eslint

.gitignore

@@ -1 +1,2 @@
 *.sw[po]
+node_modules/

UNLICENSE

@@ -0,0 +1,25 @@
+This is free and unencumbered software released into the public domain.
+
+Anyone is free to copy, modify, publish, use, compile, sell, or
+distribute this software, either in source code form or as a compiled
+binary, for any purpose, commercial or non-commercial, and by any
+means.
+
+In jurisdictions that recognize copyright laws, the author or authors
+of this software dedicate any and all copyright interest in the
+software to the public domain. We make this dedication for the benefit
+of the public at large and to the detriment of our heirs and
+successors. We intend this dedication to be an overt act of
+relinquishment in perpetuity of all present and future rights to this
+software under copyright law.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
+OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
+ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+OTHER DEALINGS IN THE SOFTWARE.
+
+For more information, please refer to <https://unlicense.org/>
+

config.js

@@ -1,3 +1,4 @@
+// DISPLAY
 export const DISPLAY_OBJECTS_INFO = false;
 export const DISPLAY_CURSOR_INFO = false;
 export const DISPLAY_CANVAS_SIZE = false;
@@ -5,36 +6,54 @@ export const DISPLAY_CURRENT_SCALE = true;
 export const DISPLAY_CURRENT_MODE = false;
 export const DISPLAY_VELOCITY_VECTORS = true;
 export const DISPLAY_ACCELERATION_VECTORS = true;
+export const DISPLAY_PATH_TRACES = false;
 
-export const MASS_CREATION_RATE = 1E1;
-export const POINTER_HISTORY_SIZE = 15;
-
+// VELOCITY
 export const VELOCITY_VECTOR_SCALE = 5E0;
 export const VELOCITY_VECTOR_COLOR = 'rgb(150, 150, 150)'; // optionally set to 'object color'
 export const VELOCITY_VECTOR_WIDTH = 1.5;
 export const VELOCITY_VECTOR_ARROWHEAD = true;
 
+// ACCELERATION
 export const ACCELERATION_VECTOR_SCALE = 5E0;
-export const ACCELERATION_VECTOR_COLOR = 'rgb(0, 255, 0)'; // optionally set to 'object color'
-// export const ACCELERATION_VECTOR_COLOR = 'object color';
+export const ACCELERATION_VECTOR_COLOR = 'rgb(0, 128, 0)'; // optionally set to 'object color'
 export const ACCELERATION_VECTOR_WIDTH = 1.5;
 export const ACCELERATION_VECTOR_ARROWHEAD = true;
 
-export const MOTION_TIME_SCALE = 1E-4;
-export const PAN_VELOCITY_SCALE_FACTOR = 1E-3;
+// PATH TRACES
+// export const PATH_TRACES_COLOR = 'rgb(128, 128, 0)'; // optionally set to 'object color'
+export const PATH_TRACES_COLOR = 'object color';
+export const PATH_TRACES_WIDTH = 1.5;
 
+// SCALING FACTORS
+export const MASS_CREATION_RATE = 1E1;
+export const POINTER_HISTORY_SIZE = 15;
+export const MOTION_TIME_SCALE = 1E-4;
+export const PAN_ACCELERATION = 1E1;
 export const ARROWHEAD_LENGTH = 7;
 export const ARROWHEAD_WIDTH = 5;
 export const OFFSCREEN_OBJECT_LINE_SCALE = 7;
 export const OFFSCREEN_OBJECT_LINE_WIDTH = 2;
 export const OFFSCREEN_OBJECT_ARROWHEAD_LENGTH = 15;
+export const ZOOM_TO_FIT_PADDING = 50;
 export const ZOOM_IN_FACTOR = 1;
 export const ZOOM_OUT_FACTOR = -1;
 export const SCALE_POWER_MAX = 8;
 export const SCALE_POWER_MIN = -8; 
 export const GRAVITATIONAL_CONSTANT = 1E5;
 
+// CSS CLASS NAMES
 export const DRAGGABLE_ELEMENT_CLASSNAME = 'lhg-draggable-element';
 
+// EVENT NAMES
+export const EVENT_MODE_LEAVE = 'lhg-mode-leave';
+export const EVENT_MODE_ENTER = 'lhg-mode-enter';
+
+// MODES
 export const MODE_MASS_GENERATION = 'mass-gen';
 export const MODE_PAN_VIEW = 'pan-view';
+
+// OPTIONS
+export const PAUSE_DURING_CREATION = true;
+export const PAUSE_DURING_SELECTION = true;
+export const MERGE_ON_COLLIDE = true;

display.js

@@ -13,8 +13,8 @@ import {
   OFFSCREEN_OBJECT_LINE_WIDTH,
   OFFSCREEN_OBJECT_ARROWHEAD_LENGTH,
   DISPLAY_CANVAS_SIZE,
-  DISPLAY_VELOCITY_VECTORS,
-  DISPLAY_ACCELERATION_VECTORS,
+  PATH_TRACES_COLOR,
+  PATH_TRACES_WIDTH,
 } from './config.js';
 
 export class Display {
@@ -63,6 +63,13 @@ export class Display {
     };
   }
 
+  simToScreen(x, y) {
+    return {
+      x: (x - this.viewOrigin.x) * this.scale,
+      y: (y - this.viewOrigin.y) * this.scale,
+    };
+  }
+
   fullscreen() {
     this.canvas.width = document.documentElement.clientWidth;
     this.canvas.height = document.documentElement.clientHeight;
@@ -77,8 +84,7 @@ export class Display {
     ctx.fillRect(this.viewOrigin.x, this.viewOrigin.y, this.width, this.height);
   }
 
