gravity/objects.js

import { MassObject } from './object.js';
import { 
  DISPLAY_OBJECTS_INFO,
  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.panning?.velocity) {
        this.sim.panning.paused = true;
      }
    }
  }

  resume() {
    if (this.paused) {
      this.sim.playing = true;
      this.paused = false;
      if (this.sim.panning?.paused) {
        this.sim.panning.paused = false;
      }
    }
  }

  // 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
    if (this.sim.getOption('pauseDuring.creation')) {
      this.pause();
    }

    if (this.sim.panning?.velocity) {
      obj.velocity = {...this.sim.panning.velocity};
    }
  }

  doneCreatingObject() {
    if (this.creatingObject !== undefined) {
      this.creatingObject = undefined;
      this.resume();
    }
  }

  object(i) {
    return this.objects[i];
  }

  selectObject(i) {
    this.selectedObject = i;
    if (this.sim.getOption('pauseDuring.selection')) {
      this.pause();
    }
  }

  deselect() {
    this.selectedObject = undefined;
    this.resume();
  }

  getSelectedOrCreating() {
    let i = this.creatingObject ?? this.selectedObject;
    if (i !== undefined) {
      return this.objects[i];
    }
  }

  get length() {
    return this.objects.length;
  }

  get boundingBox() {
    const box = this.reduce((acc, obj) => {
      if (acc.start.x === undefined) {
        acc.start = {...obj.position};
        acc.end = {...obj.position};
      } else {
        if (obj.position.x < acc.start.x) acc.start.x = obj.position.x;
        if (obj.position.x > acc.end.x) acc.end.x = obj.position.x;
        if (obj.position.y < acc.start.y) acc.start.y = obj.position.y;
        if (obj.position.y > acc.end.y) acc.end.y = obj.position.y;
      }
    }, {
      start: {x: undefined, y: undefined}, 
      end: {x: undefined, y: undefined},
    });

    box.start.x = (box.start.x ?? 0) - ZOOM_TO_FIT_PADDING;
    box.start.y = (box.start.y ?? 0) - ZOOM_TO_FIT_PADDING;
    box.end.x = (box.end.x ?? 0) + ZOOM_TO_FIT_PADDING;
    box.end.y = (box.end.y ?? 0) + ZOOM_TO_FIT_PADDING;
    return box;
  }

  objectAtLocation(x, y) {
    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) {
        idx = i;
        return null;
      }
    });
    return idx;
  }

  handlePointerDown({x, y}) {
    // If pointer is touching an object, select the object
    const touchingObject = this.objectAtLocation(x, y);
   
    if (touchingObject !== undefined) {
      this.selectObject(touchingObject);
    } else {
      // Otherwise, create a new object
      this.createObject(x, y);
    }
  }

  handlePointerUp() {
    this.doneCreatingObject();
    this.deselect();
  }

  handlePointerMove({x, y, vx, vy}) {
    // If the cursor moves while creating an object, or while an object is selected,
    // update the position and velocity of the object
    // update the position using the pointer motion but the velocity using the pointer velocity
    const obj = this.getSelectedOrCreating();
    if (obj === undefined) return;
    if (this.sim.panning?.velocity) {
      vx += this.sim.panning.velocity.x;
      vy += this.sim.panning.velocity.y;
    }
    obj.position.x = x;
    obj.position.y = y;
    obj.velocity.x = vx;
    obj.velocity.y = vy;
  }

  // cb: (obj, idx) => {}
  forEachObject(cb, {alive, startWith} = {}) {
    if (alive === undefined) alive = true;
    for (let i = startWith ?? 0; 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;
      }
    }
  }

  // cb: (acc, obj, idx) => {}
  reduce(cb, initial, opts) {
    let acc = initial;
    this.forEachObject((obj, idx) => {
      const ret = cb(acc, obj, idx);
      if (ret !== undefined) {
        acc = ret;
      }
    }, opts);
    return acc;
  }

  computeForces() {
    const gravity = this.sim.getOption('param.gravity');
    if (this.objects.length < 2) return;
    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;
        const d = Math.sqrt(dSquared);
        const F = gravity * A.mass * B.mass / dSquared;
        const Fx = F * dx / d;
        const Fy = F * dy / d;
        A.forces.push({ x: Fx, y: Fy });
        B.forces.push({ x: -Fx, y: -Fy });
      }, {alive: true, startWith: i + 1});
    });
    // Also compute acceleration
    this.forEachObject(obj => {
      obj.acceleration = obj.getAcceleration();
    });
  }

