Parallel Interface

I left a terrible bug in version 1.3 of my replacement Gina. I didn’t want to miss any test case this time, so I wrote a batch file that tests every one of them. That batch file adds a user to a group and a group to the registry. There are two possible groups in the registry, and the user can be a member of either two groups, making 2^(2+2) possibilities, 16 use cases.

After a few lines in, I realized that I would be less work to order the tests in a way that would minimize the change to the configuration between any two tests. In other words, when a n+1 test case required a change to the registry, then the user group membership should not change, and vice-versa. That would also make it easy to investigate a failed test, because only one thing would change between any two tests.

Then it hit me.

Well actually, I had to stop and think for a while. Kind of like my mind restoring a dusty old tape archive… I remembered that mathematician Richard Hamming had a code for that. It’s a numbering scheme where only 1 bit changes between any two numbers. The number of bits that change is the Hamming distance between two numbers. Using four information bits to represent each possible use case, I came up with the following table. The first two rows (MSB, in blue) are user membership to a group, and the two last rows (LSB, in green) is the presence of that group in the registry. Ordering my tests that way gave me a constant Hamming distance of 1.

The only drawback to this is that all the typing I saved writing my test batch file, I wasted on this blog post !