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Comment by camgunz | Hacker News Reader

This is pretty close to what I built for maintaining demo compatibility in Doom engines. Basically it runs a demo and dumps a save game to a file every frame. As soon as there's a divergence it says what the difference is (monster 17 is at (4, 22); should be (4, 21)) and bails. Not a ton of difference between that and diffing the stack.

https://github.com/camgunz/democomp

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What you are describing sounds like comparing traces of two supposed-to-be-identical programs to see where they first diverge. That's not what this is describing. There is no trace at all, nor any decision about what to trace or what not to trace.

There is only a decision about whether to use the old or new implementation based on the hash of the call stack at that moment. Then you binary search on hash values to identify the exact call stack hash for which the new implementation causes a problem. Then you run the program once more with the instructions "print the stack with this hash".

What you implemented is just checking output against a reference. That's an extremely common testing method.

This is significantly different because it actually changes the program that is being run. It's not common at all - probably because there aren't too many situations where it applies. You need to be processing some large input that you don't understand, and commonly change bits of your code in ways that you can turn on and off dynamically during a single run.

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