Icegrams: A fast, compact trigram library for Icelandic

Overview

Icegrams is an MIT-licensed Python 3 (>=3.9) package that encapsulates a large trigram library for Icelandic. (A trigram is a tuple of three consecutive words or tokens that appear in real-world text.)

14 million unique trigrams and their frequency counts are heavily compressed using radix tries and quasi-succinct indexes employing Elias-Fano encoding. This enables the ~43 megabyte compressed trigram file to be mapped directly into memory, with no ex ante decompression, for fast queries (typically ~10 microseconds per lookup).

The Icegrams library is implemented in Python and C/C++, glued together via CFFI.

The trigram storage approach is based on a 2017 paper by Pibiri and Venturini, also referring to Ottaviano and Venturini (2014) regarding partitioned Elias-Fano indexes.

You can use Icegrams to obtain probabilities (relative frequencies) of over a million different unigrams (single words or tokens), or of bigrams (pairs of two words or tokens), or of trigrams. You can also ask it to return the N most likely successors to any unigram or bigram.

Icegrams is useful for instance in spelling correction, predictive typing, to help disabled people write text faster, and for various text generation, statistics and modelling tasks.

The Icegrams trigram corpus is built from the 2017 edition of the Icelandic Gigaword Corpus (Risamálheild), which is collected and maintained by The Árni Magnússon Institute for Icelandic Studies. A mixed, manually vetted subset consisting of 157 documents from the corpus was used as the source of the token stream, yielding over 100 million tokens. Trigrams that only occurred once or twice in the stream were eliminated before creating the compressed Icegrams database. The creation process is further described here.

Example

>>> from icegrams import Ngrams
>>> ng = Ngrams()
>>> # Obtain the frequency of the unigram 'Ísland'
>>> ng.freq("Ísland")
42018
>>> # Obtain the probability of the unigram 'Ísland', as a fraction
>>> # of the frequency of all unigrams in the database
>>> ng.prob("Ísland")
0.0003979926900206475
>>> # Obtain the log probability (base e) of the unigram 'Ísland'
>>> ng.logprob("Ísland")
-7.8290769196308005
>>> # Obtain the frequency of the bigram 'Katrín Jakobsdóttir'
>>> ng.freq("Katrín", "Jakobsdóttir")
3517
>>> # Obtain the probability of 'Jakobsdóttir' given 'Katrín'
>>> ng.prob("Katrín", "Jakobsdóttir")
0.23298013245033142
>>> # Obtain the probability of 'Júlíusdóttir' given 'Katrín'
>>> ng.prob("Katrín", "Júlíusdóttir")
0.013642384105960274
>>> # Obtain the frequency of 'velta fyrirtækisins er'
>>> ng.freq("velta", "fyrirtækisins", "er")
4
>>> # adj_freq returns adjusted frequencies, i.e incremented by 1
>>> ng.adj_freq("xxx", "yyy", "zzz")
1
>>> # Obtain the N most likely successors of a given unigram or bigram,
>>> # in descending order by log probability of each successor
>>> ng.succ(10, "stjórnarskrá", "lýðveldisins")
[('Íslands', -1.3708244393477589), ('.', -2.2427905461504567),
    (',', -3.313814878299737), ('og', -3.4920631097060557), ('sem', -4.566577846795106),
    ('er', -4.720728526622363), ('að', -4.807739903611993), ('um', -5.0084105990741445),
    ('en', -5.0084105990741445), ('á', -5.25972502735505)]

Reference

Initializing Icegrams

After installing the icegrams package, use the following code to import it and initialize an instance of the Ngrams class:

from icegrams import Ngrams
ng = Ngrams()

Now you can use the ng instance to query for unigram, bigram and trigram frequencies and probabilities.

The Ngrams class

__init__(self)

Initializes the Ngrams instance.
freq(self, *args) -> int

Returns the frequency of a unigram, bigram or trigram.
- str[] *args A parameter sequence of consecutive unigrams to query the frequency for.
- returns An integer with the frequency of the unigram, bigram or trigram.
To query for the frequency of a unigram in the text, call ng.freq("unigram1"). This returns the number of times that the unigram appears in the database. The unigram is queried as-is, i.e. with no string stripping or lowercasing.

