Writing test-code

One of the tasks, that in my knowledge is too often neglected (well you know I do this!), is writing test-code for new functionality.

Recently in my post on an O’Caml rant, I was complaining how O’Caml made it impossible for me to do the “simplest of all computer operations” – sleep. I simply wanted to make my CPU(s) idle for a while after drawing some visualization graphics, in order to produce a fluid animation, and it seemed hopeless. I just went on and implemented (the dreaded) busy-loop. 🙂

This test-case, even though it provided visual confirmation that my dynamic friction model (well the first option anyways – I have 2 others to check still) was ok, but it was lacking something important: Error analysis – the Final Frontier for programmers, where even the uber-nerds wont go without some serious recommending.

There are a few easy things we can check out in these sort of cases:

1) Distance from constraint manifolds (for both position and velocity variables)
2) Constraint force (I actually projected the total force vector to constraint-manifold normal. subtracted gravity before projection in order to measure directly the force felt by particle due to constraint)
3) Friction force (should depend linearly on the constraint force)

Computing these values turned out to be already a task, but visualizing them over time turned out even larger task – here’s the gif-animation:

Animating particle sliding on the sine curve

Of course the classical “O’Caml list-reverse” happened, so that the plot is like a polygraph, lie-detector type of curve (For me this happens all the time with O’Caml lists – and of course any iteration through the list inverts it! :))

But furthermore the results seem good, except one tiny thing: The friction force seems to diverge when going through zero-velocity of the particle – how very odd, no? I spent already some time verifying the math – it all matches, so what’s wrong? I know my friction model in this case is not perfect, but the discrepancy should not be so noticeable.

Well, the (yet again too obvious) solution was that this model had only implemented dynamical friction!

    Rest friction not yet implemented!!

. D’Oh! 🙂 The classical – without rest friction the plot is supposed to look something like that!!!

So well, moving on to implement rest friction and then to try the two other friction models! I’ll be back with more cool things! … Like more code… And physics.. and math.. and.. Uh.. Yeah – I know – super cool!Well, I am having a glass of wine while coding now, so, I guess I’m not completely hopeless, yet? Ok? And if I play a tad on my guitar?



2 Responses to “Writing test-code”

  1. […] Maxpower3141's Blog Stuff about computers, physics and what not – the usual rants « Writing test-code […]

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