Advent of Code. Year 2015. Day 5

--- Day 5: Doesn’t He Have Intern-Elves For This? ---

In today’s puzzle, we need to help Santa figuring out which strings in his text file are naughty or nice.

Table of Contents

Part 1

A nice string is one with all of the following properties:

• It contains at least three vowels (aeiou only), like aei, xazegov, or aeiouaeiouaeiou.
• It contains at least one letter that appears twice in a row, like xx, abcdde (dd), or aabbccdd (aa, bb, cc, or dd).
• It does not contain the strings ab, cd, pq, or xy, even if they are part of one of the other requirements.

The easiest way to solve this part is to use regular expressions.

public string Part1(IEnumerable<string> input)
{
  bool HasThreeVowels(string str) =>
    Regex.Matches(str, @"[aeiou]").Count >= 3;

  bool HasDoubledLetter(string str) =>
    Regex.IsMatch(str, @"(\w)\1");

  bool ContainsNaughtyStrings(string str) =>
    Regex.IsMatch(str, @"ab|cd|pq|xy");

  bool IsNiceString(string str) =>
    HasThreeVowels(str)
    && HasDoubledLetter(str)
    && !ContainsNaughtyStrings(str);

  var niceStrings = input.Count(IsNiceString);
  return niceStrings.ToString();
}

Part 2

In the second part we have new properties:

• It contains a pair of any two letters that appears at least twice in the string without overlapping, like xyxy (xy) or aabcdefgaa (aa), but not like aaa (aa, but it overlaps).
• It contains at least one letter which repeats with exactly one letter between them, like xyx, abcdefeghi (efe), or even aaa.

The solution is very much like the first part, except now we have two methods and different regular expressions.

public string Part2(IEnumerable<string> input)
{
  bool HasPair(string str) =>
    Regex.IsMatch(str, @"(\w{2}).*\1");

  bool HasDuplicate(string str) =>
    Regex.IsMatch(str, @"(\w).\1");

  bool IsNiceString(string str) =>
    HasDuplicate(str)
    && HasPair(str);

  var niceStrings = input.Count(IsNiceString);
  return niceStrings.ToString();
}

Links:

• Puzzle
• Solution