Building an ngram frequency table and dealing with multibyte runes

Question

I am currently learning Go and am making a lot of progress. One way I do this is to port past projects and prototypes from a prior language to a new one.

Right now I am busying myself with a "language detector" I prototyped in Python a while ago. In this module, I generate an ngram frequency table, where I then calculate the difference between a given text and a known corpora.

This allows one to effectively determine which corpus is the best match by returning the cosine of two vector representations of the given ngram tables. Yay. Math.

I have a prototype written in Go that works perfectly with plain ascii characters, but I would very much like to have it working with unicode multibyte support. This is where I'm doing my head in.

Here is a quick example of what I'm dealing with: http://play.golang.org/p/2bnAjZX3r0

I've only posted the table generating logic since everything already works just fine.

As you can see by running the snippet, the first text works quite well and builds an accurate table. The second text, which is German, has a few double-byte characters in it. Due to the way I am building the ngram sequence, and due to the fact that these specific runes are made of two bytes, there appear 2 ngrams where the first byte is cut off.

Could someone perhaps post a more efficient solution or, at the very least, guide me through a fix? I'm almost positive I am over analysing this problem.

I plan on open sourcing this package and implementing it as a service using Martini, thus providing a simple API people can use for simple linguistic computation.

As ever, thanks!

Answer 1

If I understand correctly, you want chars in your Parse function to hold the last n characters in the string. Since you're interested in Unicode characters rather than their UTF-8 representation, you might find it easier to manage it as a []rune slice, and only convert back to a string when you have your ngram ready to add to the table. This way you don't need to special case non-ASCII characters in your logic.

Here is a simple modification to your sample program that does the above: http://play.golang.org/p/QMYoSlaGSv

Answer 2

By keeping a circular buffer of runes, you can minimise allocations. Also note that reading a new key from a map returns the zero value (which for int is 0), which means the unknown key check in your code is redundant.

func Parse(text string, n int) map[string]int {
    chars := make([]rune, 2 * n)
    table := make(map[string]int)
    k := 0
    for _, chars[k] = range strings.Join(strings.Fields(text), " ") + " " {
        chars[n + k] = chars[k]
        k = (k + 1) % n
        table[string(chars[k:k+n])]++
    }
    return table
}