RUDPClient/design.md at 79474b53aabe0ad1fa2bbd8359f9aa2ceda451d5

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Context

Local loopback testing shows controlled-player jitter. One root cause is ReplayPendingInputs() applying each PredictedMoveStep as a single accumulated-duration integration, while live prediction uses FixedUpdate with fixed substeps. This mismatch in integration shape causes trajectory divergence even for identical input sequences.

Tank movement kinematics: heading(t+dt) = heading(t) + turnInput * turnSpeed * dt, position(t+dt) = position(t) + forward(heading(t+dt)) * throttleSpeed * dt. Step-by-step and one-shot integration diverge at larger dt values because each step's heading affects the next step's forward direction.

Goals / Non-Goals

Goals:

ReplayPendingInputs() uses fixed-step accumulation matching server authoritative cadence
Replay produces identical trajectory to live prediction for the same input sequence
No external API changes, only internal integration method modification
Add regression test for replay vs live prediction parity
Add diagnostics for acknowledged move tick, predicted pose, authoritative pose, and correction magnitude

Non-Goals:

Do not modify server 50ms cadence
Do not fix send-interval oscillation (TODO Step 3)
Do not modify visual correction logic (TODO Step 4)

Decisions

Decision: Use server SimulationInterval (50ms) as replay substep size

Choice: Replay in 50ms fixed substeps.

Rationale:

Server integrates at 50ms cadence to produce authoritative state; client replay must match to eliminate偏差
Client FixedUpdate at 20ms is render/physics step, not server simulation granularity
Each PredictedMoveStep.SimulatedDurationSeconds may be 50ms, 100ms, etc.; stepping at 50ms handles all cases

Alternatives:

20ms step: matches client FixedUpdate but not server, still causes偏差
Use SimulatedDurationSeconds as single step: current behavior, causes non-linear divergence

Decision: Substep within ReplayPendingInputs loop without new state

Implementation:

private void ReplayPendingInputs(IReadOnlyList<PredictedMoveStep> replayInputs)
{
    const float serverStepSeconds = 0.05f;  // 50ms server SimulationInterval
    foreach (var replayInput in replayInputs)
    {
        var remaining = replayInput.SimulatedDurationSeconds;
        while (remaining > 0f)
        {
            var step = Mathf.Min(remaining, serverStepSeconds);
            ApplyTankMovementToPredictedState(
                replayInput.Input.TurnInput,
                replayInput.Input.ThrottleInput,
                step);
            remaining -= step;
        }
    }
}