physics/draw.js

class Drawing {
  constructor() {
    const div = document.createElement('div');
    const buttonsDiv = document.createElement('div');
    this.startButton = document.createElement('button');
    this.startButton.onclick = () => this.start();
    this.startButton.innerHTML = "Start";
    this.stopButton = document.createElement('button');
    this.stopButton.onclick = () => this.stop();
    this.stopButton.innerHTML = "Stop";
    this.canvas = document.createElement('canvas');
    buttonsDiv.appendChild(this.startButton);
    buttonsDiv.appendChild(this.stopButton);
    div.appendChild(buttonsDiv);
    div.appendChild(this.canvas);
    document.body.appendChild(div);
    this.ctx = this.canvas.getContext('2d');
    this.sequence = [];
    this.t = 0;
    this.rt = 0;
    this.dt = 0;
    this.points = {};
    this.stopped = true;
    this.frame = [[-50, -50], [150, 150]];
    this.scale = 1.0;
    this.canvas.width = (this.frame[1][0] - this.frame[0][0]) * this.scale;
    this.canvas.height = (this.frame[1][1] - this.frame[0][1]) * this.scale;
    this.speed = 1.0;
  }

  pixel([x, y]) {
    return [
      (x - this.frame[0][0]) * this.scale, 
      this.canvas.height - (y - this.frame[0][1]) * this.scale
    ];
  }
  
  setSpeed(speed) {
    this.speed = speed;
  }

  setFrame([x1, y1], [x2, y2]) {
    this.frame = [[x1, y1], [x2, y2]];
    this.canvas.width = (this.frame[1][0] - this.frame[0][0]) * this.scale;
    this.canvas.height = (this.frame[1][1] - this.frame[0][1]) * this.scale;
  }

  setScale(zoom) {
    this.scale = zoom;
    this.canvas.width = (this.frame[1][0] - this.frame[0][0]) * this.scale;
    this.canvas.height = (this.frame[1][1] - this.frame[0][1]) * this.scale;
  }

  setStroke(style, width) {
    this.sequence.push(() => {
      this.ctx.strokeStyle = style;
      this.ctx.lineWidth = width;
    });
  }

  setFill(style) {
    this.sequence.push(() => {
      this.ctx.fillStyle = style;
    });
  }

  line(p1, p2) {
    this.sequence.push(() => {
      this.ctx.beginPath();
      this.ctx.moveTo(...this.pixel(this.getPoint(p1)));
      this.ctx.lineTo(...this.pixel(this.getPoint(p2)));
      this.ctx.stroke();
    });
  }

  polyline(...points) {
    this.sequence.push(() => {
      this.ctx.beginPath();
      this.ctx.moveTo(...this.pixel(points[0]));
      for (let i = 1; i < points.length; i++) {
        this.ctx.lineTo(...this.pixel(points[i]));
      }
      this.ctx.stroke();
    });
  }
  
  oscillatingValue(x1, x2, period, initialPhase = 0) {
    const center = (x1 + x2) / 2;
    const amplitude = (x2 - x1) / 2;
    return center + amplitude * Math.sin(2*Math.PI*this.t/period + initialPhase);
  }

  oscillatingPoint([x1, y1], [x2, y2], period) {
    const x = this.oscillatingValue(x1, x2, period);
    const y = this.oscillatingValue(y1, y2, period);
    return [x, y];
  }

  object(p, opts, fn) {
    let history = [];
    const maxAge = opts?.trace?.age ?? 0;
    const dashLength = 5;
    let distance = 0;
    this.sequence.push(() => {
      const [x, y] = this.getPoint(p);
      let ds = 0;
      if (history.length) {
        const dx = x - history[history.length - 1].p[0];
        const dy = y - history[history.length - 1].p[1];
        ds = Math.sqrt(dx**2 + dy**2);
      }
      distance += ds;
      history.push({t: this.t, p: [x, y], distance});
      const oldest = history.findIndex(({t}) => this.t - t <= maxAge);
      if (oldest >= 0) {
        history = history.slice(oldest);
      }
      this.ctx.strokeStyle = 'black';
      this.ctx.lineWidth = 1;
      this.ctx.beginPath();
      this.ctx.moveTo(...this.pixel(history[0].p));
      for (let i = 1; i < history.length; i++) {
        if (Math.floor(history[i].distance / dashLength) % 2) {
          this.ctx.moveTo(...this.pixel(history[i].p));
        } else {
          this.ctx.lineTo(...this.pixel(history[i].p));
        }
      }
      this.ctx.stroke();
      fn(x, y);
    })
  }

  square(p, opts) {
    const size = opts?.size ?? 10;
    this.object(p, opts, (x, y) => {
      this.ctx.fillRect(...this.pixel([x - size / 2, y + size / 2]), size * this.scale, size * this.scale);
    });
  }

  circle(p, opts) {
    const radius = opts?.radius ?? 5;
    this.object(p, opts, (x, y) => {
      this.ctx.beginPath();
      this.ctx.ellipse(...this.pixel([x, y]), radius * this.scale, radius * this.scale, 0, 0, 2*Math.PI);
      this.ctx.fill();
    });
  }

  definePoint(name, fn) {
    this.points[name] = fn;
  }

  getPoint(p) {
    if (typeof p === 'string') {
      const fn = this.points[p];
      if (!fn) {
        const e = new Error;
        e.message = `Point '${p}' is not defined`;
        throw e;
      }
      return fn();
    } else {
      return p;
    }
  }
   
  render() {
    for (let action of this.sequence) {
      action();
    }
  }

  animate() {
    this.ctx.reset();
    this.render();
    if (!this.stopped) {
      requestAnimationFrame((prevt) => {
        const rt = document.timeline.currentTime;
        const elapsed = rt - prevt;
        if (this.elideInterval) {
          this.dt = 0;
          this.elideInterval = false;
        } else {
          this.dt = (rt - this.rt + elapsed) * this.speed;
        }
        this.t += this.dt;
        this.rt = rt;
        this.animate();
      });
    }
  }
  
  stop() {
    this.stopped = true;
  }
  
  start() {
    if (this.stopped) {
      this.elideInterval = true;
      this.stopped = false;
    }
    this.animate();
  }
}