-  drawObject(idx) {
-    const obj = this.sim.objects.object(idx);
+  drawObject(obj) {
     const ctx = this.ctx;
     const {r, g, b} = obj.color;
     const {x, y} = obj.position;
@@ -90,6 +96,47 @@ export class Display {
     const oy = this.viewOrigin.y;
     const cx = ox + W / 2;
     const cy = oy + H / 2;
+
+    // Draw path traces
+    if (this.sim.getOption('display.traces') && obj.history?.length) {
+      ctx.strokeStyle = PATH_TRACES_COLOR === 'object color' ? 
+         `rgb(${r}, ${g}, ${b})` : PATH_TRACES_COLOR;
+      ctx.lineWidth = PATH_TRACES_WIDTH / this.scale;
+      ctx.beginPath();
+      let dash = false;
+      const skip = 1;
+      let skipped = 0;
+      {
+        const [{position}] = obj.history;
+        const x = position.x;
+        const y = position.y;
+
+        if (Math.abs(x - cx) <= W / 2 &&
+            Math.abs(y - cy) <= H / 2) {
+          ctx.moveTo(x, y);
+          dash = true;
+        }
+      }
+      for (let i = 1; i < obj.history.length; i++) {
+        if (++skipped < skip) continue;
+        skipped = 0;
+        const {position} = obj.history[i];
+        const x = position.x;
+        const y = position.y;
+        if (dash) {
+          ctx.lineTo(x, y);
+          dash = false;
+        } else if (Math.abs(x - cx) <= W / 2 &&
+            Math.abs(y - cy) <= H / 2) {
+          ctx.moveTo(x, y);
+          dash = true;
+        }
+      }
+      ctx.stroke();
+    }
+
+    if (!obj.alive) return;
+
     // If the object is outside the display area, draw an arrow at the edge of the display
     if (Math.abs(x - cx) - radius >= W / 2 ||
         Math.abs(y - cy) - radius >= H / 2) {
@@ -146,7 +193,7 @@ export class Display {
     ctx.fill();
 
     // Draw arrow for the velocity
-    if (DISPLAY_VELOCITY_VECTORS) {
+    if (this.sim.getOption('display.velocity')) {
       const speed = Math.sqrt(vx ** 2 + vy ** 2);
       const endVx = x + VELOCITY_VECTOR_SCALE * vx / speed * Math.log(speed);
       const endVy = y + VELOCITY_VECTOR_SCALE * vy / speed * Math.log(speed);
@@ -162,7 +209,7 @@ export class Display {
     }
 
     // Draw arrow for acceleration
-    if (DISPLAY_ACCELERATION_VECTORS) {
+    if (this.sim.getOption('display.acceleration')) {
       const accelerationMagnitude = Math.sqrt(acceleration.x ** 2 + acceleration.y ** 2);
       const endAx = x + ACCELERATION_VECTOR_SCALE * acceleration.x /
         accelerationMagnitude * Math.log(accelerationMagnitude);
@@ -181,9 +228,7 @@ export class Display {
   }
 
   drawObjects() {
-    for (let i = 0; i < this.sim.objects.length; i++) {
-      this.drawObject(i);
-    }
+    this.sim.objects.forEachObject(obj => this.drawObject(obj), null);
   }
 
   drawArrow(startX, startY, endX, endY, {style, width, arrowhead, arrowheadLength, fill, ifShort}) {

eslint.config.mjs

@@ -0,0 +1,7 @@
+import js from "@eslint/js";
+import globals from "globals";
+import { defineConfig } from "eslint/config";
+
+export default defineConfig([
+  { files: ["**/*.{js,mjs,cjs}"], plugins: { js }, extends: ["js/recommended"], languageOptions: { globals: globals.browser } },
+]);

index.html

@@ -8,7 +8,7 @@ body {
   background-color: #000;
   color: #8f8;
   font-family: monospace;
-  font-size: 16pt;
+  font-size: 12pt;
   overflow: hidden;
   user-select: none;
   -webkit-user-select: none;
@@ -24,7 +24,9 @@ div[id=simulator] {
 button {
   width: 8em;
   padding-left: 0.5em;
-  padding-left: 0.5em;
+  padding-right: 0.5em;
+  padding-top: 0;
+  padding-bottom: 0;
 }
 
 button.wide {

object.js

@@ -1,5 +1,4 @@
 export class MassObject {
-  index = undefined;
   mass = 0;
   density = 1;
   position = {x: undefined, y: undefined};
@@ -8,10 +7,10 @@ export class MassObject {
   color = {r: undefined, g: undefined, b: undefined};
   created = undefined;
   forces = []; // [{x, y}]
-  active = false;
+  history = [];
+  alive = true;
 
