Matt Swanson - Worst. Bug. Ever.

Some bugs are the worst because they cost money. Some because they cost lives.

Others would cite bugs buried deep in a framework or hardware as “the worst”.

For me, the worst kind of bugs are those where the solution, in hindsight, seemed so obvious. You end up more frustrated with the bug after knowing the fix.

I encountered my worst bug during a summer internship after my sophomore year of school. I was helping a research team at Purdue write simulation tools for nanophotonics — I say this not to sound like I was some kind of genius, but to highlight that I was in over my head in a very unfamiliar domain.

A group of research scientists and grad students would work out the math needed to simulate the performance of different nano-scale lenses and I was responsible for wrapping the computations in a web interface and plotting the results.

The team had an existing set of MATLAB scripts that they used internally, but these scripts were hard to modify and distribute. But, on the bright side, I could hook into the MATLAB scripts and leverage their existing work.

When I finally got everything wired up and started comparing the results of a few test cases, they didn’t match. I did my best to debug the MATLAB script, but the math was outside of my comprehension (optics theorems, higher order integrals, and complex numbers). And when I ran the simulation with the same inputs in the stand-alone script, I would get the correct results. Hmm.

The web interface was built on a proprietary framework — it could leverage an entire grid computing cluster as the backend, but wasn’t exactly something that StackOverflow could help with.

After about of week of stepping through the code line by line (even verifying some of the calculations by hand), I finally isolated the section of code where the results diverged.

for i=1:length(LensLayers)
 d[i] = compute_diffraction_at_wavelength(LensLayers[i], WAVELENGTH)
end

It seemed pretty innocuous; loop over an array, perform a calculation on each element, store the result in another array.

Do you see the bug?

Remember when I said there were some PhD-level computations being done? Most of them dealt with complex numbers, which are natively support in MATLAB like so:

x = 2 + 3*i

Figure it out yet?

I was using i as my loop index, but as a side-effect the imaginary constant i in MATLAB was getting overwritten! So 2 + 3*i was evaluating to 5 for the first iteration, 8 for the second, etc. Sigh.

Changing the loop variable name immediately fixed the problem and the results became correct (an alternate solution is to use 3i instead of 3*i).

To this day, I’ve never run across another bug with such a frustratingly obvious solution.

It may have taken three weeks to solve, but at least I got a good “Worst. Bug. Ever.” story out of it.