 # Online Random Projections I recently encountered the problem of computing the similarity of pairs of “documents” (which, in my case, were actually graphs), where the documents arrived as a stream of individual words. The incoming stream of words could both initiate new documents and grow existing documents. Interestingly, the words of each document could also arrive out-of-order, but I will ignore this complication for now.

The problem then was to continuously compute and maintain the similarity of every pair of documents originating from the words in the stream. Strangely, I could not find any existing methods to do this even for the case of a single document pair, under the constraints of the data stream model (single-pass, bounded memory). Specifically, all the methods I surveyed needed to know my “vocabulary”: the unique words that could possibly arrive in the stream. In my case, this is never known.

In this post, I will describe the method I developed to tackle this problem, while also discussing the background needed to understand why it works. The method itself is fairly simple and, with some help from the code snippets in this post, should be easy to implement and try out yourself!

## Computing Document Similarity Computing the cosine similarity of documents D1 and D2: The documents are first split into their constituent words and a vocabulary index is assigned to each word. The documents are then represented by vectors of frequencies of the words they contain. Given these vectors, their magnitudes, dot-product and cosine-similarity can be computed.

A common way to measure how similar two documents are is by their cosine similarity. An example of this is illustrated in the figure above. All documents in the collection are first split into their constituent words, and a vocabulary index is constructed which assigns an integer to each unique word. If the vocabulary size is $$|V|$$, each document is represented by a $$|V|$$-sized vector of frequencies of the words it contains.

Given these vectors for a pair of documents, say D1 and D2, their magnitudes are given by,

\begin{align*} magn(\mathbf{D1}) &= \|\mathbf{D1}\|_2\\ magn(\mathbf{D2}) &= \|\mathbf{D2}\|_2, \end{align*}

and their cosine similarity is given by,

$sim(\mathbf{D1}, \mathbf{D2}) = cos(\mathbf{D1}, \mathbf{D2}) = \frac{\mathbf{D1} \cdot \mathbf{D2}} {\|\mathbf{D1}\|_2\|\mathbf{D2}\|_2}.$

### A Computational Problem With Large |V|

In practice, it is possible for the vocabulary to be extremely large. This increases the time needed to compute the similarity of each document pair and the memory needed to store each document. To address this issue, a popular approach is to sketch the word-frequency vectors that represent each document. Sketching replaces $$|V|$$-sized document vectors by $$k$$-sized sketch vectors, where $$k$$ is much smaller than $$|V|$$, while accurately approximating the similarity of pairs of documents.

## Sketching Document Similarity

An approach to sketching vectors that preserves their cosine similarity is using random projections, which applies some neat tricks from linear algebra and probability. Let’s say we want to compute the similarity of documents with vectors D1 and D2 having angle θ between them. We can visualize these vectors in the XY-plane. Let’s pick a vector R in the plane having the same size as D1 and D2 uniformly at random in the plane. Then define a function h that operates on any such random vector R and a document vector D as follows:

$h_{\mathbf{R}}(\mathbf{D}) = \begin{cases} +1 ~\textrm{if}~ \mathbf{D}\cdot\mathbf{R} \geq 0,\\ -1 ~\textrm{if}~ \mathbf{D}\cdot\mathbf{R} < 0. \end{cases}$

So $$h_{\mathbf{R}}(\mathbf{D1}) = h_{\mathbf{R}}(\mathbf{D2})$$ only if both D1 and D2 lie on the same side of R, and $$h_{\mathbf{R}}(\mathbf{D1}) \neq h_{\mathbf{R}}(\mathbf{D2})$$ otherwise. Now what is the probability that $$h_{\mathbf{R}}(\mathbf{D1}) = h_{\mathbf{R}}(\mathbf{D2})$$, over all random choices of R? It’s easier to see what this is with another illustration. In the red region, $$h_{\mathbf{R}}(\mathbf{D1}) \neq h_{\mathbf{R}}(\mathbf{D2})$$ since D1 and D2 will fall on opposite sides of any random vector R chosen in this region. Similarly, $$h_{\mathbf{R}}(\mathbf{D1}) = h_{\mathbf{R}}(\mathbf{D2})$$ in the yellow region. The ratio of the areas of the red and yellow regions is 2θ/2π, leading to the probability over random vectors R:

$P_{\mathbf{R}}[h_{\mathbf{R}}(\mathbf{D1}) = h_{\mathbf{R}}(\mathbf{D2})] = 1 - θ/π.$

If we could somehow estimate this probability, we could plug it into the above equation to find the angle θ, and hence, find the cosine similarity of the two document vectors:

$\begin{gather*} θ = π (1 - P_{\mathbf{R}}[h_{\mathbf{R}}(\mathbf{D1}) = h_{\mathbf{R}}(\mathbf{D2})])\\ sim(\mathbf{D1}, \mathbf{D2}) = cos(θ) \end{gather*}$