-  constructor(x, y, index) {
-    this.index = index;
+  constructor(x, y) {
     this.position.x = x;
     this.position.y = y;
     this.color.r = Math.random() * 256;

objects.js

@@ -2,32 +2,62 @@ import { MassObject } from './object.js';
 import { 
   MASS_CREATION_RATE, 
   DISPLAY_OBJECTS_INFO,
-  MOTION_TIME_SCALE,
   GRAVITATIONAL_CONSTANT,
+  ZOOM_TO_FIT_PADDING,
 } from './config.js';
 
 export class Objects {
   objects = [];
   creatingObject = undefined;
   selectedObject = undefined;
+  paused = false;
+  panVelocityPaused = undefined;
 
   constructor(sim) {
     this.sim = sim;
   }
 
+  pause() {
+    if (this.sim.playing) {
+      this.sim.playing = false;
+      this.paused = true;
+      if (this.sim.pointer.panning?.velocity) {
+        this.panVelocityPaused = {...this.sim.pointer.panning.velocity};
+        this.sim.pointer.panning = undefined;
+      }
+    }
+  }
+
+  resume() {
+    if (this.paused) {
+      this.sim.playing = true;
+      this.paused = false;
+      if (this.panVelocityPaused) {
+        this.sim.pointer.panning = {
+          velocity: {...this.panVelocityPaused},
+        };
+        this.panVelocityPaused = undefined;
+      }
+    }
+  }
+
   // Create an object with mass that grows as pointer is held down
   createObject(x, y) {
     const idx = this.objects.length;
     const obj = new MassObject(x, y, idx);
     this.creatingObject = idx;
     this.objects.push(obj);
+    // Pause the simulation during mass creation; this avoids some complex local dynamics
+    // TODO: Make this optional (toolbar item to enable)
+    if (this.sim.getOption('pauseDuring.creation')) {
+      this.pause();
+    }
   }
 
   doneCreatingObject() {
     if (this.creatingObject !== undefined) {
-      const obj = this.objects[this.creatingObject];
-      obj.active = true;
       this.creatingObject = undefined;
+      this.resume();
     }
   }
 
@@ -37,10 +67,14 @@ export class Objects {
 
   selectObject(i) {
     this.selectedObject = i;
+    if (this.sim.getOption('pauseDuring.selection')) {
+      this.pause();
+    }
   }
 
   deselect() {
     this.selectedObject = undefined;
+    this.resume();
   }
 
   getSelectedOrCreating() {
@@ -59,8 +93,7 @@ export class Objects {
       start: {x: undefined, y: undefined}, 
       end: {x: undefined, y: undefined},
     };
-    for (let i = 0; i < this.objects.length; i++) {
-      const obj = this.objects[i];
+    this.forEachObject(obj => {
       if (box.start.x === undefined) {
         box.start = {...obj.position};
         box.end = {...obj.position};
@@ -70,19 +103,25 @@ export class Objects {
         if (obj.position.y < box.start.y) box.start.y = obj.position.y;
         if (obj.position.y > box.end.y) box.end.y = obj.position.y;
       }
-    }
+    });
+    box.start.x -= ZOOM_TO_FIT_PADDING;
+    box.start.y -= ZOOM_TO_FIT_PADDING;
+    box.end.x += ZOOM_TO_FIT_PADDING;
+    box.end.y += ZOOM_TO_FIT_PADDING;
     return box;
   }
 
   objectAtLocation(x, y) {
-    for (let i = 0; i < this.objects.length; i++) {
-      const obj = this.objects[i];
+    let idx = undefined;
+    this.forEachObject((obj, i) => {
       // If distance to object is less than object's radius, we are touching the object
       const dist = Math.pow((obj.position.x - x)**2 + (obj.position.y - y)**2, 1/2);
       if (dist <= obj.radius) {
-        return i;
+        idx = i;
+        return null;
       }
-    }
+    });
+    return idx;
   }
 
   handlePointerDown({x, y}) {
@@ -97,7 +136,7 @@ export class Objects {
     }
   }
 
-  handlePointerUp({x, y}) {
+  handlePointerUp() {
     this.doneCreatingObject();
     this.deselect();
   }
@@ -106,6 +145,10 @@ export class Objects {
     // If the cursor moves while creating an object, or while an object is selected,
     // update the position and velocity of the object
     const obj = this.getSelectedOrCreating();
+    if (this.sim.pointer.panning?.velocity) {
+      vx += this.sim.pointer.panning.velocity.x;
+      vy += this.sim.pointer.panning.velocity.y;
+    }
     if (obj !== undefined) {
       obj.position.x = x;
       obj.position.y = y;
@@ -114,16 +157,25 @@ export class Objects {
     }
   }
 
+  // cb: (obj, idx) => {}
+  // TODO: Reducer
+  forEachObject(cb, alive = true, startWith = 0) {
+    for (let i = startWith; i < this.objects.length; i++) {
+      const obj = this.objects[i];
+      if (alive === null || alive == obj.alive) {
+        const ret = cb(obj, i);
+        if (ret === null) break;
+      }
+    }
+  }
+
   computeForces() {
     if (this.objects.length < 2) return;
-    for (let i = 0; i < this.objects.length; i++) {
-      this.objects[i].forces = [];
-    }
-    for (let i = 0; i < this.objects.length; i++) {
-      const A = this.objects[i];
-      for (let j = i + 1; j < this.objects.length; j++) {
-        const B = this.objects[j];
-        if (!A.active || !B.active) continue;
+    this.forEachObject(obj => {
+      obj.forces = [];
+    });
+    this.forEachObject((A, i) => {
+      this.forEachObject(B => {
         const dx = (B.position.x - A.position.x);
         const dy = (B.position.y - A.position.y);
         const dSquared = dx ** 2 + dy ** 2;
@@ -133,12 +185,15 @@ export class Objects {
         const Fy = F * dy / d;
         A.forces.push({ x: Fx, y: Fy });
         B.forces.push({ x: -Fx, y: -Fy });
-      }
-    }
+      }, true, i + 1);
+    });
+    // Also compute acceleration
+    this.forEachObject(obj => {
+      obj.acceleration = obj.getAcceleration();
+    });
   }
 
