https://mailund.dk/posts/c-refcount-list/

                                     sigma(a[i] * b[i])
cosineSimilarity(a, b) = ---------------------------------------------------------
                                     squareRoot(sigma(a[i] ^ 2)) * squareRoot(sigma(b[i] ^ 2))

$$cos\left({a,b}\right) = \frac{ \sum_{i=1}^n a_i b_i } { \sqrt{\sum_{i=1}^n a_i^2} \sqrt{\sum_{i=1}^n b_i^2} }$$