feat: basic client prediction

app/client/interpolator.js

@@ -38,7 +38,7 @@ export default class Interpolator {
       return undefined;
     }
     this.location += elapsed;
-    const fraction = this.location / this.duration;
+    const fraction = Math.min(1, this.location / this.duration);
     const [from, to] = [this.penultimate.payload.ecs, this.latest.payload.ecs];
     const interpolated = {};
     for (const {entityId, componentName, properties} of this.tracking) {

app/client/local.js

@@ -1,21 +1,51 @@
 import Client from '@/net/client.js';
 import {decode, encode} from '@/net/packets/index.js';
+import {CLIENT_INTERPOLATION, CLIENT_PREDICTION} from '@/util/constants.js';
 
 export default class LocalClient extends Client {
   server = null;
   interpolator = null;
+  predictor = null;
   async connect() {
     this.server = new Worker(
       new URL('../server/worker.js', import.meta.url),
       {type: 'module'},
     );
-    this.interpolator = new Worker(
-      new URL('./interpolator.js', import.meta.url),
-      {type: 'module'},
-    );
-    this.interpolator.addEventListener('message', (event) => {
-      this.accept(event.data);
-    });
+    if (CLIENT_INTERPOLATION) {
+      this.interpolator = new Worker(
+        new URL('./interpolator.js', import.meta.url),
+        {type: 'module'},
+      );
+      this.interpolator.addEventListener('message', (event) => {
+        this.accept(event.data);
+      });
+    }
+    if (CLIENT_PREDICTION) {
+      this.predictor = new Worker(
+        new URL('./predictor.js', import.meta.url),
+        {type: 'module'},
+      );
+      this.predictor.addEventListener('message', (event) => {
+        const [flow, packet] = event.data;
+        switch (flow) {
+          case 0: {
+            const packed = encode(packet);
+            this.throughput.$$up += packed.byteLength;
+            this.server.postMessage(packed);
+            break;
+          }
+          case 1: {
+            if (CLIENT_INTERPOLATION) {
+              this.interpolator.postMessage(packet);
+            }
+            else {
+              this.accept(packet);
+            }
+            break;
+          }
+        }
+      });
+    }
     this.server.addEventListener('message', (event) => {
       if (0 === event.data) {
         this.server.terminate();
@@ -23,16 +53,32 @@ export default class LocalClient extends Client {
         return;
       }
       this.throughput.$$down += event.data.byteLength;
-      this.interpolator.postMessage(decode(event.data));
+      const packet = decode(event.data);
+      if (CLIENT_PREDICTION) {
+        this.predictor.postMessage([1, packet]);
+      }
+      else if (CLIENT_INTERPOLATION) {
+        this.interpolator.postMessage(packet);
+      }
+      else {
+        this.accept(packet);
+      }
     });
   }
   disconnect() {
     this.server.postMessage(0);
-    this.interpolator.terminate();
+    if (CLIENT_INTERPOLATION) {
+      this.interpolator.terminate();
+    }
   }
   transmit(packet) {
-    const packed = encode(packet);
-    this.throughput.$$up += packed.byteLength;
-    this.server.postMessage(packed);
+    if (CLIENT_PREDICTION) {
+      this.predictor.postMessage([0, packet]);
+    }
+    else {
+      const packed = encode(packet);
+      this.throughput.$$up += packed.byteLength;
+      this.server.postMessage(packed);
+    }
   }
 }

