public void CorrectlyCalculateTFIDF(string documentA, string documentB, string word, double expectedTfidfOfWordInA)
    {
        var wordCollectionA = new WordCollection();

        wordCollectionA.AddWords(documentA, 10);
        wordCollectionA.InitWordPositions();
        var wordCollectionB = new WordCollection();

        wordCollectionB.AddWords(documentB, 10);
        wordCollectionB.InitWordPositions();
        var documents = new List <WordCollection> {
            wordCollectionA, wordCollectionB
        };

        var tfidfOfWordInA = wordCollectionA.CalculateTFIDF(word, documents);

        Assert.Equal(expectedTfidfOfWordInA, tfidfOfWordInA, 5);
    }