Module netcode 

Deterministic Netcode Foundation

Frame-based deterministic simulation wrapper for lockstep/rollback netcode.

Core Principle

Same Inputs + Same Initial State → Bit-Exact Same Output (on all clients)

Because ALICE-Physics uses 128-bit fixed-point arithmetic with fixed iteration counts, the simulation is fully deterministic. This means:

• No state synchronization needed — only sync player inputs
• Bandwidth: ~20 bytes/player/frame instead of ~160 bytes/body/frame
• Rollback: Save/restore snapshots for client-side prediction
• Desync detection: Compare state checksums across clients

Usage

use alice_physics::netcode::{DeterministicSimulation, FrameInput, NetcodeConfig};
use alice_physics::Vec3Fix;

// Both clients create identical simulation
let mut sim = DeterministicSimulation::new(NetcodeConfig::default());

// Each frame: collect inputs from all players, advance simulation
let inputs = vec![
    FrameInput::new(0).with_movement(Vec3Fix::from_int(1, 0, 0)),
    FrameInput::new(1).with_movement(Vec3Fix::from_int(0, 0, -1)),
];

let checksum = sim.advance_frame(&inputs);
// checksum is identical on all clients if inputs match

Bandwidth Model

Approach	Per-Frame Bandwidth (10 bodies)
State sync (traditional)	~1,600 bytes (160B × 10)
Input sync (ALICE)	~40 bytes (20B × 2 players)
Savings	97.5%

Structs

DefaultInputApplicator

Default input applicator: movement → velocity, action bit 0 → jump.

DeterministicSimulation

Frame-based deterministic simulation wrapper.

FrameInput

Player input for a single simulation frame.

NetcodeConfig

Configuration for deterministic netcode simulation

SimulationChecksum

XOR rolling checksum of the entire physics state.

SimulationSnapshot

Complete physics state snapshot for rollback.

Traits

InputApplicator

Trait for mapping player inputs to physics forces/impulses.