app/client/predictor.js

@@ -0,0 +1,172 @@
+import {LRUCache} from 'lru-cache';
+
+import Components from '@/ecs/components/index.js';
+import Ecs from '@/ecs/ecs.js';
+import Systems from '@/ecs/systems/index.js';
+import {withResolvers} from '@/util/promise.js';
+
+const cache = new LRUCache({
+  max: 128,
+});
+
+class PredictionEcs extends Ecs {
+  async readAsset(uri) {
+    if (!cache.has(uri)) {
+      const {promise, resolve, reject} = withResolvers();
+      cache.set(uri, promise);
+      fetch(uri)
+        .then((response) => resolve(response.ok ? response.arrayBuffer() : new ArrayBuffer(0)))
+        .catch(reject);
+    }
+    return cache.get(uri);
+  }
+}
+
+const Flow = {
+  UP: 0,
+  DOWN: 1,
+};
+
+const Stage = {
+  UNACK: 0,
+  ACK: 1,
+  FINISHING: 2,
+  FINISHED: 3,
+};
+
+const actions = new Map();
+
+let ecs = new PredictionEcs({Components, Systems});
+
+let mainEntityId = 0;
+
+function applyClientActions(elapsed) {
+  if (actions.size > 0) {
+    const main = ecs.get(mainEntityId);
+    const {Controlled} = main;
+    const finished = [];
+    for (const [id, action] of actions) {
+      if (Stage.UNACK === action.stage) {
+        if (!Controlled.locked) {
+          switch (action.action.type) {
+            case 'moveUp':
+            case 'moveRight':
+            case 'moveDown':
+            case 'moveLeft': {
+              Controlled[action.action.type] = action.action.value;
+              break;
+            }
+          }
+        }
+        action.steps.push(elapsed);
+      }
+      if (Stage.FINISHING === action.stage) {
+        if (!Controlled.locked) {
+          switch (action.action.type) {
+            case 'moveUp':
+            case 'moveRight':
+            case 'moveDown':
+            case 'moveLeft': {
+              Controlled[action.action.type] = 0;
+              break;
+            }
+          }
+        }
+        action.stage = Stage.FINISHED;
+      }
+      if (Stage.FINISHED === action.stage) {
+        action.steps.shift();
+        if (0 === action.steps.length) {
+          finished.push(id);
+          continue;
+        }
+      }
+      let leap = 0;
+      for (const step of action.steps) {
+        leap += step;
+      }
+      if (leap > 0) {
+        ecs.predict(main, leap);
+      }
+    }
+    for (const id of finished) {
+      actions.delete(id);
+    }
+  }
+}
+
+let downPromise;
+
+const pending = new Map();
+
+onmessage = async (event) => {
+  const [flow, packet] = event.data;
+  switch (flow) {
+    case Flow.UP: {
+      switch (packet.type) {
+        case 'Action': {
+          switch (packet.payload.type) {
+            case 'moveUp':
+            case 'moveRight':
+            case 'moveDown':
+            case 'moveLeft': {
+              if (0 === packet.payload.value) {
+                const ack = pending.get(packet.payload.type);
+                const action = actions.get(ack);
+                action.stage = Stage.FINISHING;
+                pending.delete(packet.payload.type);
+              }
+              else {
+                const tx = {
+                  action: packet.payload,
+                  stage: Stage.UNACK,
+                  steps: [],
+                };
+                packet.payload.ack = Math.random();
+                pending.set(packet.payload.type, packet.payload.ack);
+                actions.set(packet.payload.ack, tx);
+              }
+            }
+          }
+          break;
+        }
+      }
+      postMessage([0, packet]);
+      break;
+    }
+    case Flow.DOWN: {
+      downPromise = Promise.resolve(downPromise).then(async () => {
+        switch (packet.type) {
+          case 'ActionAck': {
+            const action = actions.get(packet.payload.ack);
+            action.stage = Stage.ACK;
+            return;
+          }
+          case 'Tick': {
+            for (const entityId in packet.payload.ecs) {
+              if (packet.payload.ecs[entityId]) {
+                if (packet.payload.ecs[entityId].MainEntity) {
+                  mainEntityId = parseInt(entityId);
+                }
+              }
+            }
+            await ecs.apply(packet.payload.ecs);
+            if (actions.size > 0) {
+              const main = ecs.get(mainEntityId);
+              const authoritative = structuredClone(main.toNet(main));
+              applyClientActions(packet.payload.elapsed);
+              if (ecs.diff[mainEntityId]) {
+                packet.payload.ecs[mainEntityId] = ecs.diff[mainEntityId];
+              }
+              await ecs.apply({[mainEntityId]: authoritative});
+            }
+            ecs.setClean();
+            break;
+          }
+        }
+        postMessage([1, packet]);
+      });
+      break;
+    }
+  }
+};

