var minpwlength = 4;
    var fairpwlength = 7;
    
    var STRENGTH_SHORT = 0;  // less than minpwlength 
    var STRENGTH_WEAK = 1;  // less than fairpwlength
    var STRENGTH_FAIR = 2;  // fairpwlength or over, no numbers
    var STRENGTH_STRONG = 3; // fairpwlength or over with at least one number
    
    img0 = new Image(); 
    img1 = new Image();
    img2 = new Image();
    img3 = new Image();
    
    img0.src = 'images/tooshort.gif';
    img1.src = 'images/fair.gif';
    img2.src = 'images/medium.gif';
    img3.src = 'images/strong.gif';
    
    var strengthlevel = 0;
    
    var strengthimages = Array( img0.src,
                                img1.src,
                                img2.src,
                                img3.src );
    
    function updatestrength( pw ) {
    
        if( istoosmall( pw ) ) {  
            strengthlevel = STRENGTH_SHORT;
            }
        else if( !isfair( pw ) ) { 
                strengthlevel = STRENGTH_WEAK;
            }    
        else if( hasnum( pw ) ) {
                strengthlevel = STRENGTH_STRONG;
            }
        else {
                strengthlevel = STRENGTH_FAIR;
            }
            document.getElementById( 'strength' ).src = strengthimages[ strengthlevel ];
        }
    function isfair( pw ) {
            if( pw.length < fairpwlength ) {
              return false;
            }
        else { 
              return true;
            }
        }
    function istoosmall( pw ) {
            if( pw.length < minpwlength ) {
              return true;
            }
        else {
              return false;
        }
    }
    function hasnum( pw ) {
           var hasnum = false;
          for( var counter = 0; counter < pw.length; counter ++ ) {
             if( !isNaN( pw.charAt( counter ) ) ) {
                  hasnum = true;
              }
            }
          return hasnum;
        }