RewindableRandomNumberGenerator

A random number generator that can be used inside the rollback tick loop.

An important point for writing code that works well in rollback is that it behaves the same given the same circumstances, no matter how many times it's run. It also must behave the same no matter which peer is running it.

Godot's built-in RandomNumberGenerator is not aware of rollback, so it will consistently return different numbers for each tick, and potentially on each peer.

To fix that, the RewindableRandomNumberGenerator generates the same random sequences on each peer, and for each tick.

It implements most of the same methods, so it's close to a drop-in replacement.

Creating the generator

The RewindableRandomNumberGenerator requires a seed upon initialization. This seed must be the same on all peers.

For RNG's used by singletons, hard-coding different seed values is a very simple approach:

var rng := RewindableRandomNumberGenerator.new(15)

If these objects also have names or similar, consistent identifiers, a simple hash works well too:

var rng := RewindableRandomNumberGenerator.new(hash("Exit beacon"))

For dynamically spawned objects, or just to avoid the possibility of a human error, the node path can be hashed:

var rng := RewindableRandomNumberGenerator.new(hash(get_path()))

This assumes that the same node will be spawned under the same path on all peers, which is also a requirement for RPCs to work.

Generating random numbers

The RewindableRandomNumberGenerator can be used in the same way as Godot's built-in RandomNumberGenerator. All the per-peer and per-tick consistency is ensured under the hood:

var rng := RewindableRandomNumberGenerator.new(0)

var dice_roll := rng.randi_range(1, 6)