app/client/remote.js

@@ -1,22 +1,66 @@
 import Client from '@/net/client.js';
 import {decode, encode} from '@/net/packets/index.js';
+import {CLIENT_INTERPOLATION, CLIENT_PREDICTION} from '@/util/constants.js';
 
 export default class RemoteClient extends Client {
   socket = null;
   interpolator = null;
+  predictor = null;
   async connect(host) {
     this.interpolator = new Worker(
       new URL('./interpolator.js', import.meta.url),
       {type: 'module'},
     );
-    this.interpolator.addEventListener('message', (event) => {
-      this.accept(event.data);
-    });
+    if (CLIENT_INTERPOLATION) {
+      this.interpolator = new Worker(
+        new URL('./interpolator.js', import.meta.url),
+        {type: 'module'},
+      );
+      this.interpolator.addEventListener('message', (event) => {
+        this.accept(event.data);
+      });
+    }
+    if (CLIENT_PREDICTION) {
+      this.predictor = new Worker(
+        new URL('./predictor.js', import.meta.url),
+        {type: 'module'},
+      );
+      this.predictor.addEventListener('message', (event) => {
+        const [flow, packet] = event.data;
+        switch (flow) {
+          case 0: {
+            const packed = encode(packet);
+            this.throughput.$$up += packed.byteLength;
+            this.socket.send(packed);
+            break;
+          }
+          case 1: {
+            if (CLIENT_INTERPOLATION) {
+              this.interpolator.postMessage(packet);
+            }
+            else {
+              this.accept(packet);
+            }
+            break;
+          }
+        }
+      });
+    }
     const url = new URL(`wss://${host}/ws`)
     this.socket = new WebSocket(url.href);
     this.socket.binaryType = 'arraybuffer';
     this.socket.addEventListener('message', (event) => {
-      this.interpolator.postMessage(decode(event.data));
+      this.throughput.$$down += event.data.byteLength;
+      const packet = decode(event.data);
+      if (CLIENT_PREDICTION) {
+        this.predictor.postMessage([1, packet]);
+      }
+      else if (CLIENT_INTERPOLATION) {
+        this.interpolator.postMessage(packet);
+      }
+      else {
+        this.accept(packet);
+      }
     });
     this.socket.addEventListener('close', () => {
       this.accept({type: 'ConnectionStatus', payload: 'aborted'});
@@ -27,11 +71,18 @@ export default class RemoteClient extends Client {
     this.accept({type: 'ConnectionStatus', payload: 'connected'});
   }
   disconnect() {
-    this.interpolator.terminate();
+    if (CLIENT_INTERPOLATION) {
+      this.interpolator.terminate();
+    }
   }
   transmit(packet) {
-    const packed = encode(packet);
-    this.throughput.$$up += packed.byteLength;
-    this.socket.send(packed);
+    if (CLIENT_PREDICTION) {
+      this.predictor.postMessage([0, packet]);
+    }
+    else {
+      const packed = encode(packet);
+      this.throughput.$$up += packed.byteLength;
+      this.socket.send(packed);
+    }
   }
 }

app/ecs/component.js

@@ -144,11 +144,22 @@ export default class Component {
         }
         Component.ecs.markChange(this.entity, {[Component.constructor.componentName]: values})
       }
+      toFullJSON() {
+        const {properties} = concrete;
+        const json = {};
+        for (const key in properties) {
+          json[key] = this[key];
+        }
+        return json;
+      }
       toNet(recipient, data) {
-        return data || Component.constructor.filterDefaults(this);
+        if (data) {
+          return data;
+        }
+        return this.toFullJSON();
       }
       toJSON() {
-        return Component.constructor.filterDefaults(this);
+        return this.toFullJSON();
       }
       async update(values) {
         for (const key in values) {

