let snowStorm = (function (window, document) {

  // --- common properties ---

  this.autoStart = true;          // Whether the snow should start automatically or not.
  this.flakesMax = 640;           // Limit total amount of snow made (falling + sticking)
  this.flakesMaxActive = 80;      // Limit amount of snow falling at once (less = lower CPU use)
  this.animationInterval = 40;    // Theoretical "miliseconds per frame" measurement. 20 = fast + smooth, but high CPU use. 50 = more conservative, but slower
  this.excludeMobile = true;      // Snow is likely to be bad news for mobile phones' CPUs (and batteries.) By default, be nice.
  this.flakeBottom = null;        // Integer for Y axis snow limit, 0 or null for "full-screen" snow effect
  this.followMouse = true;        // Snow movement can respond to the user's mouse
  this.snowColor = '#FFFFFF';        // Don't eat (or use?) yellow snow.
  this.snowCharacter = '•';  // • = bullet, · is square on some systems etc.
  this.snowStick = true;          // Whether or not snow should "stick" at the bottom. When off, will never collect.
  this.targetElement = null;      // element which snow will be appended to (null = document.body) - can be an element ID eg. 'myDiv', or a DOM node reference
  this.useMeltEffect = true;      // When recycling fallen snow (or rarely, when falling), have it "melt" and fade out if browser supports it
  this.useTwinkleEffect = false;  // Allow snow to randomly "flicker" in and out of view while falling
  this.usePositionFixed = false;  // true = snow does not shift vertically when scrolling. May increase CPU load, disabled by default - if enabled, used only where supported

  // --- less-used bits ---

  this.freezeOnBlur = true;       // Only snow when the window is in focus (foreground.) Saves CPU.
  this.flakeLeftOffset = 0;       // Left margin/gutter space on edge of container (eg. browser window.) Bump up these values if seeing horizontal scrollbars.
  this.flakeRightOffset = 0;      // Right margin/gutter space on edge of container
  this.flakeWidth = 8;            // Max pixel width reserved for snow element
  this.flakeHeight = 8;           // Max pixel height reserved for snow element
  this.vMaxX = 5;                 // Maximum X velocity range for snow
  this.vMaxY = 4;                 // Maximum Y velocity range for snow
  this.zIndex = 0;                // CSS stacking order applied to each snowflake

  // --- End of user section ---

  let s = this, storm = this, i,
    // UA sniffing and backCompat rendering mode checks for fixed position, etc.
    isIE = navigator.userAgent.match(/msie/i),
    isIE6 = navigator.userAgent.match(/msie 6/i),
    isWin98 = navigator.appVersion.match(/windows 98/i),
    isMobile = navigator.userAgent.match(/mobile|opera m(ob|in)/i),
    isBackCompatIE = (isIE && document.compatMode === 'BackCompat'),
    noFixed = (isMobile || isBackCompatIE || isIE6),
    screenX = null, screenX2 = null, screenY = null, scrollY = null, vRndX = null, vRndY = null,
    windOffset = 1,
    windMultiplier = 2,
    flakeTypes = 6,
    fixedForEverything = false,
    opacitySupported = (function () {
      try {
        document.createElement('div').style.opacity = '0.5';
      } catch (e) {
        return false;
      }
      return true;
    }()),
    didInit = false,
    docFrag = document.createDocumentFragment();

  this.timers = [];
  this.flakes = [];
  this.disabled = false;
  this.active = false;
  this.meltFrameCount = 20;
  this.meltFrames = [];

  this.events = (function () {

    let old = (!window.addEventListener && window.attachEvent), slice = Array.prototype.slice,
      evt = {
        add: (old ? 'attachEvent' : 'addEventListener'),
        remove: (old ? 'detachEvent' : 'removeEventListener')
      };

    function getArgs(oArgs) {
      let args = slice.call(oArgs), len = args.length;
      if (old) {
        args[1] = 'on' + args[1]; // prefix
        if (len > 3) {
          args.pop(); // no capture
        }
      } else if (len === 3) {
        args.push(false);
      }
      return args;
    }

    function apply(args, sType) {
      let element = args.shift(),
        method = [evt[sType]];
      if (old) {
        element[method](args[0], args[1]);
      } else {
        element[method].apply(element, args);
      }
    }

    function addEvent() {
      apply(getArgs(arguments), 'add');
    }

    function removeEvent() {
      apply(getArgs(arguments), 'remove');
    }

    return {
      add: addEvent,
      remove: removeEvent
    };

