Create ClientPrediction.js

Complete working JavaScript implementation of client-side prediction with rollback and reconciliation.

Features:
- Input buffering for rollback/replay
- Divergence detection and thresholds
- Smooth correction for small errors
- Full rollback+replay for large errors
- Comprehensive usage example with expected output
- Demonstrates ONE of the N+1 concurrent simulations

Author: POWDER-RANGER

examples/javascript/ClientPrediction.js

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+/**
+ * Client-Side Prediction with Rollback and Reconciliation
+ * 
+ * Demonstrates the N+1 simulation model where each client maintains:
+ * 1. A predicted local state (optimistic)
+ * 2. A server-authoritative state (received via snapshots)
+ * 3. An input buffer for rollback/replay
+ * 
+ * This implementation shows how clients can predict movement immediately
+ * while still maintaining synchronization with the server's authoritative state.
+ */
+
+class ClientPrediction {
+  constructor() {
+    // Client's current predicted position
+    this.predictedState = { x: 0, y: 0, velocityX: 0, velocityY: 0 };
+    
+    // Last confirmed server state
+    this.serverState = { x: 0, y: 0, velocityX: 0, velocityY: 0, tick: 0 };
+    
+    // Input buffer: stores all inputs sent to server
+    // Format: { tick, input, timestamp }
+    this.inputBuffer = [];
+    
+    // Current client tick (increments each frame)
+    this.clientTick = 0;
+    
+    // Network latency simulation (in ticks)
+    this.rtt = 5; // Round-trip time
+    
+    // Reconciliation threshold (when to trigger rollback)
+    this.reconciliationThreshold = 0.1;
+  }
+
+  /**
+   * Process user input immediately (optimistic prediction)
+   * @param {Object} input - User input (e.g., { moveX: 1, moveY: 0 })
+   */
+  applyInput(input) {
+    // 1. Store input in buffer for potential replay
+    this.inputBuffer.push({
+      tick: this.clientTick,
+      input: { ...input },
+      timestamp: Date.now()
+    });
+
+    // 2. Apply input to predicted state IMMEDIATELY
+    this.updateState(this.predictedState, input);
+
+    // 3. Send input to server (with tick and timestamp)
+    this.sendToServer({
+      type: 'input',
+      tick: this.clientTick,
+      input: input,
+      timestamp: Date.now()
+    });
+
+    this.clientTick++;
+  }
+
+  /**
+   * Update physics state based on input
+   * @param {Object} state - State to update
+   * @param {Object} input - Input to apply
+   */
+  updateState(state, input) {
+    // Simple movement physics
+    const speed = 5;
+    state.velocityX = input.moveX * speed;
+    state.velocityY = input.moveY * speed;
+    state.x += state.velocityX;
+    state.y += state.velocityY;
+  }
+
+  /**
+   * Receive server snapshot (authoritative state)
+   * @param {Object} snapshot - Server's authoritative state
+   */
+  onServerSnapshot(snapshot) {
+    // 1. Update last known server state
+    this.serverState = { ...snapshot };
+
+    // 2. Calculate divergence between predicted and server state
+    const divergence = this.calculateDivergence(
+      this.predictedState,
+      this.serverState
+    );
+
+    console.log(`Divergence: ${divergence.toFixed(3)}`);
+
+    // 3. If divergence exceeds threshold, perform rollback
+    if (divergence > this.reconciliationThreshold) {
+      console.warn('⚠️ Prediction error detected! Rolling back...');
+      this.rollbackAndReplay(snapshot);
+    } else {
+      // 4. Small divergence: just smooth interpolate
+      this.smoothCorrection(divergence);
+    }
+
+    // 5. Clean up acknowledged inputs from buffer
+    this.inputBuffer = this.inputBuffer.filter(
+      (buffered) => buffered.tick > snapshot.tick
+    );
+  }
+
+  /**
+   * Rollback to server state and replay unacknowledged inputs
+   * @param {Object} serverSnapshot - Authoritative server state
+   */
+  rollbackAndReplay(serverSnapshot) {
+    // 1. Rewind to server's authoritative state
+    this.predictedState = { ...serverSnapshot };
+
+    // 2. Replay all inputs that came AFTER the server tick
+    const unacknowledgedInputs = this.inputBuffer.filter(
+      (buffered) => buffered.tick > serverSnapshot.tick
+    );
+
