// Input: password
// Output:
//    on success: true
//    on failure: error message
function passwordIsValid(password)
{
  var mostCommonPasswords = ["password", "baseball", "dragon", "football", "monkey",
      "letmein", "abc123", "mustang", "access", "shadow", "master", "michael",
      "superman", "696969", "123123", "batman", "trustno1"];

  var passwordTooEasy = function(pass) {
    // --- HELPER FUNCTION --- //
    var allElementsAreEqual = function(array) { // array.length >= 1
      var firstEl = array[0];
      var allEqual = true;

      for (var i = 1; i < array.length; i++)
        if (array[i] !== firstEl)
          return false;

      return true;
    };

    // --- CHECK 1 --- //
    // check if composed of a simple sequence of 123456/qwerty combination
    //   if password length is up to 12, there are only ~80 passwords which will fail
    // q123w4e56 or similar is still considered OK (details in comments for sequence)
    //   if password length is up to 12, there are 2048 - ~80 {2 ^ 12 - ~80} passwords like this

    var firstSequence = "1234567890";
    var firstSequencePosition = 0;
    var secondSequence = "qwertyuiop";
    var secondSequencePosition = 0;

    // each element will be the number of consecutive characters from any sequence
    //   for example, pass=123qwe456rty translates to [3, 3, 3, 3]
    //   validation fails if all numbers in sequence are the same
    var sequence = [];
    var lastCharacter = -1;

    var allCharactersFromSequence = true;

    for (var i = 0; i < pass.length; i++)
    {
      var character = pass[i];
      var found = -1;
      if (character === firstSequence[firstSequencePosition])
      {
        found = 0;
        firstSequencePosition++;
      }
      else if (character === secondSequence[secondSequencePosition])
      {
        found = 1;
        secondSequencePosition++;
      }
      else // validation passed
      {
        allCharactersFromSequence = false;
      }
      // record the found character
      if (found === lastCharacter) {
        sequence[sequence.length - 1]++;
      } else {
        sequence.push(1);
        lastCharacter = found;
      }
    }
    if (allCharactersFromSequence && allElementsAreEqual(sequence))
      return true;

    // --- CHECK 2 --- //
    // check if all letters are equal
    if (allElementsAreEqual(pass))
      return true;

    // all checks passed, password is (hopefully) not too easy
    return false;
  };

  if      (password.length <= 3) return "Password too short (need 4 or more characters)";
  else if ($.inArray(password, mostCommonPasswords) != -1) return "Password too common";
  else if (passwordTooEasy(password)) return "Password too easy";
  else return true;
}