unit interpolation

src/app/components/game/game.component.ts

@@ -29,10 +29,10 @@ export class GameComponent implements OnInit {
 		const initialState: GameState = {
 			tick: 0,
 			units: [{ id: 'u1', x: 0, y: 0 }],
-            commandHistory: []
+			commandHistory: [],
 		};
 
-        const json = await fetch('/assets/gameData.json').then(r => r.json());
+		const json = await fetch('/assets/gameData.json').then((r) => r.json());
 		this.engine = new SimulationEngine(initialState, GameRules, json);
 		requestAnimationFrame(this.gameLoop.bind(this));
 	}
@@ -58,7 +58,7 @@ export class GameComponent implements OnInit {
 	}
 
 	async reloadGameData() {
-        const json = await fetch('/assets/gameData.json').then(r => r.json());
+		const json = await fetch('/assets/gameData.json').then((r) => r.json());
 		this.engine.reinitializeWithNewData(json);
 	}
 
@@ -66,17 +66,55 @@ export class GameComponent implements OnInit {
 		const cmd: Command = {
 			step: this.engine.currentStep + 1,
 			type: 'move',
-          payload: { id: 'u1', dx: 1, dy: 0 }
+			payload: { id: 'u1', dx: 1, dy: 0 },
 		};
 		this.engine.issueCommand(cmd);
 	}
 
-
 	ngAfterViewInit(): void {
 		this.resizeCanvas();
+		requestAnimationFrame(this.render);
 		window.addEventListener('resize', this.resizeCanvas.bind(this));
 	}
 
+	render = (now: number) => {
+		if (!this.engine) return;
+
+		const [prevState, currState] = this.engine.getRenderStates();
+
+		// Compute interpolation factor (0..1)
+		const lastStepTime = this.engine.isRunning ? this.engine.lastStepTime : now;
+		const t = this.engine.isRunning
+			? Math.max(Math.min((now - lastStepTime) / this.engine.interval, 1), 0)
+			: 1; // fully render the last known state
+
+		this.renderFrame(prevState, currState, t);
+
+		requestAnimationFrame(this.render);
+	};
+
+	renderFrame(prev: GameState, curr: GameState, t: number) {
+		const ctx = this.canvasRef.nativeElement.getContext('2d');
+		if (!ctx) return;
+
+		const canvas = this.canvasRef.nativeElement;
+		ctx.clearRect(0, 0, canvas.width, canvas.height);
+
+		for (let i = 0; i < curr.units.length; i++) {
+			const currUnit = curr.units[i];
+			const prevUnit =
+				prev.units.find((u) => u.id === currUnit.id) ?? currUnit;
+
+			const x = prevUnit.x + (currUnit.x - prevUnit.x) * t;
+			const y = prevUnit.y + (currUnit.y - prevUnit.y) * t;
+
+			ctx.fillStyle = 'cyan';
+			ctx.beginPath();
+			ctx.arc(x * 10, y * 10, 5, 0, Math.PI * 2);
+			ctx.fill();
+		}
+	}
+
 	resizeCanvas(): void {
 		const wrapper = this.wrapperRef.nativeElement;
 		const canvas = this.canvasRef.nativeElement;

src/app/components/game/simulationEngine.ts

@@ -4,23 +4,25 @@ import { GameRules } from './gameRules';
 import { GameState } from './gameState';
 
 export class SimulationEngine {
-	private interval = 50; // ms per step
-	private lastTime = 0;
+	interval: number = 50; // ms per step
+	lastTime = 0;
 	private frameRequestId: number | null = null;
 	private data: GameData;
+	private previousState: GameState | null = null;
 
 	currentStep = 0;
 	initialState: GameState;
 	state: GameState;
 	isRunning = false;
+	lastStepTime: number = 0;
 
 	constructor(
 		initialState: GameState,
 		private rules: typeof GameRules,
 		initialData: GameData
 	) {
-		this.initialState = JSON.parse(JSON.stringify(initialState));
-		this.state = JSON.parse(JSON.stringify(initialState));
+		this.initialState = initialState;
+		this.state = this.cloneState(this.initialState);
 		this.data = initialData;
 	}
 
@@ -40,7 +42,9 @@ export class SimulationEngine {
 	}
 
 	fastForwardToEnd() {
-        this.fastForwardTo(this.state.commandHistory[this.state.commandHistory.length - 1].step);
+		this.fastForwardTo(
+			this.state.commandHistory[this.state.commandHistory.length - 1].step
+		);
 	}
 
 	private fastForwardTo(target: number) {
@@ -51,8 +55,10 @@ export class SimulationEngine {
 	}
 
 	step() {
-		this.state = this.rules.step(this.state, this.data);
-		this.currentStep++;
+		this.previousState = this.cloneState(this.state);
+		this.rules.step(this.state, this.data);
+		this.currentStep = this.state.tick;
+		this.lastStepTime = performance.now();
 	}
 
 	start() {
@@ -72,7 +78,8 @@ export class SimulationEngine {
 
 	rewindToZero() {
 		this.stop();
-		this.state = JSON.parse(JSON.stringify(this.initialState));
+		this.state = this.cloneState(this.initialState);
+		this.previousState = this.cloneState(this.state);
 		this.currentStep = 0;
 	}
 
@@ -84,6 +91,18 @@ export class SimulationEngine {
 		this.state.commandHistory.push(command);
 	}
 
+	getRenderStates(): [GameState, GameState] {
+		return [this.previousState ?? this.state, this.state];
+	}
+
+	cloneState(state: GameState): GameState {
+		return {
+			tick: state.tick,
+			commandHistory: [...state.commandHistory],
+			units: state.units.map((u) => ({ ...u })),
+		};
+	}
+
 	private loop = () => {
 		const now = performance.now();
 		while (now - this.lastTime >= this.interval) {

src/assets/manifest.json

@@ -437,6 +437,6 @@
   {
     "path": "manifest.json",
     "type": "other",
-    "size": 7721
+    "size": 7722
   }
 ]