app/ecs/ecs.js

@@ -408,6 +408,16 @@ export default class Ecs {
     }
   }
 
+  predict(entity, elapsed) {
+    for (const systemName in this.Systems) {
+      const System = this.Systems[systemName];
+      if (!System.predict) {
+        continue;
+      }
+      System.predict(entity, elapsed);
+    }
+  }
+
   async readJson(uri) {
     const key = ['$$json', uri].join(':');
     if (!cache.has(key)) {

app/ecs/systems/apply-control-movement.js

@@ -3,6 +3,10 @@ import {normalizeVector} from '@/util/math.js';
 
 export default class ApplyControlMovement extends System {
 
+  predict(entity) {
+    this.tickSingle(entity);
+  }
+
   static get priority() {
     return {
       before: 'IntegratePhysics',
@@ -16,17 +20,25 @@ export default class ApplyControlMovement extends System {
   }
 
   tick() {
-    for (const {Controlled, Forces, Speed} of this.select('default')) {
-      if (!Controlled.locked) {
-        const movement = normalizeVector({
-          x: (Controlled.moveRight - Controlled.moveLeft),
-          y: (Controlled.moveDown - Controlled.moveUp),
-        });
-        Forces.applyImpulse({
-          x: Speed.speed * movement.x,
-          y: Speed.speed * movement.y,
-        });
-      }
+    for (const entity of this.select('default')) {
+      this.tickSingle(entity);
+    }
+  }
+
+  tickSingle(entity) {
+    const {Controlled, Forces, Speed} = entity;
+    if (!Controlled || !Forces | !Speed) {
+      return;
+    }
+    if (!Controlled.locked) {
+      const movement = normalizeVector({
+        x: (Controlled.moveRight - Controlled.moveLeft),
+        y: (Controlled.moveDown - Controlled.moveUp),
+      });
+      Forces.applyImpulse({
+        x: Speed.speed * movement.x,
+        y: Speed.speed * movement.y,
+      });
     }
   }
 

app/ecs/systems/control-direction.js

@@ -3,27 +3,39 @@ import {TAU} from '@/util/math.js';
 
 export default class ControlDirection extends System {
 
+  predict(entity) {
+    this.tickSingle(entity);
+  }
+
   tick() {
-    for (const {Controlled, Direction} of this.ecs.changed(['Controlled'])) {
-      const {locked, moveUp, moveRight, moveDown, moveLeft} = Controlled;
-      if (locked) {
-        continue;
-      }
-      if (
-        0 === moveRight
-        && 0 === moveDown
-        && 0 === moveLeft
-        && 0 === moveUp
-      ) {
-        continue;
-      }
-      Direction.direction = (
-        TAU + Math.atan2(
-          moveDown - moveUp,
-          moveRight - moveLeft,
-        )
-      ) % TAU;
+    for (const entity of this.ecs.changed(['Controlled'])) {
+      this.tickSingle(entity);
     }
   }
 
+  tickSingle(entity) {
+    const {Controlled, Direction} = entity;
+    if (!Controlled || !Direction) {
+      return;
+    }
+    const {locked, moveUp, moveRight, moveDown, moveLeft} = Controlled;
+    if (locked) {
+      return;
+    }
+    if (
+      0 === moveRight
+      && 0 === moveDown
+      && 0 === moveLeft
+      && 0 === moveUp
+    ) {
+      return;
+    }
+    Direction.direction = (
+      TAU + Math.atan2(
+        moveDown - moveUp,
+        moveRight - moveLeft,
+      )
+    ) % TAU;
+  }
+
 }

app/ecs/systems/follow-camera.js

@@ -5,6 +5,10 @@ import {System} from '@/ecs/index.js';
 
 export default class FollowCamera extends System {
 
+  predict(entity, elapsed) {
+    this.tickSingle(entity, elapsed);
+  }
+
   static get priority() {
     return {
       after: 'IntegratePhysics',
@@ -25,11 +29,15 @@ export default class FollowCamera extends System {
   }
 
   tick(elapsed) {
-    for (const {id} of this.select('default')) {
-      this.updateCamera(elapsed * 3, this.ecs.get(id));
+    for (const entity of this.select('default')) {
+      this.tickSingle(entity, elapsed);
     }
   }
 
