base2020/src/Ecs/Lockstep.ts

export const INPUT_FREQUENCY = 33; // roughly 30fps

export interface LockstepProcessor<Input, State> {
    compareInput(a: Input[], b: Input[]): boolean;
    predictInput(prev: Input[] | null, localPlayer: number, localInput: Input): Input[];
    cloneState(source: State): State;
    advanceState(state: State, input: Input[]): void;
}

export class LockstepState<Input, State> {

    private inputIndex = -1;
    private inputLog: Input[][] = [];
    private canonIndex = -1;
    private canonState: State;
    private renderIndex = -1;
    private renderState: State;

    constructor(private initialState: State, private engine: LockstepProcessor<Input, State>) {
        this.canonState = engine.cloneState(initialState);
        this.renderState = engine.cloneState(initialState);
    }

    public addCanonInput(input: Input[]): void {
        this.canonIndex++;

        // advance canonical game state
        this.engine.advanceState(this.canonState, input);

        if(this.canonIndex <= this.renderIndex) {
            // we're rendering predicted states, so if the input changes we need to invalidate the rendered state
            if(!this.engine.compareInput(this.inputLog[this.canonIndex], input)) {
                this.renderState = this.engine.cloneState(this.canonState);
                this.renderIndex = this.canonIndex;
            }
        }
        this.inputLog[this.canonIndex] = input;
    }

    /** Warning: this only supports one player input per frame. There is no support for two players using the same lockstep simulation instance. */
    public addLocalInput(player: number, input: Input): void {
        this.inputIndex++;

        // ensure that we don't overwrite the canon input with local input somehow
        // (probably only possible in situations where game is unplayable anyways? but still for sanity.)
        if(this.inputIndex > this.canonIndex) {
            this.inputLog[this.inputIndex] = this.engine.predictInput(this.inputLog[this.inputIndex - 1] ?? null, player, input);
        }
    }

    /**
     * Do any necessary simulation to catchup the predicted game state to the frame
     * that should be rendered, then return it.
     */
    public getStateToRender(): State {
        // TODO: input lag by X frames
        const targetIndex = this.inputLog.length - 1;

        while(this.renderIndex < targetIndex) {
            this.renderIndex++;
            this.engine.advanceState(this.renderState, this.inputLog[this.renderIndex]);
        }

        return this.renderState;
    }
}

export class Playback<Input, State> {
    private frame = 0;

    public constructor(
        private state: State,
        private inputLog: Input[][],
        private engine: LockstepProcessor<Input, State>,
    ) {}

    public getNextState(): State {
        if(this.frame < this.inputLog.length) {
            this.engine.advanceState(this.state, this.inputLog[this.frame]);
            this.frame++;
        }
        return this.state;
    }
}