Semantic analysis is a way to extract meaning from a written text. The written text may be a single word, a couple of words, a sentence, a paragraph or a whole book.
Explicit Semantic Analysis is a way to derive meaning based on Wikipedia. The text is transformed into a vector of Wikipedia articles. The vectors of two different texts can then be compared to assess the semantic similarity of those texts.
This implementation was written by Philip van Oosten. It takes advantage of the mature Lucene project.
This software is provided under the terms of the AGPLv3 license. If this software seems helpful to you, but you dislike the licensing, don't let it get in your way and contact the author. We can work something out.
ESA can be used as a library. You will need to make some changes to the source code to use ESA and to tweak it.
To learn how to work with it, I recommend trying a language with a small Wikipedia dump, other than English. The English wikipedia dump is very large and each step in the process of setting up ESA takes several hours to complete. A language with a smaller Wikipedia dump may not work as good as English, because there is just less data, but you will get up and running much faster.
This takes several hours.
A list of all available database dumps is available here: https://dumps.wikimedia.org/backup-index-bydb.html. Choose a download which contains all current articles without history. For English (enwiki), the download size is 13 GB at the time of writing, for Dutch (nlwiki) it is 1.3 GB. Note that Wikipedia is constantly updated, so old dumps may not contain new concepts that could be interesting for your application.
The Wikipedia article dump consists of a multi-stream BZipped xml file. That means that a straightforward way to read the bzip stream ends somewhere near the beginning of the file. You need to read the whole dump, not just the beginning. This implementation takes care of that.
This also takes several hours.
Now that the Wikipedia dump is downloaded, it must be indexed. Indexing is done with Lucene in two steps.
First, all articles are indexed to a term-to-document index. The documents are the concepts in ESA.
Second, the full-text index is inverted, so that each concept is mapped to all the terms that are important for that concept. To find that index, the terms in the first index become a document in the second index. Lucene further handles the indexing.
The class be.vanoosten.esa.Main contains an indexing method.
Using that method, you can create a term to concept index (the first index).
The same class also contains a createConceptTermIndex() method, which is a bit more involved.
That method can be used to create the second index, which maps Wikipedia articles to their tokens.
All kinds of tricks from Lucene can be used to tweak the indexing.
Maybe you will want to use a different Lucene Analyzer.
Taking a good look at Lucene documentation and the be.vanoosten.esa.WikiAnalyzer class can be a good starting point for that.
After indexing, you are ready to transform text to vectors.
Creating a concept vector from a text can be done with a Vectorizer, implemented in the class be.vanoosten.esa.tools.Vectorizer.
The vectorizer has a vectorize(String text) method, which transforms the text into a concept vector (be.vanoosten.esa.tools.ConceptVector).
Basically, the text is tokenized and searched for in the term-to-concept index.
The result is a list of Wikipedia articles, along with their numeric similarity to the vectorized text.
Two concept vectors can be easily compared to each other, using the dotProduct method.
The dot product of two concept vectors is a measure for the semantic similarity between the two texts those vectors are created from.
Calculating the semantic similarity between two texts directly is exactly what the semantic similarity tool (be.vanoosten.esa.tools.SemanticSimilarityTool) does.
Finally, the automatic brainstormer is why I went through the effort to create an ESA implementation. Starting from a text or a set of words, the brainstormer searches for words with a similar meaning. That process can be repeated a couple of times to create a network of words that can be visualized with Graphviz.
The brainstormer is available in the class be.vanoosten.esa.brainstorm.Brainstormer.
Wikipedia-based Explicit Semantic Analysis, as described by Gabrilovich and Markovitch.
ESA is well described in a scientific paper.
http://en.wikipedia.org/wiki/Explicit_semantic_analysis
http://www.cs.technion.ac.il/~gabr/resources/code/esa/esa.html
http://www.cs.technion.ac.il/~gabr/papers/ijcai-2007-sim.pdf