+  tickSingle(entity, elapsed) {
+    this.updateCamera(elapsed * 3, entity);
+  }
+
   updateCamera(portion, entity) {
     const {Camera, Position} = entity;
     if (Camera && Position) {

app/ecs/systems/integrate-physics.js

@@ -2,6 +2,10 @@ import {System} from '@/ecs/index.js';
 
 export default class IntegratePhysics extends System {
 
+  predict(entity, elapsed) {
+    this.tickSingle(entity, elapsed);
+  }
+
   static queries() {
     return {
       default: ['Position', 'Forces'],
@@ -9,10 +13,18 @@ export default class IntegratePhysics extends System {
   }
 
   tick(elapsed) {
-    for (const {Position, Forces} of this.select('default')) {
-      Position.x = Position.$$x + elapsed * (Forces.$$impulseX + Forces.$$forceX);
-      Position.y = Position.$$y + elapsed * (Forces.$$impulseY + Forces.$$forceY);
+    for (const entity of this.select('default')) {
+      this.tickSingle(entity, elapsed);
     }
   }
 
+  tickSingle(entity, elapsed) {
+    const {Forces, Position} = entity;
+    if (!Forces || !Position) {
+      return;
+    }
+    Position.x = Position.$$x + elapsed * (Forces.$$impulseX + Forces.$$forceX);
+    Position.y = Position.$$y + elapsed * (Forces.$$impulseY + Forces.$$forceY);
+  }
+
 }

app/ecs/systems/reset-forces.js

@@ -2,6 +2,10 @@ import {System} from '@/ecs/index.js';
 
 export default class ResetForces extends System {
 
+  predict(entity, elapsed) {
+    this.tickSingle(entity, elapsed);
+  }
+
   static get priority() {
     return {phase: 'post'};
   }
@@ -13,24 +17,32 @@ export default class ResetForces extends System {
   }
 
   tick(elapsed) {
-    for (const {Forces} of this.select('default')) {
-      if (0 !== Forces.forceX) {
-        const factorX = Math.pow(1 - Forces.dampingX, elapsed);
-        Forces.forceX *= factorX;
-        if (Math.abs(Forces.forceX) <= 1) {
-          Forces.forceX = 0;
-        }
-      }
-      if (0 !== Forces.forceY) {
-        const factorY = Math.pow(1 - Forces.dampingY, elapsed);
-        Forces.forceY *= factorY;
-        if (Math.abs(Forces.forceY) <= 1) {
-          Forces.forceY = 0;
-        }
-      }
-      Forces.impulseX = 0;
-      Forces.impulseY = 0;
+    for (const entity of this.select('default')) {
+      this.tickSingle(entity, elapsed);
     }
   }
 
+  tickSingle(entity, elapsed) {
+    const {Forces} = entity;
+    if (!Forces) {
+      return;
+    }
+    if (0 !== Forces.forceX) {
+      const factorX = Math.pow(1 - Forces.dampingX, elapsed);
+      Forces.forceX *= factorX;
+      if (Math.abs(Forces.forceX) <= 1) {
+        Forces.forceX = 0;
+      }
+    }
+    if (0 !== Forces.forceY) {
+      const factorY = Math.pow(1 - Forces.dampingY, elapsed);
+      Forces.forceY *= factorY;
+      if (Math.abs(Forces.forceY) <= 1) {
+        Forces.forceY = 0;
+      }
+    }
+    Forces.impulseX = 0;
+    Forces.impulseY = 0;
+  }
+
 }

app/ecs/systems/run-animations.js

@@ -2,6 +2,10 @@ import {System} from '@/ecs/index.js';
 
 export default class RunAnimations extends System {
 
+  predict(entity, elapsed) {
+    this.tickSingle(entity, elapsed);
+  }
+
   static queries() {
     return {
       default: ['Sprite'],
@@ -9,15 +13,23 @@ export default class RunAnimations extends System {
   }
 
