word2gauss

Gaussian word embeddings

Python/Cython implementation of Luke Vilnis and Andrew McCallum Word Representations via Gaussian Embedding, ICLR 2015 that represents each word as a multivariate Gaussian. Scales to (relatively) large corpora using Cython extensions and threading with asynchronous stochastic gradient descent (Adagrad).

Getting started

Installing

First install numpy, scipy and the packages in requirements.txt. Then sudo make install. make test runs the test suite.

NOTE: The performance sensitive parts of the code have been carefully written in a way that allows gcc to auto-vectorize all the important loops. Accordingly we recommend using gcc to compile and setting these flags for building:

export CFLAGS="-ftree-vectorizer-verbose=2 -O3 -ffast-math"
sudo -E bash -c "make install"

If you are using a Mac, gcc compiled code runs approximately 2.5X faster than the default clang compiler. You can force the build to use gcc instead of clang with:

# change these to the location of gcc -- note that /usr/bin/gcc is really
# clang in a default XCode installation
export CC=/usr/local/bin/gcc
export CXX=/usr/local/bin/g++
export CFLAGS="-ftree-vectorizer-verbose=2 -O3 -ffast-math"
sudo -E bash -c "make install"

Code overview

The GaussianEmbedding class is the main workhorse for most tasks. It stores the model data, deals with serialization to/from files and learns the parameters. To allow embedding of non-word types like hierarchies and entailment relations, GaussianEmbedding has no knowledge of any vocabulary and operates only on unique IDs. Each ID is a uint32 from 0 .. N-1 with -1 signifying an OOV token.

For learning word embeddings, the token - id mapping is off-loaded to a Vocabulary class that translates streams of documents into token_id lists and provides the ability to draw a random token_id from the token distribution. The random generator is necessary for the negative sampling needed to generate training positive/negative training pairs from a sentence (see details below). We assume the Vocabulary has these methods:

tokenize(text) returns a numpy array of token IDs in the text or -1 for OOV tokens
random_ids(N) returns a length N numpy array of random IDs
word2id(string) and id2word(uint32) map from string <-> id
__len__(self) returns number of words in vocabulary

Learning embeddings

To learn embeddings, you will need a suitable corpus and an implementation of the Vocabulary interface.

import logging
logging.basicConfig(format='%(asctime)s : %(levelname)s : %(message)s', level=logging.INFO)

from gzip import GzipFile

from word2gauss import GaussianEmbedding, iter_pairs
from vocab import Vocabulary

# load the vocabulary
vocab = Vocabulary(...)

# create the embedding to train
# use 100 dimensional spherical Gaussian with KL-divergence as energy function
embed = GaussianEmbedding(len(vocab), 100,
    covariance_type='spherical', energy_type='KL')

# open the corpus and train with 8 threads
# the corpus is just an iterator of documents, here a new line separated
# gzip file for example
with GzipFile('location_of_corpus', 'r') as corpus:
    embed.train(iter_pairs(corpus, vocab), n_workers=8)

# save the model for later
embed.save('model_file_location', vocab=vocab.id2word, full=True)

Examining trained models

from word2gauss import GaussianEmbedding
from vocab import Vocabulary

# load in a previously trained model and the vocab
vocab = Vocabulary(...)
embed = GaussianEmbedding.load('model_file_location')

# find nearest neighbors to 'rock'
embed.nearest_neighbors('rock', vocab=vocab)

# find nearest neighbors to 'rock' sorted by covariance
embed.nearest_neighbors('rock', num=100, vocab=vocab, sort_order='sigma')

# solve king + woman - man = ??
embed.nearest_neighbors([['king', 'woman'], ['man']], num=10, vocab=vocab)

Background details

Instead of representing a word as a vector as in word2vec, word2gauss represents each word as a multivariate Gaussian. Assuming some dictionary of known tokens w[i], i = 0 .. N-1, each word is represented as a probability P[i], a K dimensional Gaussian parameterized by

   P[i] ~ N(x; mu[i], Sigma[i])

Here, mu[i] and Sigma[i] are the mean and co-variance matrix for word i. The mean is a vector of length K and in the most general case Sigma[i] is a (K, K) matrix. The paper makes one of two approximations to simplify Sigma[i]:

'diagonal' in which case Sigma[i] is a vector length K
'spherical' in which case Sigma[i] is a single float

To learn the probabilities, first define an energy function E(P[i], P[j]) that returns a similarity like measure of the two probabilities. Both the symmetric Expected Likelihood Inner Product and asymmetric KL-divergence are implemented.

Given a pair of "positive" and "negative" indices, define Delta E = E(P[posi], P[posj]) - E(P[negi], P[negj]). Intuitively the training process optimizes the parameters to make Delta E positive. Formally, use a max-margin loss:

    loss = max(0, Closs - Delta E)

and optimize the parameters to minimize the sum of the loss over the entire training set of positive/negative pairs.

To generate the training pairs, use co-occuring words as the positive examples and a randomly sampled words as the negative examples. Since the energy function is potentially asymmetric, for each co-occuring word pair randomly sample both the left and right tokens for negative examples. In addition, we allow the option to generate several sets of training pairs from each word. In pseudo-code:

for sentence in corpus:
    for i in len(sentence):
        for k in 1..window_size:
            for nsample in 1..number_of_samples_per_word:
                positive pair = (left, right) = (sentence[i], sentence[i + k])
                negative pairs = [(left, random ID), (random ID, right)]
                update model weights

kalaidin / word2gauss