/**
  * 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();
 }