   tick(elapsed) {
-    for (const {Sprite} of this.select('default')) {
-      if (0 === Sprite.speed || !Sprite.isAnimating) {
-        continue;
-      }
-      Sprite.elapsed += elapsed / Sprite.speed;
-      while (Sprite.elapsed > 1) {
-        Sprite.elapsed -= 1;
-        Sprite.frame += 1;
-      }
+    for (const entity of this.select('default')) {
+      this.tickSingle(entity, elapsed);
+    }
+  }
+
+  tickSingle(entity, elapsed) {
+    const {Sprite} = entity;
+    if (!Sprite) {
+      return;
+    }
+    if (0 === Sprite.speed || !Sprite.isAnimating) {
+      return;
+    }
+    Sprite.elapsed += elapsed / Sprite.speed;
+    while (Sprite.elapsed >= 1) {
+      Sprite.elapsed -= 1;
+      Sprite.frame += 1;
     }
   }
 

app/ecs/systems/sprite-direction.js

@@ -2,6 +2,10 @@ import {System} from '@/ecs/index.js';
 
 export default class SpriteDirection extends System {
 
+  predict(entity) {
+    this.tickSingle(entity);
+  }
+
   static get priority() {
     return {
       after: 'ControlDirection',
@@ -15,35 +19,43 @@ export default class SpriteDirection extends System {
   }
 
   tick() {
-    for (const {Controlled, Direction, Sprite} of this.select('default')) {
-      const parts = [];
-      if (Controlled) {
-        const {locked, moveUp, moveRight, moveDown, moveLeft} = Controlled;
-        if (locked) {
-          continue;
-        }
-        if ((moveUp > 0 || moveRight > 0 || moveDown > 0 || moveLeft > 0)) {
-          parts.push('moving');
-        }
-        else {
-          parts.push('idle');
-        }
+    for (const entity of this.select('default')) {
+      this.tickSingle(entity);
+    }
+  }
+
+  tickSingle(entity) {
+    const parts = [];
+    const {Controlled, Direction, Sprite} = entity;
+    if (!Sprite) {
+      return;
+    }
+    if (Controlled) {
+      const {locked, moveUp, moveRight, moveDown, moveLeft} = Controlled;
+      if (locked) {
+        return;
       }
-      if (Direction) {
-        if (!Sprite.rotates) {
-          const name = {
-            0: 'right',
-            1: 'down',
-            2: 'left',
-            3: 'up',
-          };
-          parts.push(name[Direction.quantize(4)]);
-        }
+      if ((moveUp > 0 || moveRight > 0 || moveDown > 0 || moveLeft > 0)) {
+        parts.push('moving');
       }
-      if (parts.length > 0) {
-        if (Sprite.hasAnimation(parts.join(':'))) {
-          Sprite.animation = parts.join(':');
-        }
+      else {
+        parts.push('idle');
+      }
+    }
+    if (Direction) {
+      if (!Sprite.rotates) {
+        const name = {
+          0: 'right',
+          1: 'down',
+          2: 'left',
+          3: 'up',
+        };
+        parts.push(name[Direction.quantize(4)]);
+      }
+    }
+    if (parts.length > 0) {
+      if (Sprite.hasAnimation(parts.join(':'))) {
+        Sprite.animation = parts.join(':');
       }
     }
   }

app/server/engine.js

@@ -425,7 +425,7 @@ export default class Engine {
         this.update(this.updateElapsed);
         this.setClean();
         this.frame += 1;
-        this.updateElapsed -= UPS_PER_S;
+        this.updateElapsed = this.updateElapsed % UPS_PER_S;
       }
       this.handle = setTimeout(loop, 1000 / TPS);
     };

app/util/constants.js

@@ -2,6 +2,8 @@ export const CHUNK_SIZE = 32;
 
 export const CLIENT_LATENCY = 0;
 
+export const CLIENT_INTERPOLATION = true;
+
 export const CLIENT_PREDICTION = true;
 
 export const IRL_MINUTES_PER_GAME_DAY = 20;
@@ -15,4 +17,4 @@ export const SERVER_LATENCY = 0;
 
 export const TPS = 60;
 
-export const UPS = 30;
+export const UPS = 15;