) in the corpus's primary key and that contains one or more text fields that are indexed.
• document frequency (dFt) is number of documents in the corpus that contain a term.

• Flesch reading ease score - a readibility measure calculated from sentence length and word length on a 100-point scale. The higher the score, the easier it is to understand the document (Wikipedia(6)).
• Flesch-Kincaid grade level - a readibility measure relative to U.S. school grade level. It is also calculated from sentence length and word length (Wikipedia(6)).

• index - an inverted index used to look up document references from the corpus against a vocabulary of terms.
• index-elimination - selecting a subset of the entries in an index where the term is in the collection of terms in a search phrase.
• inverse document frequency or iDft is equal to log (N / dft), where N is the total number of terms in the index. The IdFt of a rare term is high, whereas the [IdFt] of a frequent term is likely to be low.
• Jaccard index measures similarity between finite sample sets, and is defined as the size of the intersection divided by the size of the union of the sample sets (from Wikipedia).
• JSON is an acronym for "Java Script Object Notation", a common format for persisting data.
• k-gram - a sequence of (any) k consecutive characters from a term. A k-gram can start with "$", denoting the start of the term, and end with "$", denoting the end of the term. The 3-grams for "castle" are { $ca, cas, ast, stl, tle, le$ }.
• lemmatizer - lemmatisation (or lemmatization) in linguistics is the process of grouping together the inflected forms of a word so they can be analysed as a single item, identified by the word's lemma, or dictionary form (from Wikipedia).
• postings - a separate index that records which documents the vocabulary occurs in. In a positional index, the postings also records the positions of each term in the text to create a positional inverted index.
• postings list - a record of the positions of a term in a document. A position of a term refers to the index of the term in an array that contains all the terms in the text. In a zoned index, the postings lists records the positions of each term in the text a zone.
• term - a word or phrase that is indexed from the corpus. The term may differ from the actual word used in the corpus depending on the tokenizer used.
• term filter - filters unwanted terms from a collection of terms (e.g. stopwords), breaks compound terms into separate terms and / or manipulates terms by invoking a stemmer and / or lemmatizer.
• stemmer - stemming is the process of reducing inflected (or sometimes derived) words to their word stem, base or root form—generally a written word form (from Wikipedia).
• stopwords - common words in a language that are excluded from indexing.
• term frequency (Ft) is the frequency of a term in an index or indexed object.
• term position is the zero-based index of a term in an ordered array of terms tokenized from the corpus.
• text - the indexable content of a document.
• token - representation of a term in a text source returned by a tokenizer. The token may include information about the term such as its position(s) (term position) in the text or frequency of occurrence (term frequency).
• token filter - returns a subset of tokens from the tokenizer output.
• tokenizer - a function that returns a collection of tokens from text, after applying a character filter, term filter, stemmer and / or lemmatizer.
• vocabulary - the collection of terms indexed from the corpus.
• zone is the field or zone of a document that a term occurs in, used for parametric indexes or where scoring and ranking of search results attribute a higher score to documents that contain a term in a specific zone (e.g. the title rather that the body of a document).

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References

• Manning, Raghavan and Schütze, "Introduction to Information Retrieval", Cambridge University Press, 2008
• University of Cambridge, 2016 "Information Retrieval", course notes, Dr Ronan Cummins, 2016
• Wikipedia (1), "Inverted Index", from Wikipedia, the free encyclopedia
• Wikipedia (2), "Lemmatisation", from Wikipedia, the free encyclopedia
• Wikipedia (3), "Stemming", from Wikipedia, the free encyclopedia
• Wikipedia (4), "Synonym", from Wikipedia, the free encyclopedia
• Wikipedia (5), "Jaccard Index", from Wikipedia, the free encyclopedia
• Wikipedia (6), "Flesch–Kincaid readability tests", from Wikipedia, the free encyclopedia

Issues

If you find a bug please fill an issue.

This project is a supporting package for a revenue project that has priority call on resources, so please be patient if we don't respond immediately to issues or pull requests.

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