To query for the frequency of a bigram in the text, call ng.freq("unigram1", "unigram2").

To query for the frequency of a trigram in the text, call ng.freq("unigram1", "unigram2", "unigram3").

If you pass more than 3 arguments to ng.freq(), only the last 3 are significant, and the query will be treated as a trigram query.

Examples:
```
>>>> ng.freq("stjórnarskrá")
2973
>>>> ng.freq("stjórnarskrá", "lýðveldisins")
39
>>>> ng.freq("stjórnarskrá", "lýðveldisins", "Íslands")
12
>>>> ng.freq("xxx", "yyy", "zzz")
0
```
adj_freq(self, *args) -> int

Returns the adjusted frequency of a unigram, bigram or trigram.
- str[] *args A parameter sequence of consecutive unigrams to query the frequency for.
- returns An integer with the adjusted frequency of the unigram, bigram or trigram. The adjusted frequency is the actual frequency plus 1. The method thus never returns 0.
To query for the frequency of a unigram in the text, call ng.adj_freq("unigram1"). This returns the number of times that the unigram appears in the database, plus 1. The unigram is queried as-is, i.e. with no string stripping or lowercasing.

To query for the frequency of a bigram in the text, call ng.adj_freq("unigram1", "unigram2").

To query for the frequency of a trigram in the text, call ng.adj_freq("unigram1", "unigram2", "unigram3").

If you pass more than 3 arguments to ng.adj_freq(), only the last 3 are significant, and the query will be treated as a trigram query.

Examples:
```
>>>> ng.adj_freq("stjórnarskrá")
2974
>>>> ng.adj_freq("stjórnarskrá", "lýðveldisins")
40
>>>> ng.adj_freq("stjórnarskrá", "lýðveldisins", "Íslands")
13
>>>> ng.adj_freq("xxx", "yyy", "zzz")
1
```
prob(self, *args) -> float

Returns the probability of a unigram, bigram or trigram.
- str[] *args A parameter sequence of consecutive unigrams to query the probability for.
- returns A float with the probability of the given unigram, bigram or trigram.
The probability of a unigram is the frequency of the unigram divided by the sum of the frequencies of all unigrams in the database.

The probability of a bigram (u1, u2) is the frequency of the bigram divided by the frequency of the unigram u1, i.e. how likely u2 is to succeed u1.

The probability of a trigram (u1, u2, u3) is the frequency of the trigram divided by the frequency of the bigram (u1, u2), i.e. how likely u3 is to succeed u1 u2.

If you pass more than 3 arguments to ng.prob(), only the last 3 are significant, and the query will be treated as a trigram probability query.

Examples:
```
>>>> ng.prob("stjórnarskrá")
2.8168929772755334e-05
>>>> ng.prob("stjórnarskrá", "lýðveldisins")
0.01344989912575655
>>>> ng.prob("stjórnarskrá", "lýðveldisins", "Íslands")
0.325
```
logprob(self, *args) -> float

Returns the log probability of a unigram, bigram or trigram.
- str[] *args A parameter sequence of consecutive unigrams to query the log probability for.
- returns A float with the natural logarithm (base e) of the probability of the given unigram, bigram or trigram.
The probability of a unigram is the adjusted frequency of the unigram divided by the sum of the frequencies of all unigrams in the database.

The probability of a bigram (u1, u2) is the adjusted frequency of the bigram divided by the adjusted frequency of the unigram u1, i.e. how likely u2 is to succeed u1.

The probability of a trigram (u1, u2, u3) is the adjusted frequency of the trigram divided by the adjusted frequency of the bigram (u1, u2), i.e. how likely u3 is to succeed u1 u2.

If you pass more than 3 arguments to ng.logprob(), only the last 3 are significant, and the query will be treated as a trigram probability query.

