Advent of Code. Year 2015. Day 11

--- Day 11: Corporate Policy ---

In today’s puzzle, we need to help Santa to choose his next password. Santa has a method to create the next password based on previous. He does that by incrementing his old password string. Also because of corporate policy, a password should pass additional requirements like: Passwords may not contain the letters i, o, or l.

Table of Contents

Part 1

To solve this puzzle we implemented a recursive method called Increment. It takes one string argument and based on that argument it computes incremented string and returns it.

private static string Increment(string s)
{
  if (s == null || (s.Length == 0))
    return "a";

  var lastChar = s[s.Length - 1];
  var fragment = s.Substring(0, s.Length - 1);
  if (lastChar >= 'z')
    return Increment(fragment) + 'a';

  ++lastChar;
  return fragment + lastChar;
}

Next, we implemented requirements in different methods:

• Passwords must include one increasing straight of at least three letters, like abc, bcd, cde, and so on, up to xyz. They cannot skip letters; abd doesn’t count.

private static bool Increasing(string input)
{
  for (var i = 0; i < input.Length - 2; i++)
  {
    var firstAndSecond = input[i] - input[i + 1] == -1;
    var secondAndThird = input[i + 1] - input[i + 2] == -1;
    if (firstAndSecond && secondAndThird)
      return true;
  }

  return false;
}

• Passwords may not contain the letters i, o, or l, as these letters can be mistaken for other characters and are therefore confusing.

private static bool NotContaining(string input)
{
  var badChars = new[] {'l', 'o', 'i'};
  return !badChars.Any(input.Contains);
}

• Passwords must contain at least two different, non-overlapping pairs of letters, like aa, bb, or zz.

private static bool HasAtLeastTwoDifferentPairs(string input)
{
  var pairs = new List<string>();
  for (var i = 0; i < input.Length - 1; i++)
  {
    pairs.Add( string.Concat(input[i], input[i + 1]));
  }

  return pairs.Distinct().Count(r => r[0] == r[1]) >= 2;
}

After that, we created the method IsValid to check if the password is meeting requirements or not.

private static bool IsValid(string input)
{
  return Increasing(input) &&
         NotContaining(input) &&
         HasAtLeastTwoDifferentPairs(input);
}

Finally, all we needed to do is to call the Increment method until the password became valid.

public string Part1(IEnumerable<string> lines)
{
  var input = GetInput(lines);
  while (!IsValid(input))
    input = Increment(input);

  return input;
}

Part 2

In the second part after we got the password, we just need to increment it again and keep incrementing it until the password is valid.

public string Part2(IEnumerable<string> lines)
{
  var input = GetInput(lines);
  while (!IsValid(input))
    input = Increment(input);

  input = Increment(input);
  while (!IsValid(input))
    input = Increment(input);
  return input;
}

Links:

• Puzzle
• Solution