using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        // Advent of Code 2019 - day 4

        // https://adventofcode.com/2019/day/4

        Console.WriteLine("PART 1");

        var okPasswords = FindOkPasswords(138307, 654504, false);

        foreach (int okPassword in okPasswords)

        {

            Console.WriteLine(String.Format("{0,6} is ok", okPassword));

        }

        Console.WriteLine("Total ok passwords found (part 1): " + okPasswords.Count);

        Console.WriteLine("================================");

        Console.WriteLine("PART 2");

        okPasswords = FindOkPasswords(138307, 654504, true);

        foreach (int okPassword in okPasswords)

        {

            Console.WriteLine(String.Format("{0,6} is ok", okPassword));

        }

        Console.WriteLine("Total ok passwords found (part 2): " + okPasswords.Count);

    }

    // onlyAllowAdjacentsThatAreNotPartOfLargerGroup is TRUE for part 2, FALSE for part 1.

    private static List<int> FindOkPasswords(int beginRange, int endRange, bool onlyAllowAdjacentsThatAreNotPartOfLargerGroup)

    {

        var retval = new List<int>();

        for (int i=beginRange; i<=endRange; i++)

        {

            if (IsOkPassword(i.ToString(), onlyAllowAdjacentsThatAreNotPartOfLargerGroup)) retval.Add(i);

        }

        return retval;

    }

    // onlyAllowAdjacentsThatAreNotPartOfLargerGroup is TRUE for part 2, FALSE for part 1.

    private static bool IsOkPassword(string password, bool onlyAllowAdjacentsThatAreNotPartOfLargerGroup)

    {

        bool foundAnyRepeating = false;

        bool foundAdjacentsThatAreNotPartOfLargerGroup = false;

        char previousChar = ' ';

        int consecutiveRepeating = 0;

        foreach (char character in password)

        {

            if (character.Equals(previousChar))

            {

                foundAnyRepeating = true;

                consecutiveRepeating++;

            }

            else

            {

                if (consecutiveRepeating == 1) foundAdjacentsThatAreNotPartOfLargerGroup = true;

                consecutiveRepeating = 0;

            }

            if (character < previousChar) return false; // Found decreasing digit

            previousChar = character;

        }

        if (consecutiveRepeating == 1) foundAdjacentsThatAreNotPartOfLargerGroup = true;

        return onlyAllowAdjacentsThatAreNotPartOfLargerGroup ? foundAdjacentsThatAreNotPartOfLargerGroup : foundAnyRepeating;

    }

}