HTML5 Canvas实现圆弧荡秋千动画效果

<!DOCTYPE html>
<html>
 <head>
  <title>demo</title>
  <style type="text/css">
   #canvas {
    margin:50px; border:5px solid gray; box-shadow:0px 0px 5px 5px #494949;
   }
  </style>
 </head>
 <body>
  <canvas id="canvas" width="500px" height="500px"></canvas>
 </body>
 <script type="text/javascript">
  var canvas = document.getElementById("canvas");
  var ctx = canvas.getContext("2d");
  var r_x = 250, r_y = 0;
  var offset_h = 250;
  var offset_w = 500;
  var count = 0;
  var mode = "up";
  var temp = 0;
  var getRPoint = function(x, y) {
   var r = (Math.pow(x, 2) + Math.pow(y, 2)) / (2 * y);
   var point = {
    x: x,
    y: Math.abs(250 - (r - y)),
    r: r
   };
   return point;
  };
  function arc(attrs) {
   ctx.beginPath();
   ctx.arc(attrs.x, attrs.y, attrs.r, attrs.startAngle || 0, attrs.endAngle || Math.PI);
   ctx.stroke();
  }
  var interval = setInterval(function() { 
   count++;
   switch(mode) {
    case "up":
     temp = count;
     ctx.clearRect(0, 0, 500, 500);
     if(count%18 == 0) {
      mode = "down";
      return;
     } 
    break;
    case "down":
     temp = 36 - count;
     ctx.clearRect(0, 0, 500, 500);
     if(count%36 == 0) {
      mode = "default";
      return;
     }     
    break;
    case "default":
     temp = count - 36;
     if(count%54 == 0) {
      mode = "up";
      count = 0;
      return;
     }
   }
   arc(getRPoint(250, 250-8*temp));
  }, 100);
 </script>
</html>