  }());

  function rnd(n, min) {
    if (isNaN(min)) {
      min = 0;
    }
    return (Math.random() * n) + min;
  }

  function plusMinus(n) {
    return (parseInt(rnd(2), 10) === 1 ? n * -1 : n);
  }

  this.randomizeWind = function () {
    let i;
    vRndX = plusMinus(rnd(s.vMaxX, 0.2));
    vRndY = rnd(s.vMaxY, 0.2);
    if (this.flakes) {
      for (i = 0; i < this.flakes.length; i++) {
        if (this.flakes[i].active) {
          this.flakes[i].setVelocities();
        }
      }
    }
  };

  this.scrollHandler = function () {
    let i;
    // "attach" snowflakes to bottom of window if no absolute bottom value was given
    scrollY = (s.flakeBottom ? 0 : parseInt(window.scrollY || document.documentElement.scrollTop || document.body.scrollTop, 10));
    if (isNaN(scrollY)) {
      scrollY = 0; // Netscape 6 scroll fix
    }
    if (!fixedForEverything && !s.flakeBottom && s.flakes) {
      for (i = s.flakes.length; i--;) {
        if (s.flakes[i].active === 0) {
          s.flakes[i].stick();
        }
      }
    }
  };

  this.resizeHandler = function () {
    if (window.innerWidth || window.innerHeight) {
      screenX = window.innerWidth - 16 - s.flakeRightOffset;
      screenY = (s.flakeBottom ? s.flakeBottom : window.innerHeight);
    } else {
      screenX = (document.documentElement.clientWidth || document.body.clientWidth || document.body.scrollWidth) - (!isIE ? 8 : 0) - s.flakeRightOffset;
      screenY = s.flakeBottom ? s.flakeBottom : (document.documentElement.clientHeight || document.body.clientHeight || document.body.scrollHeight);
    }
    screenX2 = parseInt(screenX / 2, 10);
  };

  this.resizeHandlerAlt = function () {
    screenX = s.targetElement.offsetLeft + s.targetElement.offsetWidth - s.flakeRightOffset;
    screenY = s.flakeBottom ? s.flakeBottom : s.targetElement.offsetTop + s.targetElement.offsetHeight;
    screenX2 = parseInt(screenX / 2, 10);
  };

  this.freeze = function () {
    // pause animation
    let i;
    if (!s.disabled) {
      s.disabled = 1;
    } else {
      return false;
    }
    for (i = s.timers.length; i--;) {
      clearInterval(s.timers[i]);
    }
  };

  this.resume = function () {
    if (s.disabled) {
      s.disabled = 0;
    } else {
      return false;
    }
    s.timerInit();
  };

  this.toggleSnow = function () {
    if (!s.flakes.length) {
      // first run
      s.start();
    } else {
      s.active = !s.active;
      if (s.active) {
        s.show();
        s.resume();
      } else {
        s.stop();
        s.freeze();
      }
    }
  };

  this.stop = function () {
    let i;
    this.freeze();
    for (i = this.flakes.length; i--;) {
      this.flakes[i].o.style.display = 'none';
    }
    s.events.remove(window, 'scroll', s.scrollHandler);
    s.events.remove(window, 'resize', s.resizeHandler);
    if (s.freezeOnBlur) {
      if (isIE) {
        s.events.remove(document, 'focusout', s.freeze);
        s.events.remove(document, 'focusin', s.resume);
      } else {
        s.events.remove(window, 'blur', s.freeze);
        s.events.remove(window, 'focus', s.resume);
      }
    }
  };

  this.show = function () {
    let i;
    for (i = this.flakes.length; i--;) {
      this.flakes[i].o.style.display = 'block';
    }
  };

  this.SnowFlake = function (parent, type, x, y) {
    let s = this, storm = parent;
    this.type = type;
    this.x = x || parseInt(rnd(screenX - 20), 10);
    this.y = (!isNaN(y) ? y : -rnd(screenY) - 12);
    this.vX = null;
    this.vY = null;
    this.vAmpTypes = [1, 1.2, 1.4, 1.6, 1.8]; // "amplification" for vX/vY (based on flake size/type)
    this.vAmp = this.vAmpTypes[this.type];
    this.melting = false;
    this.meltFrameCount = storm.meltFrameCount;
    this.meltFrames = storm.meltFrames;
    this.meltFrame = 0;
    this.twinkleFrame = 0;
    this.active = 1;
    this.fontSize = (10 + (this.type / 5) * 10);
    this.o = document.createElement('div');
    this.o.innerHTML = storm.snowCharacter;
    this.o.style.color = storm.snowColor;
    this.o.style.position = (fixedForEverything ? 'fixed' : 'absolute');
    this.o.style.width = storm.flakeWidth + 'px';
    this.o.style.height = storm.flakeHeight + 'px';
    this.o.style.fontFamily = 'arial,verdana';
    this.o.style.cursor = 'default';
    this.o.style.overflow = 'hidden';
    this.o.style.fontWeight = 'normal';
    this.o.style.zIndex = storm.zIndex;
    docFrag.appendChild(this.o);

