var funParabola = function(element, target, options) {
    /*
     * 网页模拟现实需要一个比例尺
     * 如果按照1像素就是1米来算，显然不合适，因为页面动不动就几百像素
     * 页面上，我们放两个物体，200~800像素之间，我们可以映射为现实世界的2米到8米，也就是100:1
     * 不过，本方法没有对此有所体现，因此不必在意
     */

    var defaults = {
        speed: 166.67, // 每帧移动的像素大小，每帧（对于大部分显示屏）大约16~17毫秒
        curvature: 0.001,  // 实际指焦点到准线的距离，你可以抽象成曲率，这里模拟扔物体的抛物线，因此是开口向下的
        progress: function() {},
        complete: function() {}
    };

    var params = {}; options = options || {};

    for (var key in defaults) {
        params[key] = options[key] || defaults[key];
    }

    var exports = {
        mark: function() { return this; },
        position: function() { return this; },
        move: function() { return this; },
        init: function() { return this; }
    };

    /* 确定移动的方式
     * IE6-IE8 是margin位移
     * IE9+使用transform
     */
    var moveStyle = "margin", testDiv = document.createElement("div");
    if ("oninput" in testDiv) {
        ["", "ms", "webkit"].forEach(function(prefix) {
            var transform = prefix + (prefix? "T": "t") + "ransform";
            if (transform in testDiv.style) {
                moveStyle = transform;
            }
        });
    }

    // 根据两点坐标以及曲率确定运动曲线函数（也就是确定a, b的值）
    /* 公式： y = a*x*x + b*x + c;
     */
    var a = params.curvature, b = 0, c = 0;

    // 是否执行运动的标志量
    var flagMove = true;

    if (element && target && element.nodeType == 1 && target.nodeType == 1) {
        var rectElement = {}, rectTarget = {};

        // 移动元素的中心点位置，目标元素的中心点位置
        var centerElement = {}, centerTarget = {};

        // 目标元素的坐标位置
        var coordElement = {}, coordTarget = {};

        // 标注当前元素的坐标
        exports.mark = function() {
            if (flagMove == false) return this;
            if (typeof coordElement.x == "undefined") this.position();
            element.setAttribute("data-center", [coordElement.x, coordElement.y].join());
            target.setAttribute("data-center", [coordTarget.x, coordTarget.y].join());
            return this;
        }

        exports.position = function() {
            if (flagMove == false) return this;

            var scrollLeft = document.documentElement.scrollLeft || document.body.scrollLeft,
                scrollTop = document.documentElement.scrollTop || document.body.scrollTop;

            // 初始位置
            if (moveStyle == "margin") {
                element.style.marginLeft = element.style.marginTop = "0px";
            } else {
                element.style[moveStyle] = "translate(0, 0)";
            }

            // 四边缘的坐标
            rectElement = element.getBoundingClientRect();
            rectTarget = target.getBoundingClientRect();

            // 移动元素的中心点坐标
            centerElement = {
                x: rectElement.left + (rectElement.right - rectElement.left) / 2 + scrollLeft,
                y: rectElement.top + (rectElement.bottom - rectElement.top) / 2    + scrollTop
            };

            // 目标元素的中心点位置
            centerTarget = {
                x: rectTarget.left + (rectTarget.right - rectTarget.left) / 2 + scrollLeft,
                y: rectTarget.top + (rectTarget.bottom - rectTarget.top) / 2 + scrollTop
            };

            // 转换成相对坐标位置
            coordElement = {
                x: 0,
                y: 0
            };
            coordTarget = {
                x: -1 * (centerElement.x - centerTarget.x),
                y:  -1 * (centerElement.y - centerTarget.y)
            };

            /*
             * 因为经过(0, 0), 因此c = 0
             * 于是：
             * y = a * x*x + b*x;
             * y1 = a * x1*x1 + b*x1;
             * y2 = a * x2*x2 + b*x2;
             * 利用第二个坐标：
             * b = (y2+ a*x2*x2) / x2
             */
            // 于是
            b = (coordTarget.y - a * coordTarget.x * coordTarget.x) / coordTarget.x;

            return this;
        };