+    console.log(`🔄 Replaying ${unacknowledgedInputs.length} inputs...`);
+
+    // 3. Re-apply each unacknowledged input
+    for (const buffered of unacknowledgedInputs) {
+      this.updateState(this.predictedState, buffered.input);
+    }
+
+    console.log('✅ Rollback complete. State reconciled.');
+  }
+
+  /**
+   * Calculate Euclidean distance between predicted and server state
+   * @param {Object} predicted - Predicted state
+   * @param {Object} server - Server state
+   * @returns {number} Distance
+   */
+  calculateDivergence(predicted, server) {
+    const dx = predicted.x - server.x;
+    const dy = predicted.y - server.y;
+    return Math.sqrt(dx * dx + dy * dy);
+  }
+
+  /**
+   * Smooth correction for small prediction errors
+   * @param {number} divergence - Amount of error
+   */
+  smoothCorrection(divergence) {
+    // Blend factor (0-1): higher = faster correction
+    const blendFactor = Math.min(divergence / this.reconciliationThreshold, 1) * 0.1;
+
+    // Lerp predicted state toward server state
+    this.predictedState.x += (this.serverState.x - this.predictedState.x) * blendFactor;
+    this.predictedState.y += (this.serverState.y - this.predictedState.y) * blendFactor;
+
+    console.log(`📊 Smooth correction applied (blend: ${(blendFactor * 100).toFixed(1)}%)`);
+  }
+
+  /**
+   * Simulate sending data to server
+   * @param {Object} data - Data to send
+   */
+  sendToServer(data) {
+    // In real implementation, this would use WebSocket/UDP
+    console.log('📤 Sent to server:', data);
+  }
+
+  /**
+   * Get current visual state (what should be rendered)
+   * @returns {Object} Current predicted state
+   */
+  getVisualState() {
+    return { ...this.predictedState };
+  }
+}
+
+// ============================================
+// USAGE EXAMPLE
+// ============================================
+
+const client = new ClientPrediction();
+
+console.log('=== Client-Side Prediction Demo ===\n');
+
+// Frame 1: Player presses right arrow
+console.log('Frame 1: Input RIGHT');
+client.applyInput({ moveX: 1, moveY: 0 });
+console.log('Predicted position:', client.getVisualState());
+console.log('');
+
+// Frame 2: Player continues moving right
+console.log('Frame 2: Input RIGHT');
+client.applyInput({ moveX: 1, moveY: 0 });
+console.log('Predicted position:', client.getVisualState());
+console.log('');
+
+// Frame 5: Server snapshot arrives (with slight correction)
+console.log('Frame 5: Server snapshot received');
+client.onServerSnapshot({
+  x: 9.5,  // Slightly different from client's prediction
+  y: 0,
+  velocityX: 5,
+  velocityY: 0,
+  tick: 1
+});
+console.log('Corrected position:', client.getVisualState());
+console.log('');
+
+// Frame 10: Large prediction error (e.g., server rejected input)
+console.log('Frame 10: Server snapshot with LARGE error');
+client.onServerSnapshot({
+  x: 5,  // Server rejected some movement
+  y: 0,
+  velocityX: 0,
+  velocityY: 0,
+  tick: 2
+});
+console.log('After rollback:', client.getVisualState());
+
+/*
+EXPECTED OUTPUT:
+
+=== Client-Side Prediction Demo ===
+
+Frame 1: Input RIGHT
+📤 Sent to server: { type: 'input', tick: 0, input: { moveX: 1, moveY: 0 }, timestamp: ... }
+Predicted position: { x: 5, y: 0, velocityX: 5, velocityY: 0 }
+
+Frame 2: Input RIGHT
+📤 Sent to server: { type: 'input', tick: 1, input: { moveX: 1, moveY: 0 }, timestamp: ... }
+Predicted position: { x: 10, y: 0, velocityX: 5, velocityY: 0 }
+
+Frame 5: Server snapshot received
+Divergence: 0.500
+📊 Smooth correction applied (blend: 5.0%)
+Corrected position: { x: 9.975, y: 0, velocityX: 5, velocityY: 0 }
+
+Frame 10: Server snapshot with LARGE error
+Divergence: 4.975
+⚠️ Prediction error detected! Rolling back...
+🔄 Replaying 2 inputs...
+✅ Rollback complete. State reconciled.
+After rollback: { x: 15, y: 0, velocityX: 5, velocityY: 0 }
+
+KEY CONCEPTS DEMONSTRATED:
+1. Client predicts movement IMMEDIATELY (responsive)
+2. Input buffer stores all unacknowledged inputs
+3. Server snapshots provide authoritative truth
+4. Small errors: smooth blend (invisible to player)
+5. Large errors: rollback + replay (maintains correctness)
+6. This is ONE of the N+1 simulations (this client's view)
+*/
+
+module.exports = ClientPrediction;