Fixed Timestep Loop

Almost every hand-written loop advances physics by the frame's delta. It runs fine at a steady 60fps on the machine it was written on, then physics changes with framerate, one long frame teleports objects through walls, and replays desync. None of it announces itself.

Two guards, because they fail differently. maxStepsPerFrame stops the spiral of death: a slow frame queues extra steps, running them makes the next frame slower, which queues more. The game does not crash, it grinds to a halt while pinning the CPU. maxFrameSeconds stops a restored tab reporting a 600-second frame and queueing 36,000 steps.

Time dropped by either guard is REPORTED, never absorbed. Silently swallowing simulated time is how a multiplayer desync becomes undebuggable.

VERIFIED: 17 tests, and 7 deliberate defects applied to the real source — all 7 caught. The set ships in mutations.json, so you can re-run it. The tests are differential -- the same simulation at different framerates must produce a bit-identical world at equal step counts. Two of them exist because mutation testing found gaps in my own suite.

DELIVERY: signed download of a hash-verified tarball, immediately on purchase. Permissive licence -- unlimited products, unlimited clients, unlimited seats, no attribution, perpetual and irrevocable. One restriction: do not republish the source as source.

Interface

What you call, and what comes back. Types and signatures only — the implementation ships with the source.

  export function lerp(previous: number, current: number, alpha: number): number;