Apex random string generator

apex random string

trigger ContactGuidGenerator on Contact (before insert) {
    for( Contact c: Trigger.new ) {
 
        final String chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789abcdefghijklmnopqrstuvwxyz';
        String guid = '';
        while (guid.length() < 16) {
           Integer idx = Math.mod(Math.abs(Crypto.getRandomInteger()), chars.length());
           guid += chars.substring(idx, idx+1);
        }
        c.guid__c = guid; 
    }    
}

Maybe check c.guid__c if the created random string already exist and create a new random string or not depending on that, which would be better IMO