Archive for April, 2014

Bitwise Operators for Kdb+ Database

Kdb does not have built-in functions for bitwise and,or,xor operations, we are going to create a C DLL extension that provides bitwise operators.

To download the source and definitions for and/or/xor bitwise operations click here
If your new to extending Kdb+ see this tutorial on writing a DLL for Kdb+

I also thought this would be a good opportunity to look at using q-code to generate C code, to generate Kdb Dll’s. When converting an existing library to work with Kdb or writing similar functions for different types, there’s a lot of repetitive coding. Either you can use macros or you can use one language to generate another, to save a lot of typing. Let’s look at bitwise operations as an example of what I mean:

If I wanted to write a bitwise and function band that takes two lists and performs a corresponding operation, we could write the C functions like so:

Notice the similarity between the bandJJ and bandII functions. Plus if we want to handle the case where the second argument is an atom we need an entire other set of C functions similar to bandJ to handle that. This is soon beginning to spiral beyond an easy copy paste job. Instead I used the following 20 lines of q code to generate the 130 lines of C code:

This generates my C code, a .def file exporting the functions that I want to provide and creates a .q file that loads those functions into kdb. Which means any time I add a function or want to support a new type, everything is done for me. let’s look at our functions in action:

Yes the q code is messy (it could be improved) but the concept of dynamically generating your imports can be a real time saver.

This code is not intended for reliability or performance, use at your own risk! If however bitwise operations are a topic that interest you I recommend this article on: Writing a fast vectorised OR function for Kdb.