   computeFrame(elapsedTime) {
-    elapsedTime = elapsedTime * MOTION_TIME_SCALE;
     // If we're creating an object, increment its mass
     // with the mass creation rate accelerating over time
     if (this.creatingObject !== undefined) {
@@ -151,51 +206,86 @@ export class Objects {
     this.computeForces();
 
     if (this.sim.playing) {
-      // Generate predicted positions (Velocity verlet method)
-      for (let i = 0; i < this.objects.length; i++) {
-        const obj = this.objects[i];
-        obj.currentAcceleration = obj.getAcceleration();
+      // Predict positions (Velocity verlet method)
+      this.forEachObject(obj => {
+        obj.currentAcceleration = {...obj.acceleration};
+
         obj.position.x += elapsedTime *
             (obj.velocity.x + 1/2 * obj.currentAcceleration.x * elapsedTime);
         obj.position.y += elapsedTime *
             (obj.velocity.y + 1/2 * obj.currentAcceleration.y * elapsedTime);
+      });
+
+      // Collisions
+      if (this.sim.getOption('collision.merge')) {
+        this.forEachObject((A, i) => {
+          this.forEachObject((B, j) => {
+            const dx = (B.position.x - A.position.x);
+            const dy = (B.position.y - A.position.y);
+            const dSquared = dx ** 2 + dy ** 2;
+            const d = Math.sqrt(dSquared);
+            if (d < A.radius + B.radius) {
+              // Merge B into A:
+              // Set position = center of mass 
+              // Set velocity = total momentum / total mass
+              // Combine forces
+              // Add masses
+              // Average color
+              // If B was selected or being created, select A instead
+              // Remove B using splice
+              A.position.x = (A.position.x * A.mass + B.position.x * B.mass) / (A.mass + B.mass);
+              A.position.y = (A.position.y * A.mass + B.position.y * B.mass) / (A.mass + B.mass);
+              A.velocity.x = (A.velocity.x * A.mass + B.velocity.x * B.mass) / (A.mass + B.mass);
+              A.velocity.y = (A.velocity.y * A.mass + B.velocity.y * B.mass) / (A.mass + B.mass);
+              A.forces.push(...B.forces);
+              A.mass += B.mass;
+              A.color = {
+                r: (A.mass * A.color.r + B.mass * B.color.r) / (A.mass + B.mass),
+                g: (A.mass * A.color.g + B.mass * B.color.g) / (A.mass + B.mass),
+                b: (A.mass * A.color.b + B.mass * B.color.b) / (A.mass + B.mass),
+              };
+              if (this.creatingObject === j) this.creatingObject = i;
+              if (this.selectedObject === j) this.selectedObject = i;
+              B.alive = false;
+              B.forces = [];
+            }
+          }, true, i + 1);
+        });
       }
       
       // Recompute forces
       this.computeForces();
 
-      // Generated predicted velocities
-      for (let i = 0; i < this.objects.length; i++) {
-        const obj = this.objects[i];
-        const acceleration = obj.getAcceleration();
+      // Predict velocities
+      this.forEachObject(obj => {
+        const acceleration = {...obj.acceleration};
         obj.acceleration = {
           x: (obj.currentAcceleration.x + acceleration.x) / 2,
           y: (obj.currentAcceleration.y + acceleration.y) / 2,
         };
         obj.velocity.x += obj.acceleration.x * elapsedTime;
         obj.velocity.y += obj.acceleration.y * elapsedTime;
-      }
-    } else {
-      for (let i = 0; i < this.objects.length; i++) {
-        const obj = this.objects[i];
-        obj.acceleration = obj.getAcceleration();
-      }
+
+        // Append to object history
+        // TODO: enforce object history length
+        // TODO: store object color changes in history
+        obj.history.push({position: {...obj.position}});
+      });
     }
 
     // Display objects info
     if (DISPLAY_OBJECTS_INFO) {
-      for (let i = 0; i < this.objects.length; i++) {
-        const obj = this.objects[i];
+      this.forEachObject((obj, i) => {
         const speed = Math.pow(obj.velocity.x ** 2 + obj.velocity.y ** 2, 1/2);
         // Invert y so that the angle is counterclockwise from x-axis
         const direction = Math.atan2(-obj.velocity.y, obj.velocity.x) * 180 / Math.PI;
         this.sim.info[`Object ${i}`] = [
-          `${obj.position.x}, `, 
-          `${obj.position.y}, `, 
+          `${obj.position.x.toPrecision(6)}, `, 
+          `${obj.position.y.toPrecision(6)}, `, 
           `${obj.mass.toPrecision(6)} kg, `,
           `${speed.toPrecision(2)} m/s, ${direction.toPrecision(2)}°`, 
         ];
-      }
+      });
     }
   }
 }

overlay.js

@@ -10,6 +10,7 @@ export class Overlay {
     this.sim.div.appendChild(infoBox);
     this.infoBox = infoBox;
     infoBox.style.position = 'relative';
+    infoBox.style.display = 'inline-block';
     infoBox.style.top = 0;
     infoBox.style.left = 0;
     infoBox.width = 'fit-content';

package.json

@@ -0,0 +1,21 @@
+{
+  "name": "gravity",
+  "version": "0.2.0",
+  "description": "Gravity Simulator",
+  "main": "simulator.js",
+  "scripts": {
+    "test": "echo \"Error: no test specified\" && exit 1",
+    "lint": "npx eslint"
+  },
+  "repository": {
+    "type": "git",
+    "url": "git@git.laddhoffman.com:lentil/gravity.git"
+  },
+  "author": "Ladd Hoffman <lentil@laddhoffman.com>",
+  "license": "Unlicense",
+  "devDependencies": {
+    "@eslint/js": "^9.39.2",
+    "eslint": "^9.39.2",
+    "globals": "^16.5.0"
+  }
+}

pointer.js

@@ -1,19 +1,13 @@
 import {
-  POINTER_HISTORY_SIZE,
-  ZOOM_IN_FACTOR,
-  ZOOM_OUT_FACTOR,
   DISPLAY_CURSOR_INFO,
   DRAGGABLE_ELEMENT_CLASSNAME,
   MODE_MASS_GENERATION,
   MODE_PAN_VIEW,
-  PAN_VELOCITY_SCALE_FACTOR,
+  POINTER_HISTORY_SIZE,
+  ZOOM_IN_FACTOR,
+  ZOOM_OUT_FACTOR,
 } from './config.js';
 
