Jia Tan with the XZ backdoor was caught because some performance obsessed person noticed a tiny delay... I'm sure they learned their lesson and are ensuring their next backdoor doesn't impact performance.
That's a fascinating parallel! both involve separating timeless constraints (value function / elliptic equation) from temporal dynamics (policy / flux evolution).
Trying to timestep through a constraint that should hold instantaneously creates artificial numerical difficulties. The Bellman equation's value iteration is actually another example of this same pattern...
This aligns with what I've observed in computational physics.
Trying to handle instantaneous constraints and propagating modes with a single timestepper is often suboptimal.
I developed a framework that systematically separates these components: using direct elliptic solvers for constraints and explicit methods for flux evolution. The resulting algorithms are both more stable and more efficient than unified implicit approaches.
The key insight is that many systems (EM, GR, fluids, quantum mechanics) share the same pattern:
- An elliptic constraint equation (solved directly, not timestepped)
- A continuity law for charge/mass/probability flux
- Wave-like degrees of freedom (handled with explicit methods)
Witgh this structure, you can avoid the stiffness issues entirely rather than trying to power through them with implicit methods.
https://github.com/MidnightBSD/src/commit/7d956a27123f2d77a0...