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Using Lambda expressions to step through algorithms

Published:: 2009-03-22 23:02:15
Posted from:: Home
Tags:: C#, Simulation,

Debugging number-crunching algorithms can be hard, and when the algorithm actually ends up as something visual (as in computer graphics), it feels somewhat awkward to break the executing program and use the debugger to look at numeric data.

To debug my rigid body simulator I wanted a way to break/step/continue through the algorithm while still running the graphics loop so that I can inspect the collisions visually. I came up with a nice way to do this in C# using Action delegates and Lambda expressions.

This is the original algorithm (simplified here as it is not the focus of this blog post):

for (int i=0; i<5; i++) {
  advance(true,false,false);
  detectCollisions();
  collisionResponse();
}
advance(true,true,false);
detectCollisions();
contactResolution();
advance(true,true,false);

What I did was that I encapsulated each call in a lambda expression and stored them in a list:

Action [] myAlgorithm = new Action []{
  () => i = 0,
  () => advance(true,false,false),
  () => detectCollisions(),
  () => collisionResponse(),
  () => { if (++i < 5) executionPointer = 1; },
  () => advance(true,true,false),
  () => detectCollisions(),
  () => contactResolution(),
  () => advance(true,true,false),
};

Then I wrote a simple class with a few methods for executing this sequence. A method called Run which runs the whole sequence once (from the current position), a method Step which just executes the next statement (perhaps I should call it method or even expression, but it’s really just a wrapped statement).

AlgorithmStepper stepper = new AlgorithmStepper();
stepper.Algorithm = myAlgorithm;
...
while (Running) {
  renderScene();
  if (UserPressedAMagicButton)
    stepper.Step();
  if (UserPressedAnotherMagicButton)
    stepper.Run();
}

This way I can have a fully interactive application, with a frozen simulation in it. I can visualize the current state of objects, their velocities and any found collision points, collision normals and penetration depths, and just press a button to execute the next part of the algorithm (to see what is going wrong…).

One problem here is that I need to step a whole lot of times before something interesting happens. To make things a little bit easier I added a third method to the AlgorithmStepper class, namely RunToBreak(), which runs the algorithm until a fourth method Break() is called. I could then add a conditional break to the algorithm:

Action [] myAlgorithm = new Action []{
  () => i = 0,
  () => advance(true,false,false),
  () => detectCollisions(),
  () => collisionResponse(),
  () => if (CollisionsFound) stepper.Break();
  () => { if (++i < 5) executionPointer = 1; },
  () => advance(true,true,false),
  () => detectCollisions(),
  () => contactResolution(),
  () => advance(true,true,false),
};

This does not in any way replace looking at numerical data, but it has been a really nice complement on the road to understand problems in the simulation. And yet another reason to be happy I made this in C#.

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Using Lambda expressions to step through algorithms

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