-function dispatchEvent(target, eventType, data) {
-  const ev = new CustomEvent(eventType, {detail: data});
-  target.dispatchEvent(ev);
-}
-
 export class Pointer {
   sim = undefined;
 
@@ -43,7 +37,7 @@ export class Pointer {
         this.handlePointerMove({x: e.clientX, y: e.clientY});
       }
     });
-   
+
     el.addEventListener('pointerdown', e => {
       // If this is a child of a draggable element, handle dragging
       let target = e.target;
@@ -85,10 +79,10 @@ export class Pointer {
 
     // Monitor wheel events
     el.addEventListener('wheel', e => {
-      const factor = e.deltaY > 0 ? ZOOM_IN_FACTOR : ZOOM_OUT_FACTOR;
-      this.sim.scheduleZoom({x: e.clientX, y: e.clientY}, factor);
+      const factor = e.deltaY < 0 ? ZOOM_IN_FACTOR : ZOOM_OUT_FACTOR;
+      const {x, y} = this.sim.screenToSim(e.clientX, e.clientY);
+      this.sim.scheduleZoom({x, y}, factor);
     });
-
   }
 
   getPointerVelocity() {
@@ -99,9 +93,25 @@ export class Pointer {
     const start = this.pointerHistory[0];
     const end = this.pointerHistory[this.pointerHistory.length - 1];
     const dt = (end.t - start.t) / 1000;
+    // Bonus scale factor for pointer power
     return {
-      x: (end.x - start.x) / dt,
-      y: (end.y - start.y) / dt,
+      x: (end.x - start.x) / dt * this.sim.display.scale,
+      y: (end.y - start.y) / dt * this.sim.display.scale,
+      dt
+    };
+  }
+
+  getPointerAcceleration() {
+    // Average over pointer history
+    if (this.pointerHistory.length < 2) {
+      return {x: 0, y: 0, dt: 1};
+    }
+    const start = this.pointerHistory[0];
+    const end = this.pointerHistory[this.pointerHistory.length - 1];
+    const dt = (end.t - start.t) / 1000;
+    return {
+      x: (end.v.x - start.v.x) / dt,
+      y: (end.v.y - start.v.y) / dt,
       dt
     };
   }
@@ -112,25 +122,25 @@ export class Pointer {
 
   updatePointer({x, y}) {
     const t = document.timeline.currentTime;
-    this.pointerHistory.push({x, y, t});
-    if (this.pointerHistory.length > POINTER_HISTORY_SIZE) {
+    while (this.pointerHistory.length >= POINTER_HISTORY_SIZE) {
       this.pointerHistory.shift();
     }
+    const v = this.getPointerVelocity();
+    const a = this.getPointerAcceleration();
+    this.pointerHistory.push({t, x, y, v, a});
   }
 
   handlePointerDown({x: clientX, y: clientY}) {
+    this.updatePointer({x: clientX, y: clientY});
     if (this.sim.isCurrentMode(MODE_MASS_GENERATION)) {
+      // this.panning = undefined;
       const {x, y} = this.sim.screenToSim(clientX, clientY)
       this.sim.objects.handlePointerDown({x, y});
 
     } else if (this.sim.isCurrentMode(MODE_PAN_VIEW)) {
-      this.panning = {
-        gathering: true,
-        viewOriginStart: this.sim.display.viewOrigin,
-        pointerStart: {x: clientX, y: clientY},
-        pointerCurrent: {x: clientX, y: clientY},
-        velocity: this.getPointerVelocity(),
-      };
+      this.panning = this.panning || {};
+      this.panning.gathering = true;
+      this.panning.velocity = {x: 0, y: 0};
     }
   }
 
@@ -145,14 +155,14 @@ export class Pointer {
       if (this.panning?.gathering) {
         this.panning.gathering = false;
       }
-    } 
+    }
   }
 
   // Handle cursor (mouse or touch) movement
   // TODO: If e.touches.length > 1, user may be engaging pinch to zoom
   handlePointerMove({x: clientX, y: clientY}) {
     if (this.sim.isCurrentMode(MODE_MASS_GENERATION)) {
-      this.updatePointer({ x: clientX, y: clientY });
+      this.updatePointer({x: clientX, y: clientY});
       const {x, y} = this.sim.screenToSim(clientX, clientY);
       const velocity = this.getPointerVelocity();
       // Convert pointer velocity to sim internal scale
@@ -162,9 +172,13 @@ export class Pointer {
 
