Returns an array of n different variants of the input string str. The variants are computed by randomly rotating one random character.
function computeInputVariants(str, n) {
  var variants = [ str ];
  for (var i = 1; i < n; i++) {
    var pos = Math.floor(Math.random() * str.length);
    var chr = String.fromCharCode((str.charCodeAt(pos) + Math.floor(Math.random() * 128)) % 128);
    variants[i] = str.substring(0, pos) + chr + str.substring(pos + 1, str.length);
  }
  return variants;
}