    this.refresh = function () {
      if (isNaN(s.x) || isNaN(s.y)) {
        // safety check
        return false;
      }
      s.o.style.left = s.x + 'px';
      s.o.style.top = s.y + 'px';
    };

    this.stick = function () {
      if (noFixed || (storm.targetElement !== document.documentElement && storm.targetElement !== document.body)) {
        s.o.style.top = (screenY + scrollY - storm.flakeHeight) + 'px';
      } else if (storm.flakeBottom) {
        s.o.style.top = storm.flakeBottom + 'px';
      } else {
        s.o.style.display = 'none';
        s.o.style.top = 'auto';
        s.o.style.bottom = '0px';
        s.o.style.position = 'fixed';
        s.o.style.display = 'block';
      }
    };

    this.vCheck = function () {
      if (s.vX >= 0 && s.vX < 0.2) {
        s.vX = 0.2;
      } else if (s.vX < 0 && s.vX > -0.2) {
        s.vX = -0.2;
      }
      if (s.vY >= 0 && s.vY < 0.2) {
        s.vY = 0.2;
      }
    };

    this.move = function () {
      let vX = s.vX * windOffset, yDiff;
      s.x += vX;
      s.y += (s.vY * s.vAmp);
      if (s.x >= screenX || screenX - s.x < storm.flakeWidth) { // X-axis scroll check
        s.x = 0;
      } else if (vX < 0 && s.x - storm.flakeLeftOffset < -storm.flakeWidth) {
        s.x = screenX - storm.flakeWidth - 1; // flakeWidth;
      }
      s.refresh();
      yDiff = screenY + scrollY - s.y;
      if (yDiff < storm.flakeHeight) {
        s.active = 0;
        if (storm.snowStick) {
          s.stick();
        } else {
          s.recycle();
        }
      } else {
        if (storm.useMeltEffect && s.active && s.type < 3 && !s.melting && Math.random() > 0.998) {
          // ~1/1000 chance of melting mid-air, with each frame
          s.melting = true;
          s.melt();
          // only incrementally melt one frame
          // s.melting = false;
        }
        if (storm.useTwinkleEffect) {
          if (!s.twinkleFrame) {
            if (Math.random() > 0.9) {
              s.twinkleFrame = parseInt(Math.random() * 20, 10);
            }
          } else {
            s.twinkleFrame--;
            s.o.style.visibility = (s.twinkleFrame && s.twinkleFrame % 2 === 0 ? 'hidden' : 'visible');
          }
        }
      }
    };

    this.animate = function () {
      // main animation loop
      // move, check status, die etc.
      s.move();
    };

    this.setVelocities = function () {
      s.vX = vRndX + rnd(storm.vMaxX * 0.12, 0.1);
      s.vY = vRndY + rnd(storm.vMaxY * 0.12, 0.1);
    };

    this.setOpacity = function (o, opacity) {
      if (!opacitySupported) {
        return false;
      }
      o.style.opacity = opacity;
    };

    this.melt = function () {
      if (!storm.useMeltEffect || !s.melting) {
        s.recycle();
      } else {
        if (s.meltFrame < s.meltFrameCount) {
          s.setOpacity(s.o, s.meltFrames[s.meltFrame]);
          s.o.style.fontSize = s.fontSize - (s.fontSize * (s.meltFrame / s.meltFrameCount)) + 'px';
          s.o.style.lineHeight = storm.flakeHeight + 2 + (storm.flakeHeight * 0.75 * (s.meltFrame / s.meltFrameCount)) + 'px';
          s.meltFrame++;
        } else {
          s.recycle();
        }
      }
    };

    this.recycle = function () {
      s.o.style.display = 'none';
      s.o.style.position = (fixedForEverything ? 'fixed' : 'absolute');
      s.o.style.bottom = 'auto';
      s.setVelocities();
      s.vCheck();
      s.meltFrame = 0;
      s.melting = false;
      s.setOpacity(s.o, 1);
      s.o.style.padding = '0px';
      s.o.style.margin = '0px';
      s.o.style.fontSize = s.fontSize + 'px';
      s.o.style.lineHeight = (storm.flakeHeight + 2) + 'px';
      s.o.style.textAlign = 'center';
      s.o.style.verticalAlign = 'baseline';
      s.x = parseInt(rnd(screenX - storm.flakeWidth - 20), 10);
      s.y = parseInt(rnd(screenY) * -1, 10) - storm.flakeHeight;
      s.refresh();
      s.o.style.display = 'block';
      s.active = 1;
    };

    this.recycle(); // set up x/y coords etc.
    this.refresh();

