/**
 * Singleton
 * @param {Object} [options]
 * @param {String} [options.axis] --- x | y | xy
 * @param {Number} [options.acceleration]
 * @param {Function} [options.callback]
 * @return {Object}
 */
function scrollInitial(options) {
    options = Object(options);
    options.axis = options.axis || "y";
    options.acceleration = Math.abs(options.acceleration) || 0.1;

    var requestID;
    var speed = 1 + options.acceleration;
    var defaultView = document.defaultView;
    var ix = options.axis === "x" || options.axis === "xy";
    var iy = options.axis === "y" || options.axis === "xy";

    if (!ix && !iy) {
        throw new TypeError("Unexpected token");
    }

    var requestScrollFrame = function () {
        var x1 = 0, y1 = 0;
        var x2 = 0, y2 = 0;
        var x3 = 0, y3 = 0;
        var x, y, xMax, yMax, inProgress;

        if (document.body) {
            x1 = document.body.scrollLeft || 0;
            y1 = document.body.scrollTop || 0;
        }

        if (document.documentElement) {
            x2 = document.documentElement.scrollLeft || 0;
            y2 = document.documentElement.scrollTop || 0;
        }

        x3 = defaultView.scrollX || 0;
        y3 = defaultView.scrollY || 0;

        xMax = Math.max(x1, x2, x3);
        yMax = Math.max(y1, y2, y3);

        if (ix && iy) {
            inProgress = xMax > 0 || yMax > 0;
        } else if (ix) {
            inProgress = xMax > 0;
        } else if (iy) {
            inProgress = yMax > 0;
        }

        x = ix ? Math.floor(xMax / speed) : xMax;
        y = iy ? Math.floor(yMax / speed) : yMax;
        defaultView.scroll(x, y);

        if (inProgress) {
            requestID = requestAnimationFrame(requestScrollFrame);
        } else {
            typeof options.callback === "function" && options.callback();
        }
    };

    var cancelScrollFrame = function () {
        requestID && cancelAnimationFrame(requestID);
    };


    return {
        requestScrollFrame: requestScrollFrame,
        cancelScrollFrame: cancelScrollFrame,
    };
}