Santa's previous password expired, and he needs help choosing a new one.

To help him remember his new password after the old one expires, Santa has devised a method of coming up with a password based on the previous one. Corporate policy dictates that passwords must be exactly eight lowercase letters (for security reasons), so he finds his new password by incrementing his old password string repeatedly until it is valid.

Read the full puzzle.

using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace AdventOfCode.Y2015.Day11;

[ProblemName("Corporate Policy")]
class Solution : Solver {

    public object PartOne(string input) => Passwords(input).First();
    public object PartTwo(string input) => Passwords(input).Skip(1).First();

    IEnumerable<string> Passwords(string pwd) =>
        from word in Words(pwd) 
        let straigth = Enumerable.Range(0, word.Length - 2).Any(i => word[i] == word[i + 1] - 1 && word[i] == word[i + 2] - 2)
        let reserved = "iol".Any(ch => word.Contains(ch))
        let pairs = Enumerable.Range(0, word.Length - 1).Select(i => word.Substring(i, 2)).Where(sword => sword[0] == sword[1]).Distinct()
        where straigth && !reserved && pairs.Count() > 1
        select word;
    
    IEnumerable<string> Words(string word) {
        while (true) {
            var sb = new StringBuilder();
            for (var i = word.Length - 1; i >= 0; i--) {
                var ch = word[i] + 1;
                if (ch > 'z') {
                    ch = 'a';
                    sb.Insert(0, (char)ch);
                } else {
                    sb.Insert(0, (char)ch);
                    sb.Insert(0, word.Substring(0, i));
                    i = 0;
                }
            }
            word = sb.ToString();
            yield return word;
        }
    }
}

Please ☆ my repo if you like it!