It turns out that it’s easy to estimate this probability: simply pick $$k$$ random vectors $$\mathbf{R}_1, \dots, \mathbf{R}_k$$, evaluate $$h_{\mathbf{R_i}}(\mathbf{D1})$$ and $$h_{\mathbf{R_i}}(\mathbf{D2})$$ for each of these vectors and then count the fraction of function values for which the two documents agree (i.e. function values are both +1 or both -1),

$P_{\mathbf{R}}[h_{\mathbf{R}}(\mathbf{D1}) = h_{\mathbf{R}}(\mathbf{D2})] \approx \frac{\sum_{i=1}^k \mathbb{1}(h_{\mathbf{R_i}}(\mathbf{D1}) = h_{\mathbf{R_i}}(\mathbf{D2}))} {k}.$

In practice, the random vectors turn out to be sufficiently random if each of their elements are chosen uniformly from $$\{+1, -1\}$$.

By fixing a set of random vectors $$\mathbf{R}_1, \dots, \mathbf{R}_k$$, every $$|V|$$-sized document vector D can be replaced by a $$k$$-sized sketch vector, where element $$i$$ of the sketch vector is either +1 or -1 and is given by $$h_{\mathbf{R_i}}(\mathbf{D})$$. Thus, each sketch vector is essentially a bit vector and consumes very little space. Document similarity can then be estimated using just these sketch vectors. The accuracy of estimation improves with increasing $$k$$.

### A Computational Problem With Unknown |V|

Notice that in the random projections technique just described, we need to know $$|V|$$ to construct $$|V|$$-sized random vectors. In a streaming scenario, $$|V|$$ is never known, and may grow with time!

## Streaming Document Similarity

Let’s say we could pick $$k$$ functions $$h_1, \dots, h_k$$ at random from a family of functions $$\mathcal{H}$$, where each $$h \in \mathcal{H}$$ maps a word to an element of $$\{+1, -1\}$$. Then we could replace each random vector $$\mathbf{R}_i$$ with a function $$h_i \in \mathcal{H}$$, as long as $$\mathcal{H}$$ obeys the following properties:

1. It should be equally probable for a given word to map to +1 or -1, over randomly chosen $$h \in \mathcal{H}$$.

This captures the equivalence of the function $$h_i$$ with $$\mathbf{R}_i$$ whose elements are each uniformly chosen from $$\{+1, -1\}$$.

However, requiring only this property permits trival families such as $$\mathcal{H} = \{h_1(w) = +1, ~h_2(w) = -1, \forall ~\textrm{words}~ w\}$$. This family satisfies property (1) since the probability that any function randomly picked from $$\mathcal{H}$$ maps a word to +1 or -1 is 1/2; note that the randomness originates from picking the function and not from computing the mapped value of a word. But each of the functions in this trivial family map all words to the same value, which is not very useful.

So we need the following additional property:

2. For a given $$h \in \mathcal{H}$$, values in {+1, -1} are equally probable over all words.

This ensures that a function in the family cannot map all words to the same value, and should evenly distribute the mapped values across all words between +1 and -1. This still does not prevent families such as $$\mathcal{H} = \{h_1(w), ~h_2(w) = -h_1(w), \forall ~\textrm{words}~ w\}$$ where $$h_1$$ and $$h_2$$ both satisfy property (2). This family is not very useful either, since the second function is correlated with the first.

This leads to our final desired property:

3. All the functions in $$\mathcal{H}$$ are pairwise-independent.

It turns out that a family satisfying these requirements is a a 2-universal (strongly universal) hash function family by definition. Many such families exist that work on strings. One example is the multilinear family:

$h(w) = m_0 + \sum_{i = 1}^n m_i w_i,$

where $$w_i$$ is the integer representation of the $$i^\textrm{th}$$ character of the word $$w$$, and $$m_0, \dots, m_n$$ are uniformly random integers. The function above returns an integer, but this can be easily mapped to an element of $$\{+1, -1\}$$ by extracting the MSB and some algebra.

### Implementation

To sample $$k$$ functions from the strongly-universal multilinear family, it is only necessary to estimate the maximum word length $$m = |w|_{\textrm{max}}$$. Then, for each function $$h_1, \dots, h_k$$, $$m + 1$$ random integers are generated and stored.

The following C++ 11 snippet initializes $$h_1, \dots, h_k$$ as $$k$$ hash functions sampled uniformly at random from the multilinear family:

Then the mapping for a word $$w$$ using $$h_i$$ can be computed as:

Notice the bitwise-and when computing the summation: this is to take care of the sign-extension performed when a 1-byte character is cast to a 4-byte unsigned integer.

## Streaming Heterogenous Graph Similarity

The technique described above was used to perform streaming clustering and anomaly detection on heterogenous graphs, where the graphs were arriving as a stream of individual edges. Further details are available in the paper and code at the project website.