RUDPClient/openspec/specs/local-player-reconciliation/spec.md

local-player-reconciliation Specification

Purpose

Define how the local (controlled) player reconciles client-side prediction with server authoritative state. This capability ensures that when the client receives a server PlayerState, it treats that snapshot as the latest gameplay baseline, rebuilds local prediction from still-unacknowledged inputs, and smooths visible presentation toward the rebuilt predicted pose.

Requirements

Requirement: Reconcile applies authoritative state and replays unconfirmed inputs in correct order

The controlled-client reconciliation path SHALL treat a newly accepted authoritative PlayerState as the latest gameplay baseline, prune only pending movement inputs whose tick is less than or equal to AcknowledgedMoveTick, replay all still-unacknowledged pending inputs from that authoritative baseline using fixed-step substeps matching the server authoritative movement cadence, and publish the replay result as the controlled player's latest predicted simulation pose. Presentation smoothing MUST consume that rebuilt predicted pose as a target without redefining the gameplay truth of the replay result.

Scenario: Authoritative acceptance rebuilds prediction from the latest baseline

• WHEN the controlled player accepts an authoritative PlayerState whose acknowledged movement-input tick is N
• THEN the reconciliation treats the received authoritative position and rotation as the new gameplay baseline
• THEN the reconciliation replays all pending inputs with tick greater than N using 50ms fixed-step substeps
• THEN the replay result becomes the controlled player's latest predicted simulation pose for that authoritative update
• THEN the presentation layer receives that predicted pose as its smoothing target

Scenario: Repeated authoritative updates rebuild prediction without consuming pending inputs

• WHEN the controlled player accepts two increasing authoritative PlayerState snapshots before all pending inputs have been acknowledged
• THEN each accepted snapshot rebuilds the predicted simulation pose from its own authoritative baseline
• THEN only inputs acknowledged by the newer snapshot are pruned from the pending-input buffer
• THEN still-unacknowledged inputs remain available for replay against later authoritative snapshots

Scenario: Visible smoothing does not redefine rebuilt gameplay truth

• WHEN the controlled player has a rebuilt predicted simulation pose and a visible presentation pose that has not yet converged
• THEN gameplay logic continues to treat the rebuilt predicted pose as the latest client prediction truth
• THEN the visible pose may temporarily differ while smoothing converges
• THEN a large divergence may still hard-snap the visible pose directly to the rebuilt predicted pose

Requirement: Bounded correction handles residual error after replay

The controlled-client reconciliation SHALL compare the controlled player's visible presentation pose against the rebuilt predicted simulation pose after replay, interpolate the visible pose toward that predicted pose for small residual error, and snap the visible pose directly to the predicted pose when divergence exceeds the configured snap threshold.

Scenario: Small residual error uses presentation interpolation

• WHEN the controlled player completes replay and the remaining distance between visible pose and rebuilt predicted pose is within the configured snap threshold
• THEN the client keeps the rebuilt predicted pose as presentation target
• THEN the visible pose converges toward that target through presentation smoothing across later frames
• THEN the replayed predicted pose remains unchanged as gameplay truth during that convergence

Scenario: Large divergence snaps visible pose to rebuilt predicted pose

• WHEN the controlled player completes replay and the remaining distance between visible pose and rebuilt predicted pose exceeds the configured snap threshold
• THEN the client snaps the visible position and rotation directly to the rebuilt predicted pose
• THEN any previous presentation-only smoothing state is cleared or replaced
• THEN later local prediction continues from the rebuilt predicted baseline