  };

  this.snow = function () {
    let active = 0, used = 0, waiting = 0, flake = null, i;
    for (i = s.flakes.length; i--;) {
      if (s.flakes[i].active === 1) {
        s.flakes[i].move();
        active++;
      } else if (s.flakes[i].active === 0) {
        used++;
      } else {
        waiting++;
      }
      if (s.flakes[i].melting) {
        s.flakes[i].melt();
      }
    }
    if (active < s.flakesMaxActive) {
      flake = s.flakes[parseInt(rnd(s.flakes.length), 10)];
      if (flake.active === 0) {
        flake.melting = true;
      }
    }
  };

  this.mouseMove = function (e) {
    if (!s.followMouse) {
      return true;
    }
    let x = parseInt(e.clientX, 10);
    if (x < screenX2) {
      windOffset = -windMultiplier + (x / screenX2 * windMultiplier);
    } else {
      x -= screenX2;
      windOffset = (x / screenX2) * windMultiplier;
    }
  };

  this.createSnow = function (limit, allowInactive) {
    let i;
    for (i = 0; i < limit; i++) {
      s.flakes[s.flakes.length] = new s.SnowFlake(s, parseInt(rnd(flakeTypes), 10));
      if (allowInactive || i > s.flakesMaxActive) {
        s.flakes[s.flakes.length - 1].active = -1;
      }
    }
    storm.targetElement.appendChild(docFrag);
  };

  this.timerInit = function () {
    s.timers = (!isWin98 ? [setInterval(s.snow, s.animationInterval)] : [setInterval(s.snow, s.animationInterval * 3), setInterval(s.snow, s.animationInterval)]);
  };

  this.init = function () {
    let i;
    for (i = 0; i < s.meltFrameCount; i++) {
      s.meltFrames.push(1 - (i / s.meltFrameCount));
    }
    s.randomizeWind();
    s.createSnow(s.flakesMax); // create initial batch
    s.events.add(window, 'resize', s.resizeHandler);
    s.events.add(window, 'scroll', s.scrollHandler);
    if (s.freezeOnBlur) {
      if (isIE) {
        s.events.add(document, 'focusout', s.freeze);
        s.events.add(document, 'focusin', s.resume);
      } else {
        s.events.add(window, 'blur', s.freeze);
        s.events.add(window, 'focus', s.resume);
      }
    }
    s.resizeHandler();
    s.scrollHandler();
    if (s.followMouse) {
      s.events.add(isIE ? document : window, 'mousemove', s.mouseMove);
    }
    s.animationInterval = Math.max(20, s.animationInterval);
    s.timerInit();
  };

  this.start = function (bFromOnLoad) {
    if (!didInit) {
      didInit = true;
    } else if (bFromOnLoad) {
      // already loaded and running
      return true;
    }
    if (typeof s.targetElement === 'string') {
      let targetID = s.targetElement;
      s.targetElement = document.getElementById(targetID);
      if (!s.targetElement) {
        throw new Error('Snowstorm: Unable to get targetElement "' + targetID + '"');
      }
    }
    if (!s.targetElement) {
      s.targetElement = (!isIE ? (document.documentElement ? document.documentElement : document.body) : document.body);
    }
    if (s.targetElement !== document.documentElement && s.targetElement !== document.body) {
      s.resizeHandler = s.resizeHandlerAlt; // re-map handler to get element instead of screen dimensions
    }
    s.resizeHandler(); // get bounding box elements
    s.usePositionFixed = (s.usePositionFixed && !noFixed); // whether or not position:fixed is supported
    fixedForEverything = s.usePositionFixed;
    if (screenX && screenY && !s.disabled) {
      s.init();
      s.active = true;
    }
  };

  function doDelayedStart() {
    window.setTimeout(function () {
      s.start(true);
    }, 20);
    // event cleanup
    s.events.remove(isIE ? document : window, 'mousemove', doDelayedStart);
  }

  function doStart() {
    if (!s.excludeMobile || !isMobile) {
      if (s.freezeOnBlur) {
        s.events.add(isIE ? document : window, 'mousemove', doDelayedStart);
      } else {
        doDelayedStart();
      }
    }
    // event cleanup
    s.events.remove(window, 'load', doStart);
  }

  // hooks for starting the snow
  if (s.autoStart) {
    s.events.add(window, 'load', doStart, false);
  }

  return this;

}(window, document));