function getCookie(cookiename) {
  if (document.cookie.length > 0) {
    cookiestart = document.cookie.indexOf(cookiename + "=");
    if (cookiestart != -1) { 
      cookiestart = cookiestart + cookiename.length + 1 ;
      cookieend = document.cookie.indexOf(";", cookiestart);
      if (cookieend == -1) cookieend = document.cookie.length;
      return unescape(document.cookie.substring(cookiestart, cookieend));
      } 
    }
  return "";
}

function setCookie(cookiename, value, expiredays) {
  var exdate = new Date();
  exdate.setDate(exdate.getDate() + expiredays);
  document.cookie=cookiename + "=" + escape(value) + ((expiredays==null) ? "" : "; expires=" + exdate.toUTCString());
}

function checkCookie() {
  logostate = getCookie('logostate');
  if (logostate != null && logostate != "") {
    //--- Cookie found, read and set data ---
    if (logostate == "./afbeeldingen/sp_01.gif") {
      var logofilename = "./afbeeldingen/sp_02.gif";
      setCookie('logostate', logofilename, 1);
    } else if (logostate == "./afbeeldingen/sp_02.gif") {
      var logofilename = "./afbeeldingen/sp_01.gif";
      setCookie('logostate', logofilename, 1);
    }
    return logofilename;
  } else {
    //--- No cookie found, create a new one ---
    var logofilename = "./afbeeldingen/sp_01.gif";
    setCookie('logostate', logofilename, 1);
    return logofilename;
  }
}