     } else if (this.sim.isCurrentMode(MODE_PAN_VIEW)) {
       if (this.panning?.gathering) {
-        this.updatePointer({ x: clientX, y: clientY });
-        this.panning.pointerCurrent = {x: clientX, y: clientY};
-        this.panning.velocity = this.getPointerVelocity();
+        this.updatePointer({x: clientX, y: clientY});
+        const velocity = this.getPointerVelocity();
+        const acceleration = this.getPointerAcceleration();
+        // Convet to sim coordinates
+        // Let's try incorporating pointer acceleration
+        this.panning.velocity.x = velocity.x + acceleration.x * velocity.dt;
+        this.panning.velocity.y = velocity.y + acceleration.y * velocity.dt;
       }
     }
   }
@@ -177,20 +191,11 @@ export class Pointer {
     }
 
     // Apply update to viewOrigin based on panning
-    if (!this.sim.isCurrentMode(MODE_PAN_VIEW)) {
-      this.panning = undefined;
-      return;
-    }
     if (this.panning) {
-      const {pointerStart, pointerCurrent, viewOriginStart, velocity} = this.sim.pointer.panning;
+      const {velocity} = this.panning;
       // Convert pointer velocity to sim internal scale
-      const vx = velocity.x / this.sim.display.scale;
-      const vy = velocity.y / this.sim.display.scale;
-      this.panning.viewOriginStart = this.sim.display.viewOrigin;
-      this.panning.pointerStart = pointerCurrent;
-      // const speed = Math.sqrt(velocity.x ** 2 + velocity.y ** 2);
-      this.sim.display.viewOrigin.x -= vx * elapsedTime * PAN_VELOCITY_SCALE_FACTOR;
-      this.sim.display.viewOrigin.y -= vy * elapsedTime * PAN_VELOCITY_SCALE_FACTOR;
+      this.sim.display.viewOrigin.x -= velocity.x * elapsedTime;
+      this.sim.display.viewOrigin.y -= velocity.y * elapsedTime;
     }
   }
 }

simulator.js

@@ -6,11 +6,14 @@ import { Toolbar } from './toolbar.js';
 import { PlayPause } from './tool/play-pause.js';
 import { Zoom } from './tool/zoom.js';
 import { ModeSwitch } from './tool/modes.js';
+import { Options } from './tool/options.js';
 import { 
   SCALE_POWER_MAX, 
   SCALE_POWER_MIN,
   DISPLAY_CURRENT_SCALE,
   DISPLAY_CURRENT_MODE,
+  MOTION_TIME_SCALE,
+  MODE_PAN_VIEW,
 } from './config.js';
 
 export class Sim {
@@ -25,6 +28,7 @@ export class Sim {
   objects = undefined;
 
   isCurrentMode = () => false;
+  getOption = () => undefined;
 
   init(divId) {
     this.divId = divId;
@@ -34,13 +38,15 @@ export class Sim {
     this.display = new Display(this);
     this.overlay = new Overlay(this);
     this.objects = new Objects(this);
-    this.pointer = new Pointer(this);
     this.toolbar = new Toolbar(this);
 
     // Set up toolbar
     this.toolbar.addTool(new Zoom(this.toolbar));
     this.toolbar.addTool(new PlayPause(this.toolbar));
     this.toolbar.addTool(new ModeSwitch(this.toolbar));
+    this.toolbar.addTool(new Options(this.toolbar));
+
+    this.pointer = new Pointer(this);
 
     // Initiate main loop
     this.time = document.timeline.currentTime;
@@ -49,16 +55,19 @@ export class Sim {
 
   // It's better not to change the scale in the middle of possible frame calculations,
   // so use this to schedule it and let the event loop pick it up.
-  scheduleZoom({x, y}, factor) {
-    this.nextZoom = {x, y, factor};
+  // velocity should be in Sim coordinate scale
+  scheduleZoom({x, y}, factor, velocity) {
+    this.nextZoom = {x, y, factor, velocity};
   }
 
-  zoom({x: screenX, y: screenY, factor}) {
-    const {x, y} = this.screenToSim(screenX, screenY);
+  // x, y should be in Sim coordinates
+  // velocity should be in Sim coordinate scale
+  zoom({x, y, factor, velocity}) {
     // x, y are the mouse coordinates, which should be the center of the new view frame
     // the new view origin should be x, y minus half the new view width and height
     // compute new scale
     this.display.scalePower += factor;
+    // TODO: Lossy rescaling to expand zoom range
     if (this.display.scalePower > SCALE_POWER_MAX) this.display.scalePower = SCALE_POWER_MAX;
     if (this.display.scalePower < SCALE_POWER_MIN) this.display.scalePower = SCALE_POWER_MIN;
     // compute coordinates of new view frame
@@ -66,9 +75,17 @@ export class Sim {
     this.display.viewOrigin.y = y - this.display.height / 2;
 
     this.pointer.clearPointerHistory();
-    if (this.pointer.panning) {
-      this.pointer.panning = undefined;
-      // TODO: Maybe rescale velocity
+
+    if (this.playing && velocity) {
+      // Switch to pan mode
+      this.setCurrentMode(MODE_PAN_VIEW);
+
+      this.pointer.panning = { 
+        velocity: {
+          x: -velocity.x,
+          y: -velocity.y,
+        }
+      };
     }
   }
 
@@ -87,7 +104,7 @@ export class Sim {
 
   // Main loop
   loop(currentTime) {
-    const elapsedTime = currentTime - this.time;
+    const elapsedTime = (currentTime - this.time) * MOTION_TIME_SCALE;
     this.time = currentTime;
 
     if (DISPLAY_CURRENT_MODE) {

tool/modes.js

@@ -3,6 +3,8 @@ import { Tool } from '../tool.js';
 import {
   MODE_MASS_GENERATION,
   MODE_PAN_VIEW,
+  EVENT_MODE_LEAVE,
+  EVENT_MODE_ENTER,
 } from '../config.js';
 
 export class ModeSwitch extends Tool {
@@ -16,9 +18,6 @@ export class ModeSwitch extends Tool {
   constructor(toolbar) {
     super(toolbar);
 
