/**
 * js抛物线动画
 * @param {[object]} origin [起点元素]
 * @param {[object]} target [目标点元素]
 * @param {[object]} element [要运动的元素]
 * @param {[number]} a [抛物线弧度]
 * @param {[number]} time [动画执行时间]
 * @param {[function]} callback [抛物线执行完成后回调]
 */
 var parabola = function(config){
 var b = 0,
  INTERVAL = 15,
  timer = null,
  x1,y1,x2,y2,originx,originy,diffx,diffy;

 this.config = config || {};
 // 起点
 this.origin = $(this.config.origin)||null;
 // 终点
 this.target = $(this.config.target)||null;
 // 运动的元素
 this.element = $(this.config.element)||null;
 // 曲线弧度
 this.a = this.config.a || 0.004;
 // 运动时间(ms)
 this.time = this.config.time || 1000;

 this.init = function(){
  x1 = this.origin.offset().left;
  y1 = this.origin.offset().top;
  x2 = this.target.offset().left;
  y2 = this.target.offset().top;
  originx = x1;
  originy = y1;
  diffx = x2-x1;
  diffy = y2-y1,
  speedx = diffx/this.time;

  // 已知a, 根据抛物线函数 y = a*x*x + b*x + c 将抛物线起点平移到坐标原点[0, 0]，终点随之平移，那么抛物线经过原点[0, 0] 得出c = 0;
  // 终点平移后得出：y2-y1 = a*(x2 - x1)*(x2 - x1) + b*(x2 - x1)
  // 即 diffy = a*diffx*diffx + b*diffx;
  // 可求出常数b的值
  b = (diffy - this.a*diffx*diffx)/diffx;
  this.element.css({
  left: x1,
  top: y1
  })
  return this;
 }

 // 确定动画方式
 this.moveStyle = function(){
  var moveStyle = 'position',
  testDiv = document.createElement('div');
  if('placeholder' in testDiv){
  ['','ms','moz','webkit'].forEach(function(pre){
   var transform = pre + (pre ? 'T' : 't') + 'ransform';
   if(transform in testDiv.style){
   moveStyle = transform;
   }
  })
  }
  return moveStyle;
 }

 this.move = function(){
  var start = new Date().getTime(),
  moveStyle = this.moveStyle(),
  _this = this;
  timer = setInterval(function(){
  if(new Date().getTime() - start > _this.time){
   clearInterval(timer);
   _this.element.css({
   left: x2,
   top: y2
   })
   typeof _this.config.callback === 'function' && _this.config.callback(_this.element);
   return;
  }
  x = speedx * (new Date().getTime() - start);
  y = _this.a*x*x + b*x;
  if(moveStyle === 'position'){
   _this.element.css({
   left: x+originx,
   top: y+originy
   })
  }else{
   if(window.requestAnimationFrame){
   window.requestAnimationFrame(_this.element[0].style[moveStyle] = 'translate('+x+'px,'+y+'px)');
   }else{
   _this.element[0].style[moveStyle] = 'translate('+x+'px,'+y+'px)';
   }
  }
  },INTERVAL)
  return this;
 }

 this.init();
 }