Examples:
```
>>>> ng.logprob("stjórnarskrá")
-10.477290968535172
>>>> ng.logprob("stjórnarskrá", "lýðveldisins")
-4.308783672906165
>>>> ng.logprob("stjórnarskrá", "lýðveldisins", "Íslands")
-1.1239300966523995
```
succ(self, n, *args) -> list[tuple]

Returns the N most probable successors of a unigram or bigram.
- int n A positive integer specifying how many successors, at a maximum, should be returned.
- str[] *args One or two string parameters containing the unigram or bigram to query the successors for.
- returns A list of tuples of (successor unigram, log probability), in descending order of probability.
If you pass more than 2 string arguments to ng.succ(), only the last 2 are significant, and the query will be treated as a bigram successor query.

Examples:
```
>>>> ng.succ(2, "stjórnarskrá")
[('.', -1.8259625296091855), ('landsins', -2.223111581475692)]
>>>> ng.succ(2, "stjórnarskrá", "lýðveldisins")
[('Íslands', -1.1239300966523995), ('og', -1.3862943611198904)]
>>>> # The following is equivalent to ng.succ(2, "lýðveldisins", "Íslands")
>>>> ng.succ(2, "stjórnarskrá", "lýðveldisins", "Íslands")
[('.', -1.3862943611198908), (',', -1.6545583477145702)]
```

Notes

Icegrams is built with a sliding window over the source text. This means that a sentence such as "Maðurinn borðaði ísinn." results in the following trigrams being added to the database:

("", "", "Maðurinn")
("", "Maðurinn", "borðaði")
("Maðurinn", "borðaði", "ísinn")
("borðaði", "ísinn", ".")
("ísinn", ".", "")
(".", "", "")

The same sliding window strategy is applied for bigrams, so the following bigrams would be recorded for the same sentence:

("", "Maðurinn")
("Maðurinn", "borðaði")
("borðaði", "ísinn")
("ísinn", ".")
(".", "")

You can thus obtain the N unigrams that most often start a sentence by asking for ng.succ(N, "").

And, of course, four unigrams are also added, one for each token in the sentence.

The tokenization of the source text into unigrams is done with the Tokenizer package and uses the rules documented there. Importantly, tokens other than words, abbreviations, entity names, person names and punctuation are replaced by placeholders. This means that all numbers are represented by the token [NUMBER], amounts by [AMOUNT], dates by [DATEABS] and [DATEREL], e-mail addresses by [EMAIL], etc. For the complete mapping of token types to placeholder strings, see the documentation for the Tokenizer package.

Prerequisites

This package runs on CPython 3.9 or newer, and on PyPy 3.9 or newer. It has been tested on Linux (gcc on x86-64 and ARMhf), macOS (clang) and Windows (MSVC).

If a binary wheel package isn't available on PyPI for your system, you may need to have the python3-dev package (or its Windows equivalent) installed on your system to set up Icegrams successfully. This is because a source distribution install requires a C++ compiler and linker:

# Debian or Ubuntu:
sudo apt-get install python3-dev

Installation

To install this package:

$ pip install icegrams

If you want to be able to edit the source, do like so (assuming you have git installed):

$ git clone https://github.com/mideind/Icegrams
$ cd Icegrams
$ # [ Activate your virtualenv here if you have one ]
$ python setup.py develop

The package source code is now in ./src/icegrams.

Tests

To run the built-in tests, install pytest, cd to your Icegrams subdirectory (and optionally activate your virtualenv), then run:

$ python -m pytest

Changelog

Version 1.1.3: Minor tweaks. Support for Python 3.13. Now requires Python 3.9+. (2024-08-27)
Version 1.1.2: Minor bug fixes. Cross-platform wheels provided. Now requires Python 3.7+. (2022-12-14)
Version 1.1.0: Python 3.5 support dropped; macOS builds fixed; PyPy wheels generated
Version 1.0.0: New trigram database sourced from the Icelandic Gigaword Corpus (Risamálheild) with improved tokenization. Replaced GNU GPLv3 with MIT license.
Version 0.6.0: Python type annotations added
Version 0.5.0: Trigrams corpus has been spell-checked

Copyright and licensing

This software is licensed under the MIT License:

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.