-    const [[currentModeID, _]] = this.modes;
-    this.currentMode = currentModeID;
-
     const modesDiv = document.createElement('div');
     const titleDiv = document.createElement('div');
 
@@ -41,18 +40,19 @@ export class ModeSwitch extends Tool {
       button.innerHTML = `<h3>${modeTitle}</h3>`;
       button.classList.add('wide');
 
-      button.addEventListener('click', (e) => {
-        if (this.currentMode !== modeID) {
-          this.currentMode = modeID;
-          this.setModesOpacity();
-        }
-      });
+      button.addEventListener('click', () => this.setCurrentMode(modeID));
     }
-    this.setModesOpacity();
 
-    // Add global method to get current mode / check mode
+    // First listed mode is the default
+    const [[currentModeID, ]] = this.modes;
+    this.setCurrentMode(currentModeID);
+
+    // Add global method to set/get current mode
+    this.sim.setCurrentMode = (modeID) => this.setCurrentMode(modeID);
     this.sim.getCurrentMode = () => this.currentMode;
     this.sim.isCurrentMode = (modeID) => modeID === this.currentMode;
+    this.sim.onModeLeave = (modeID, cb) => this.onModeLeave(modeID, cb);
+    this.sim.onModeEnter = (modeID, cb) => this.onModeEnter(modeID, cb);
   }
 
   setModesOpacity() {
@@ -60,7 +60,33 @@ export class ModeSwitch extends Tool {
       button.style.opacity = button.modeID === this.currentMode ? '50%' : '100%';
     }
   }
+
+  setCurrentMode(modeID) {
+    if (modeID === this.currentMode) return;
+    const leave = new CustomEvent(EVENT_MODE_LEAVE, {detail: {modeID: this.currentMode}});
+    const enter = new CustomEvent(EVENT_MODE_LEAVE, {detail: {modeID}});
+    this.currentMode = modeID;
+    this.setModesOpacity();
+    this.div.dispatchEvent(leave);
+    this.div.dispatchEvent(enter);
+  }
   
-  // TODO: on enter / on leave mode / some sort of callbacks on mode transitions
+  // cb: () => {}
+  onModeLeave(modeID, cb) {
+    this.div.addEventListener(EVENT_MODE_LEAVE, (e) => {
+      if (e.detail?.modeID === modeID) {
+        cb();
+      }
+    });
+  }
+
+  // cb: () => {}
+  onModeEnter(modeID, cb) {
+    this.div.addEventListener(EVENT_MODE_ENTER, (e) => {
+      if (e.detail?.modeID === modeID) {
+        cb();
+      }
+    });
+  }
 
 }

tool/options.js

@@ -0,0 +1,87 @@
+// Options picker
+import { Tool } from '../tool.js';
+import {
+  DISPLAY_ACCELERATION_VECTORS,
+  DISPLAY_VELOCITY_VECTORS,
+  PAUSE_DURING_CREATION,
+  PAUSE_DURING_SELECTION,
+  MERGE_ON_COLLIDE,
+} from '../config.js';
+
+export class Options extends Tool {
+  options = [{
+    type: 'group', name: 'pauseDuring', title: 'Pause During',
+    items: [
+      { type: 'boolean', name: 'creation', title: 'Creation', default: PAUSE_DURING_CREATION },
+      { type: 'boolean', name: 'selection', title: 'Selection', default: PAUSE_DURING_SELECTION },
+  ]}, {
+    type: 'group', name: 'display', title: 'Display',
+    items: [
+      { type: 'boolean', name: 'velocity', title: 'Velocity', default: DISPLAY_VELOCITY_VECTORS },
+      { type: 'boolean', name: 'acceleration', title: 'Acceleration', default: DISPLAY_ACCELERATION_VECTORS },
+      { type: 'boolean', name: 'traces', title: 'Trace Path', default: DISPLAY_ACCELERATION_VECTORS },
+  ]}, {
+    type: 'group', name: 'collision', title: 'Collisions',
+    items: [
+      { type: 'boolean', name: 'merge', title: 'Merge', default: MERGE_ON_COLLIDE },
+    ]},
+  ];
+
+  values = {};
+
+  visitItem(item, path) {
+    path = [path, item.name].filter(x => !!x).join('.');
+    switch (item.type) {
+      case 'group': {
+        const group = document.createElement('div');
+        const heading = document.createElement('h3');
+        heading.innerHTML = item.title;
+        group.appendChild(heading);
+        for (const next of item.items) {
+          const child = this.visitItem(next, path);
+          group.appendChild(child);
+        }
+        return group;
+      }
+      case 'boolean': {
+        const button = document.createElement('button');
+        button.innerHTML = `<h4>${item.title}</h4>`;
+        this.setOption(path, item.default);
+        button.style.opacity = this.values[path] ? '100%' : '50%';
+        button.addEventListener('click', () => {
+            this.setOption(path, !this.getOption(path));
+            button.style.opacity = this.values[path] ? '100%' : '50%';
+        });
+        return button;
+      }
+      default:
+        throw new Error('unknown option type');
+    }
+  }
+
+  constructor(toolbar) {
+    super(toolbar);
+    const heading = document.createElement('h2');
+    heading.innerHTML = 'Options';
+    this.div.appendChild(heading);
+    this.div.style.paddingTop = '0px';
+    for (const item of this.options) {
+      const child = this.visitItem(item);
+      this.div.appendChild(child);
+    }
+
+    // Global methods to get/set current option values
+    this.sim.getOption = (path) => this.getOption(path);
+    this.sim.setOption = (path, value) => this.setOption(path, value);
+  }
+
+  getOption(path) {
+    const val = this.values[path];
+    return val;
+  }
+
+  setOption(path, value) {
+    this.values[path] = value;
+    // TODO Update button opacity in case this is called from elsewhere
+  }
+}

