.. Copyright (C) 2001-2021 NLTK Project
.. For license information, see LICENSE.TXT

=======================================
Demonstrate word embedding using Gensim
=======================================

    >>> from nltk.test.gensim_fixt import setup_module
    >>> setup_module()

We demonstrate three functions:
- Train the word embeddings using brown corpus;
- Load the pre-trained model and perform simple tasks; and
- Pruning the pre-trained binary model.

    >>> import gensim

---------------
Train the model
---------------

Here we train a word embedding using the Brown Corpus:

    >>> from nltk.corpus import brown
    >>> train_set = brown.sents()[:10000]
    >>> model = gensim.models.Word2Vec(train_set)

It might take some time to train the model. So, after it is trained, it can be saved as follows:

    >>> model.save('brown.embedding')
    >>> new_model = gensim.models.Word2Vec.load('brown.embedding')

The model will be the list of words with their embedding. We can easily get the vector representation of a word.

    >>> len(new_model.wv['university'])
    100

There are some supporting functions already implemented in Gensim to manipulate with word embeddings.
For example, to compute the cosine similarity between 2 words:

    >>> new_model.wv.similarity('university','school') > 0.3
    True

---------------------------
Using the pre-trained model
---------------------------

NLTK includes a pre-trained model which is part of a model that is trained on 100 billion words from the Google News Dataset.
The full model is from https://code.google.com/p/word2vec/ (about 3 GB).

    >>> from nltk.data import find
    >>> word2vec_sample = str(find('models/word2vec_sample/pruned.word2vec.txt'))
    >>> model = gensim.models.KeyedVectors.load_word2vec_format(word2vec_sample, binary=False)

We pruned the model to only include the most common words (~44k words).

    >>> len(model)
    43981

Each word is represented in the space of 300 dimensions:

    >>> len(model['university'])
    300

Finding the top n words that are similar to a target word is simple. The result is the list of n words with the score.

    >>> model.most_similar(positive=['university'], topn = 3)
    [('universities', 0.70039...), ('faculty', 0.67809...), ('undergraduate', 0.65870...)]

Finding a word that is not in a list is also supported, although, implementing this by yourself is simple.

    >>> model.doesnt_match('breakfast cereal dinner lunch'.split())
    'cereal'

Mikolov et al. (2013) figured out that word embedding captures much of syntactic and semantic regularities. For example,
the vector 'King - Man + Woman' is close to 'Queen' and 'Germany - Berlin + Paris' is close to 'France'.

    >>> model.most_similar(positive=['woman','king'], negative=['man'], topn = 1)
    [('queen', 0.71181...)]

    >>> model.most_similar(positive=['Paris','Germany'], negative=['Berlin'], topn = 1)
    [('France', 0.78840...)]

We can visualize the word embeddings using t-SNE (https://lvdmaaten.github.io/tsne/). For this demonstration, we visualize the first 1000 words.

|    import numpy as np
|    labels = []
|    count = 0
|    max_count = 1000
|    X = np.zeros(shape=(max_count,len(model['university'])))
|
|    for term in model.index_to_key:
|        X[count] = model[term]
|        labels.append(term)
|        count+= 1
|        if count >= max_count: break
|
|    # It is recommended to use PCA first to reduce to ~50 dimensions
|    from sklearn.decomposition import PCA
|    pca = PCA(n_components=50)
|    X_50 = pca.fit_transform(X)
|
|    # Using TSNE to further reduce to 2 dimensions
|    from sklearn.manifold import TSNE
|    model_tsne = TSNE(n_components=2, random_state=0)
|    Y = model_tsne.fit_transform(X_50)
|
|    # Show the scatter plot
|    import matplotlib.pyplot as plt
|    plt.scatter(Y[:,0], Y[:,1], 20)
|
|    # Add labels
|    for label, x, y in zip(labels, Y[:, 0], Y[:, 1]):
|        plt.annotate(label, xy = (x,y), xytext = (0, 0), textcoords = 'offset points', size = 10)
|
|    plt.show()

------------------------------
Prune the trained binary model
------------------------------

Here is the supporting code to extract part of the binary model (GoogleNews-vectors-negative300.bin.gz) from https://code.google.com/p/word2vec/
We use this code to get the `word2vec_sample` model.

|    import gensim
|    # Load the binary model
|    model = gensim.models.KeyedVectors.load_word2vec_format('GoogleNews-vectors-negative300.bin.gz', binary = True)
|
|    # Only output word that appear in the Brown corpus
|    from nltk.corpus import brown
|    words = set(brown.words())
|    print(len(words))
|
|    # Output presented word to a temporary file
|    out_file = 'pruned.word2vec.txt'
|    with open(out_file,'w') as f:
|        word_presented = words.intersection(model.index_to_key)
|        f.write('{} {}\n'.format(len(word_presented),len(model['word'])))
|
|        for word in word_presented:
|            f.write('{} {}\n'.format(word, ' '.join(str(value) for value in model[word])))