function getCookieVal(offset) {
    var endstr = document.cookie.indexOf(";", offset);
    if (endstr == -1) {
        endstr = document.cookie.length;
    }
    return unescape(document.cookie.substring(offset, endstr));
}
var arg = "Pre=";                   //Cookie名
var alen = arg.length;
var clen = document.cookie.length;
var i = 0;
var getCookieValue;
while (i < clen) {
    var j = i + alen;
    if (document.cookie.substring(i, j) == arg) {
        getCookieValue = getCookieVal(j);
        break;
    }
    i = document.cookie.indexOf(" ", i) + 1;
    if (i == 0) {
        break;
    }
}
if (getCookieValue == undefined) {//写入Cookie
//    var Days = 1;
//    var exp = new Date();
//    exp.setTime(exp.getTime() + Days * 24 * 60 * 60 * 1000);
    if (document.referrer == "") {
        document.cookie = "Pre=" + document.location + ";";
    }
    else {
        document.cookie = "Pre=" + document.referrer + ";";//expires=" + exp.toGMTString();    
    }
}