        // 按照这个曲线运动
        exports.move = function() {
            // 如果曲线运动还没有结束，不再执行新的运动
            if (flagMove == false) return this;

            var startx = 0, rate = coordTarget.x > 0? 1: -1;

            var step = function() {
                // 切线 y‘=2ax+b
                var tangent = 2 * a * startx + b; // = y / x
                // y*y + x*x = speed
                // (tangent * x)^2 + x*x = speed
                // x = Math.sqr(speed / (tangent * tangent + 1));
                startx = startx + rate * Math.sqrt(params.speed / (tangent * tangent + 1));

                // 防止过界
                if ((rate == 1 && startx > coordTarget.x) || (rate == -1 && startx < coordTarget.x)) {
                    startx = coordTarget.x;
                }
                var x = startx, y = a * x * x + b * x;

                // 标记当前位置，这里有测试使用的嫌疑，实际使用可以将这一行注释
                element.setAttribute("data-center", [Math.round(x), Math.round(y)].join());

                // x, y目前是坐标，需要转换成定位的像素值
                if (moveStyle == "margin") {
                    element.style.marginLeft = x + "px";
                    element.style.marginTop = y + "px";
                } else {
                    element.style[moveStyle] = "translate("+ [x + "px", y + "px"].join() +")";
                }

                if (startx !== coordTarget.x) {
                    params.progress(x, y);
                    window.requestAnimationFrame(step);
                } else {
                    // 运动结束，回调执行
                    params.complete();
                    flagMove = true;
                }
            };
            window.requestAnimationFrame(step);
            flagMove = false;

            return this;
        };

        // 初始化方法
        exports.init = function() {
            this.position().mark().move();
        };
    }

    return exports;
};

// 这是很简单的拖拽方法，与本demo主旨无关，方便演示使用
var funDrag = function(element, callback) {
    callback = callback || function() {};
    var params = {
        left: 0,
        top: 0,
        currentX: 0,
        currentY: 0,
        flag: false
    };
    //获取相关CSS属性
    var getCss = function(o,key){
        return o.currentStyle? o.currentStyle[key] : document.defaultView.getComputedStyle(o,false)[key];
    };

    //拖拽的实现
    if(getCss(element, "left") !== "auto"){
        params.left = getCss(element, "left");
    }
    if(getCss(element, "top") !== "auto"){
        params.top = getCss(element, "top");
    }
    //o是移动对象
    element.onmousedown = function(event){
        params.flag = true;
        event = event || window.event;
        params.currentX = event.clientX;
        params.currentY = event.clientY;
    };
    document.onmouseup = function(){
        params.flag = false;
        if(getCss(element, "left") !== "auto"){
            params.left = getCss(element, "left");
        }
        if(getCss(element, "top") !== "auto"){
            params.top = getCss(element, "top");
        }
        callback();
    };
    document.onmousemove = function(event){
        event = event || window.event;
        if(params.flag){
            var nowX = event.clientX, nowY = event.clientY;
            var disX = nowX - params.currentX, disY = nowY - params.currentY;
            element.style.left = parseInt(params.left) + disX + "px";
            element.style.top = parseInt(params.top) + disY + "px";
        }
    }
};
(function() {
    var lastTime = 0;
    var vendors = [‘webkit‘, ‘moz‘];
    for(var x = 0; x < vendors.length && !window.requestAnimationFrame; ++x) {
        window.requestAnimationFrame = window[vendors[x] + ‘RequestAnimationFrame‘];
        window.cancelAnimationFrame = window[vendors[x] + ‘CancelAnimationFrame‘] ||    // name has changed in Webkit
            window[vendors[x] + ‘CancelRequestAnimationFrame‘];
    }

    if (!window.requestAnimationFrame) {
        window.requestAnimationFrame = function(callback, element) {
            var currTime = new Date().getTime();
            var timeToCall = Math.max(0, 16.7 - (currTime - lastTime));
            var id = window.setTimeout(function() {
                callback(currTime + timeToCall);
            }, timeToCall);
            lastTime = currTime + timeToCall;
            return id;
        };
    }
    if (!window.cancelAnimationFrame) {
        window.cancelAnimationFrame = function(id) {
            clearTimeout(id);
        };
    }
}());