physics/draw.js

class Drawing {
  constructor(div) {
    if (div instanceof HTMLElement) {
      this.div = div;
    } else {
      this.div = document.getElementById(div);
    }
    this.div.classList.add('drawing');
    this.div.classList.add('container');
    this.buttonsDiv = document.createElement('div');
    this.titleDiv = document.createElement('div');
    this.titleDiv.classList.add('title');
    this.captionDiv = document.createElement('div');
    this.captionDiv.classList.add('caption');
    this.canvas = document.createElement('canvas');
    this.div.appendChild(this.titleDiv);
    this.div.appendChild(this.buttonsDiv);
    this.div.appendChild(this.canvas);
    this.div.appendChild(this.captionDiv);
    this.ctx = this.canvas.getContext('2d');
    this.sequence = [];
    this.t = 0;
    this.rt = 0;
    this.dt = 0;
    this.points = {};
    this.stopped = true;
    this.frame = [[-10, -10], [110, 110]];
    this.frameMargin = 10;
    this.scale = 1.0;
    this.speed = 1.0;
    this.rendered = false;
    this.updateCanvasSize();
  }

  addButtons() {
    if (!this.buttonsAdded) {
      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.buttonsDiv.appendChild(this.startButton);
      this.buttonsDiv.appendChild(this.stopButton);
      this.buttonsAdded = true;
      this.updateButtonStates();
    }
  }

  updateButtonStates() {
    if (this.stopped) {
      this.stopButton.disabled = true;
      this.startButton.disabled = false;
    } else {
      this.stopButton.disabled = false;
      this.startButton.disabled = true;
    }
  }

  setTitle(title) {
    const h = document.createElement('h4');
    h.innerHTML = title;
    this.titleDiv.appendChild(h);
  }

  setCaption(caption) {
    this.captionDiv.innerHTML = caption;
  }

  start() {
    if (this.stopped) {
      this.elideInterval = true;
      this.stopped = false;
    }
    if (this.onStartFn) {
      this.onStartFn();
    }
    this.addButtons();
    this.updateButtonStates();
    this.animate();
  }

  stop() {
    this.stopped = true;
    this.updateButtonStates();
  }

  render() {
    for (let action of this.sequence) {
      action();
    }
    this.rendered = true;
  }

  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();
      });
    }
  }

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

  onStart(fn) {
    this.onStartFn = fn;
  }
  
  setSpeed(speed) {
    this.speed = speed;
  }

  updateCanvasSize() {
    this.canvas.width = (this.frame[1][0] - this.frame[0][0]) * this.scale + 2 * this.frameMargin;
    this.canvas.height = (this.frame[1][1] - this.frame[0][1]) * this.scale + 2 * this.frameMargin;
    if (this.rendered) {
      this.render();
    }
  }

  setFrame([x1, y1], [x2, y2]) {
    this.frame = [[x1, y1], [x2, y2]];
    this.updateCanvasSize();
  }

  setFrameMargin(frameMargin) {
    this.frameMargin = frameMargin;
    this.updateCanvasSize();
  }

  setScale(zoom) {
    this.scale = zoom;
    this.updateCanvasSize();
  }

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

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

  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;
    }
  }
     
  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();
    });
  }
  
  arrow(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 = 'gray';
      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);
    });
    return {
      reset: () => {
        history = [];
        distance = 0;
      }
    };
  }

  square(p, opts) {
    const size = opts?.size ?? 10;
    return 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;
    return 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();
    });
  }

  func(opts, fn) {
    const origin = opts?.origin ?? [0, 0];
    const domain = opts?.domain ?? [this.frame[0][0], this.frame[1][0]];
    const step = opts?.step ?? 1;
    this.sequence.push(() => {
      this.ctx.beginPath();
      this.ctx.moveTo(...this.pixel([domain[0], fn(domain[0])]));
      for (let x = domain[0] + step; x <= domain[1]; x += step) {
        this.ctx.lineTo(...this.pixel([x, fn(x)]));
      }
      this.ctx.lineTo(...this.pixel([domain[1], fn(domain[1])]));
      this.ctx.stroke();
    })
  }
  
  static fromText(node) {
    const div = document.createElement('div');
    node.parentNode.parentNode.insertBefore(div, node.parentNode);
    node.style.display = 'none';
    const d = new Drawing(div);
    const lines = node.innerText.split('\n');
    for (let i = 0; i < lines.length; i++) {
      const line = lines[i];
      if (line.startsWith("#")) continue;
      const [cmd, ...args] = line.split(' ');
      // console.log({cmd, args});
      switch (cmd) {
        case 'title': {
          d.setTitle(args.join(' '));
        } break;
        case 'caption': {
          d.setCaption(args.join(' '));
        } break;
        case 'buttons': {
          d.addButtons();
        } break;
        case 'frame': {
          const [x1, y1, x2, y2] = args.map(x => parseInt(x));
          d.setFrame([x1, y1], [x2, y2]);
        } break;
        case 'axes': {
          const [x, y] = args.map(x => parseInt(x));
          d.polyline([0, y], [0, 0], [x, 0]);
        } break;
        case 'stroke': {
          const style = args[0];
          const width = parseInt(args[1]);
          d.setStroke(style, width);
        } break;
        case 'fill': {
          const style = args[0];
          d.setFill(style);
        } break;
        case 'point': {
          const [name, ...rest] = args;
          let body = rest.join(' ');
          while (lines[i + 1].startsWith(' ')) {
            body += lines[i + 1];
            i += 1;
          }
          d.definePoint(name, () => {
            return (function() {
              return eval(body);
            }).call(d);
          });
        } break;
        case 'circle': 
        case 'square': {
          const p = args[0];
          const traceAge = args[1] ? parseInt(args[1]) : 0;
          d[cmd](p, {trace: {age: traceAge}});
        } break;
        case 'start': {
          d.start();
        } break;
      }
    }
    d.render();
  }
}