FA-87014 / Physics integrator stability / Member archive

RK4 fourth stage advances only half a step · case 04

End-of-step slope is evaluated at the midpoint and trajectories drift.

Member previewVariant 4 · 3 implementations · 8 checks per implementation

Case contract

solve(x0, v0, k, c, F, wf, dt, steps): unit mass x''=-k*x-c*v+F*cos(wf*t) with wf in rad/s, t starting at 0, integrated with classical RK4. Return [x, v, t] rounded to 6.

Why this case matters

Game and robotics physics loops depend on integrator update order, step control and stabilization terms; a wrong decision point turns a stable simulation into drifting or exploding motion.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
case 0	`[0.06492, 1.575427, 0.6]`	`[0.065151, 1.550767, 0.6]`	Failed

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This record includes three runnable implementations, regression fixtures, execution results, and source hashes.

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