  computeFrame(elapsedTime) {
    // If we're creating an object, increment its mass
    // with the mass creation rate accelerating over time
    const massCreationRate = this.sim.getOption('param.massCreationRate');
    if (this.creatingObject !== undefined) {
      const obj = this.objects[this.creatingObject];
      const rate = massCreationRate * obj.age;
      // TODO: After objects merge during creation, mass creation rate can accelerate
      obj.mass += rate * elapsedTime;
    }

    // Calculate forces due to gravity. 
    this.computeForces();

    if (this.sim.playing) {
      // 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) {
              let S, T;
              // Merge the older into the newer, in order to provide mass creation rate continuity
              if (A.age > B.age) {
                // A merges into B; B survives
                S = B;
                T = A;
                // If A was selected or being created, select S instead
                if (this.creatingObject === i) this.creatingObject = j;
                if (this.selectedObject === i) this.selectedObject = j;
              } else {
                // B merges into A; A survives
                S = A;
                T = B;
                // If B was selected or being created, select S instead
                if (this.creatingObject === j) this.creatingObject = i;
                if (this.selectedObject === j) this.selectedObject = i;
              }
              // Merge T into S:
              // Set position = center of mass 
              // Set velocity = total momentum / total mass
              // Combine forces
              // Sdd masses
              // Sverage color
              S.position.x = (S.position.x * S.mass + T.position.x * T.mass) / (S.mass + T.mass);
              S.position.y = (S.position.y * S.mass + T.position.y * T.mass) / (S.mass + T.mass);
              S.velocity.x = (S.velocity.x * S.mass + T.velocity.x * T.mass) / (S.mass + T.mass);
              S.velocity.y = (S.velocity.y * S.mass + T.velocity.y * T.mass) / (S.mass + T.mass);
              S.forces.push(...T.forces);
              S.mass += T.mass;
              S.color = {
                r: (S.mass * S.color.r + T.mass * T.color.r) / (S.mass + T.mass),
                g: (S.mass * S.color.g + T.mass * T.color.g) / (S.mass + T.mass),
                b: (S.mass * S.color.b + T.mass * T.color.b) / (S.mass + T.mass),
              };
              T.alive = false;
              T.forces = [];
            }
          }, {alive: true, startWith: i + 1});
        });
      }
      
      // Recompute forces
      this.computeForces();

      // 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;

        // 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) {
      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.toPrecision(6)}, `, 
          `${obj.position.y.toPrecision(6)}, `, 
          `${obj.mass.toPrecision(6)} kg, `,
          `${speed.toPrecision(2)} m/s, ${direction.toPrecision(2)}°`, 
        ];
      });
    }
  }

  computeSystemCenter() {
    // Determine center of mass
    const { totalMass, count, totalMassLocation } = 
      this.sim.objects.reduce((acc, obj) => ({
        count: acc.count + 1,
        totalMass: acc.totalMass + obj.mass,
        totalMassLocation: {
          x: acc.totalMassLocation.x + obj.position.x * obj.mass,
          y: acc.totalMassLocation.y + obj.position.y * obj.mass,
        },
      }), {
        totalMassLocation: {x: 0, y: 0},
        totalMass: 0,
        count: 0,
      });

    if (!count) return;

    const centerOfMass = {
      x: totalMassLocation.x / totalMass,
      y: totalMassLocation.y / totalMass,
    };
  
    return { totalMass, count, totalMassLocation, centerOfMass };
  }

  computeSystemAngularMomentum(centerOfMass) {
    return this.reduce((acc, obj) => {
      // Angular momentum for each object is m * s / d
      // where d is the distance of the object from the global center of mass
      // and s is the magnitude of the cross product of v and r
      const r = {
        x: obj.position.x - centerOfMass.x,
        y: obj.position.y - centerOfMass.y,
      };
      const v = obj.velocity;
      const s = v.x * r.y - v.y * r.x;
      const d = Math.sqrt(r.x ** 2 + r.y ** 2);
      return acc + obj.mass * s / d;
    }, 0);
  }
}