(function($){
  var SNOW_INTERVAL_KEY = 'snow-interval';

  $.fn.snow = function(option){
    // 销毁支持
    if(option==='destroy'){
      return this.each(function(){
        var id = $(this).data(SNOW_INTERVAL_KEY);
        if(id) clearInterval(id);
        $('.snow-flake').remove();
      });
    }

    var opt = $.extend({
      minSize: 8,
      maxSize: 18,
      newOn: 400,
      flakeColor: "#fff"
    }, option);

    // 生成雪花并下落
    var timer = setInterval(function(){
      var ww = window.innerWidth, wh = window.innerHeight;
      var size = opt.minSize + Math.random() * (opt.maxSize - opt.minSize);
      var startLeft = Math.random() * ww;
      var endLeft = startLeft + (Math.random() - .5) * 200;
      var dur = wh * 8 + Math.random() * 2500;

      $('<span class="snow-flake">&#10052;</span>').css({
        position:'fixed',
        top:-size+'px',
        left:startLeft,
        zIndex:9999,
        fontSize: size,
        color: opt.flakeColor,
        opacity: .4 + Math.random() * .6,
        pointerEvents:'none'
      }).appendTo('body').animate({
        top: wh+20,
        left: endLeft,
        opacity: 0.15
      }, dur, 'linear', function(){$(this).remove();});
    }, opt.newOn);

    // 返回自身，方便链式；记录定时器id以支持销毁
    return this.each(function(){$(this).data(SNOW_INTERVAL_KEY, timer);});
  };
})(jQuery);

// 初始化
$(function(){
  $(document).snow({
    minSize: 6,
    maxSize: 28,
    newOn: 350
  });
  // 销毁方法示例:
  // setTimeout(()=>{$(document).snow('destroy');}, 10000);
});