tool/play-pause.js

@@ -21,6 +21,7 @@ export class PlayPause extends Tool {
 
     pauseButton.addEventListener('click', (e) => {
       e.stopPropagation();
+      this.sim.pointer.panning = undefined;
       if (this.playing) {
         this.playing = false;
         pauseButton.style.opacity = '50%';
@@ -28,7 +29,7 @@ export class PlayPause extends Tool {
       }
     });
 
-    playButton.addEventListener('click', (e) => {
+    playButton.addEventListener('click', () => {
       if (!this.playing) {
         this.playing = true;
         pauseButton.style.opacity = '100%';
@@ -42,6 +43,6 @@ export class PlayPause extends Tool {
   }
 
   set playing(playing) {
-    return this.sim.playing = playing;
+    this.sim.playing = playing;
   }
 }

tool/zoom.js

@@ -16,6 +16,7 @@ export class Zoom extends Tool {
     this.div.appendChild(zoomIn);
     this.div.appendChild(document.createElement('br'));
     this.div.appendChild(zoomAll);
+    this.div.appendChild(document.createElement('br'));
 
     zoomAll.classList.add('wide');
 
@@ -23,37 +24,48 @@ export class Zoom extends Tool {
     zoomIn.innerHTML = '<h2>Zoom<br>In</h2>';
     zoomAll.innerHTML = '<h2>Zoom to Fit</h2>';
 
-    zoomOut.addEventListener('click', (e) => {
+    zoomOut.addEventListener('click', () => {
       // Aim at center of view
       const x = this.sim.display.width * this.sim.display.scale / 2;
       const y = this.sim.display.height * this.sim.display.scale / 2;
-      this.sim.scheduleZoom({x, y}, ZOOM_OUT_FACTOR);
+      this.sim.scheduleZoom(this.sim.screenToSim(x, y), ZOOM_OUT_FACTOR);
     });
 
-    zoomIn.addEventListener('click', (e) => {
+    zoomIn.addEventListener('click', () => {
       // Aim at center of view
       const x = this.sim.display.width * this.sim.display.scale / 2;
       const y = this.sim.display.height * this.sim.display.scale / 2;
-      this.sim.scheduleZoom({x, y}, ZOOM_IN_FACTOR);
+      this.sim.scheduleZoom(this.sim.screenToSim(x, y), ZOOM_IN_FACTOR);
     });
 
-    zoomAll.addEventListener('click', (e) => {
+    zoomAll.addEventListener('click', () => {
       // Determine bounding box
       const box = this.sim.objects.boundingBox;
       const x = (box.start.x + box.end.x) / 2;
       const y = (box.start.y + box.end.y) / 2;
-      if (box.start.x !== box.end.x && box.start.y !== box.end.y) {
-        const widthRatio = Math.abs(box.start.x - box.end.x) / this.sim.display.width;
-        const heightRatio = Math.abs(box.start.y - box.end.y) / this.sim.display.height;
-        const biggerRatio = Math.max(widthRatio, heightRatio);
-        if (biggerRatio <= 1) {
-          const base2Ratio = Math.log(1/biggerRatio) / Math.log(2);
-          this.sim.scheduleZoom({x, y}, Math.floor(base2Ratio));
-        } else {
-          const base2Ratio = Math.log(1/biggerRatio) / Math.log(2);
-          this.sim.scheduleZoom({x, y}, Math.ceil(base2Ratio));
-        }
+      const widthRatio = Math.abs(box.start.x - box.end.x) / this.sim.display.width;
+      const heightRatio = Math.abs(box.start.y - box.end.y) / this.sim.display.height;
+      const biggerRatio = Math.max(widthRatio, heightRatio);
+      const base2factor = Math.log(1/biggerRatio) / Math.log(2) - 0.5;
+      const factor = Math.floor(base2factor);
+      // Determine average momentum and set panning velocity to match
+      const netMomentum = {x: 0, y: 0};
+      let totalMass = 0;
+      let count = 0;
+      this.sim.objects.forEachObject(obj => {
+        count++;
+        netMomentum.x += obj.mass * obj.velocity.x;
+        netMomentum.y += obj.mass * obj.velocity.y;
+        totalMass += obj.mass;
+      });
+      if (!count) {
+        return;
       }
+      const netVelocity = {
+        x: netMomentum.x / totalMass,
+        y: netMomentum.y / totalMass,
+      };
+      this.sim.scheduleZoom({x, y}, factor, netVelocity)
     });
   }
 }

toolbar.js

@@ -10,7 +10,7 @@ export class Toolbar {
     this.div = div;
     this.sim.div.appendChild(div);
     div.style.position = 'relative';
-    div.style.width = 'fit-content';
+    div.style.width = '20EM';
     div.style.top = 0;
     div.style.left = 0;
     div.style.zIndex = 2;
@@ -25,7 +25,7 @@ export class Toolbar {
   frame() {
     for (let tool in this.tools) {
       // TODO: tool.frame()
+      tool.frame();
     }
   }
-
 }