How to Fix Fixed-Point Determinism Drift in Lockstep Simulation

A lockstep RTS where two clients gradually disagree about unit positions is suffering from float nondeterminism. Moving the simulation to fixed-point math keeps every machine bit-identical. Here is how.

How to fix it

1. Use a fixed-point type

Represent positions and velocities as integers scaled by a fixed factor (for example 1/65536) and do arithmetic on the integers. Integer math is exact and identical on every platform.

2. Avoid platform-variant functions

Standard sin, cos, and sqrt can differ across libraries. Use deterministic table-based or fixed-point implementations shared by all clients.

3. Keep floats out of the simulation

Use floats only for rendering and interpolation, never for anything that affects simulation state, so divergence cannot enter the authoritative game logic.

Catching the ones you can't reproduce

The hardest version of this to fix is the one you can't reproduce — it only happens on a player's hardware, OS, driver, or save state, under conditions that simply aren't present on your machine. A report that says “it crashed” or “it froze” gives you nothing to act on, so the bug survives release after release while quietly costing you players.

Automatic error capture closes that gap. Each failure arrives with its full stack trace, the device and OS, the build number, and a breadcrumb trail of what the player did right before it broke, so even a failure you have never seen becomes a specific, reproducible issue. Fold identical failures into one signature ranked by how many players each hits, and your worklist sorts itself worst-first instead of arriving as a stream of vague complaints.

This is where a tool like Bugnet earns its place. Its SDK captures every error automatically with the full stack trace plus device, OS, memory, build, and game-state context, folds duplicates into one grouped issue with an occurrence count, and ties each to the build it first appeared on — so you fix the problem that hurts the most players first and confirm it is gone when its signature disappears from the next release.