reformat move.ts

move.ts

@@ -4,14 +4,12 @@ import { drawCircle, resizeCanvas } from "./common.js";
 const SEGMENT_SPACING = 100;
 
 function initTrail(pos: Vec2d, len: number) {
-  return Array.from({ length: len }, (_, i) =>
+  return Array.from({ length: len }, (_, i) => pos.add(new Vec2d(-1, 0).scale(i * SEGMENT_SPACING)));
-    pos.add(new Vec2d(0, -1).scale(i * SEGMENT_SPACING)),
-  );
 }
 
 // state
 let target: Vec2d | undefined = undefined;
-let trail = initTrail(new Vec2d(200, 200), 5);
+let trail = initTrail(new Vec2d(500, 500), 5);
 let velocity = new Vec2d(500, 500);
 let pause = false;
 let mode: "follow" | "bounce" | "circle" = "follow";
@@ -23,26 +21,19 @@ function update(ctx: CanvasRenderingContext2D, timestamp: number) {
   lastFrameTime = timestamp;
   if (!pause) {
   switch (mode) {
-      case "bounce":
+        case "bounce": updateBounce(ctx, dt); break;
-        updateBounce(ctx, dt);
+        case "follow": updateFollow(ctx, dt); break;
-        break;
+        case "circle": updateCircle(ctx, dt); break;
-      case "follow":
+        default: throw new Error(`Unknown mode: ${mode}`);
-        updateFollow(ctx, dt);
-        break;
-      case "circle":
-        updateCircle(ctx, dt);
-        break;
-      default:
-        throw new Error(`Unknown mode: ${mode}`);
     }
   }
 
   ctx.clearRect(0, 0, ctx.canvas.width, ctx.canvas.height);
   for (const pos of trail) {
-    drawCircle(ctx, pos, 20, 0xff00ff);
+    drawCircle(ctx, pos, 20, 0xFF00FF);
   }
 
-  window.requestAnimationFrame((t) => update(ctx, t));
+  window.requestAnimationFrame(t => update(ctx, t));
 }
 
 function updateFollow(ctx: CanvasRenderingContext2D, dt: number) {
@@ -52,7 +43,9 @@ function updateFollow(ctx: CanvasRenderingContext2D, dt: number) {
     // frame, so this is framerate dependent:
     // trail[0] = trail[0].lerp(target, 0.1);
     // use exponential decay instead, this is framerate independent:
-    const alpha = Math.exp(-10 * dt);
+    // a = a + (b - a) * t  lerp formula
+    // a = a + (b - a) * e^(-k * dt)  substitute t with e^(-k * dt)
+    const alpha = Math.exp(-10 * dt)
     trail[0] = trail[0].lerp(target, alpha);
     updateTrail();
   }
@@ -62,32 +55,19 @@ function updateBounce(ctx: CanvasRenderingContext2D, dt: number) {
     // update head pos
     // P_t+1 = P_t + V * t
     const newPos = trail[0].add(velocity.scale(dt));
-  if (newPos.x > ctx.canvas.width - 100) {
+    if (newPos.x > ctx.canvas.width - 100) { velocity.x *= -1; newPos.x = ctx.canvas.width - 100; }
-    velocity.x *= -1;
+    if (newPos.y > ctx.canvas.height - 100) { velocity.y *= -1; newPos.y = ctx.canvas.height - 100; }
-    newPos.x = ctx.canvas.width - 100;
+    if (newPos.x < 100) { velocity.x *= -1; newPos.x = 100; }
-  }
+    if (newPos.y < 100) { velocity.y *= -1; newPos.y = 100; }
-  if (newPos.y > ctx.canvas.height - 100) {
-    velocity.y *= -1;
-    newPos.y = ctx.canvas.height - 100;
-  }
-  if (newPos.x < 100) {
-    velocity.x *= -1;
-    newPos.x = 100;
-  }
-  if (newPos.y < 100) {
-    velocity.y *= -1;
-    newPos.y = 100;
-  }
     trail[0] = newPos;
     updateTrail();
 }
 
 function updateCircle(ctx: CanvasRenderingContext2D, dt: number) {
   const center = new Vec2d(ctx.canvas.width / 2, ctx.canvas.height / 2);
-  const rel = trail[0].sub(center);
+  const rel = trail[0].sub(center); // distance from center
-  const radius = rel.length();
+  const { r, theta } = rel.toPolar();
-  const theta = Math.atan2(rel.y, rel.x) + dt;
+  trail[0] = center.add(Vec2d.fromPolar(r, theta + dt));
-  trail[0] = center.add(Vec2d.fromPolar(radius, theta));
   updateTrail();
 }
 
@@ -117,22 +97,22 @@ function init() {
     canvas.onmousemove = (evt) => {
         const {clientX, clientY} = evt;
         target = new Vec2d(clientX, clientY);
-  };
+    }
 
     canvas.onclick = () => {
         pause = !pause;
-  };
+    }
 
     window.onkeydown = (evt) => {
       if (mode === "bounce") mode = "follow";
       else if (mode === "follow") mode = "circle";
       else if (mode === "circle") mode = "bounce";
       console.log(mode);
-  };
+    }
 
-  window.addEventListener("resize", () => resizeCanvas(ctx));
+    window.addEventListener('resize', () => resizeCanvas(ctx));
 
-  window.requestAnimationFrame((t) => update(ctx, t));
+    window.requestAnimationFrame(t => update(ctx, t));
 }
 
 init();

vec.ts

@@ -25,6 +25,13 @@ export default class Vec2d implements Point {
     return `[${this.x}, ${this.y}]`;
   }
 
+  toPolar() {
+    return {
+      r: this.length(),
+      theta: Math.atan2(this.y, this.x),
+    };
+  }
+
   normalize(): Vec2d {
     const len = this.length();
     return new Vec2d(this.x / len, this.y / len);