advent-of-code/old/2021/10/Program.cs

using System.Numerics;
using System.Text;

namespace Day10
{
    class Program
    {
        static List<char> openingCharacters = new List<char>() { '(', '[', '{', '<' };
        static List<char> closingCharacters = new List<char>() { ')', ']', '}', '>' };

        static void Main(string[] args)
        {
            string[] lines = File.ReadAllLines("input.txt");


            //Part1(lines);
            Part2(lines);
        }

        static void Part1(string[] lines)
        {
            List<string> uncorruptedLines = new List<string>();
            int syntaxErrorScore = 0;

            foreach (string line in lines)
            {
                if (!IsLineCorrupted(line, ref syntaxErrorScore))
                {
                    uncorruptedLines.Add(line);
                }
            }

            Console.WriteLine($"Syntax error score: {syntaxErrorScore}");
        }

        static void Part2(string[] lines)
        {
            List<string> uncorruptedLines = new List<string>();
            int syntaxErrorScore = 0;

            foreach (string line in lines)
            {
                if (!IsLineCorrupted(line, ref syntaxErrorScore))
                {
                    uncorruptedLines.Add(line);
                }
            }

            // Now we just have the incomplete lines, work out how to complete them

            List<BigInteger> autoCompleteLineScores = new List<BigInteger>();

            foreach (string line in uncorruptedLines)
            {
                BigInteger totalLineScore = 0;
                string characters = GetCharactersToCompleteLine(line);

                // Now calculate the score for the line
                foreach (char character in characters)
                {
                    totalLineScore = totalLineScore * 5;

                    switch (character)
                    {
                        case ')':
                            totalLineScore += 1;
                            break;
                        case ']':
                            totalLineScore += 2;
                            break;
                        case '}':
                            totalLineScore += 3;
                            break;
                        case '>':
                            totalLineScore += 4;
                            break;
                        default:
                            break;
                    }
                }

                autoCompleteLineScores.Add(totalLineScore);

                Console.WriteLine($"{characters} - {totalLineScore} total points.");
            }

            // the winner is found by sorting all of the scores and then taking the middle score. (There will always be an odd number of scores to consider.)
            autoCompleteLineScores.Sort();
            BigInteger middleScore = autoCompleteLineScores.Skip(autoCompleteLineScores.Count / 2).Take(1).First();

            Console.WriteLine($"Middle score: {middleScore}");
        }

        static bool IsLineCorrupted(string line, ref int syntaxErrorScore)
        {
            Stack<char> stackOfExpectedClosingCharacters = new Stack<char>();

            for (int i = 0; i < line.Length; i++)
            {
                if (openingCharacters.Contains(line[i]))
                {
                    // If it's an opening character then add the expected closing character to the stack
                    stackOfExpectedClosingCharacters.Push(GetExpectedClosingCharacter(line[i]));
                }
                else if (closingCharacters.Contains(line[i]))
                {
                    // If it's a closing character, then check if it's the one we're expecting
                    // Make sure there's items in the stack so we don't get an InvalidOperationException
                    if (stackOfExpectedClosingCharacters.Count > 0)
                    {
                        char expectedClosingCharacter = stackOfExpectedClosingCharacters.Pop();
                        if (line[i] != expectedClosingCharacter)
                        {
                            // It's not the one we're expecting so the line is corrupted
                            Console.WriteLine($"Expected {expectedClosingCharacter}, but found {line[i]} instead.");

                            // Update the syntax error score
                            switch (line[i])
                            {
                                case ')':
                                    syntaxErrorScore += 3;
                                    break;
                                case ']':
                                    syntaxErrorScore += 57;
                                    break;
                                case '}':
                                    syntaxErrorScore += 1197;
                                    break;
                                case '>':
                                    syntaxErrorScore += 25137;
                                    break;
                                default:
                                    break;
                            }

                            // Report the corrupted line
                            return true;
                        }

                        // If it is the one we're expecting we can safely carry on with the line
                    }
                }
            }

            // Having reached the end of the line with no wrong closing characters the line cannot be corrupted
            return false;
        }

        static string GetCharactersToCompleteLine(string line)
        {
            Stack<char> stackOfExpectedClosingCharacters = new Stack<char>();

            for (int i = 0; i < line.Length; i++)
            {
                if (openingCharacters.Contains(line[i]))
                {
                    // If it's an opening character then add the expected closing character to the stack
                    stackOfExpectedClosingCharacters.Push(GetExpectedClosingCharacter(line[i]));
                }
                else if (closingCharacters.Contains(line[i]))
                {
                    // If it's a closing character, then it must be the one we're expecting as we've already got rid of the corrupt lines
                    // Make sure there's items in the stack so we don't get an InvalidOperationException, then pop it off the stack
                    if (stackOfExpectedClosingCharacters.Count > 0)
                    {
                        stackOfExpectedClosingCharacters.Pop();
                    }
                }
            }

            // The stack should now contain all the closing characters needed, in order, so get them into a string
            StringBuilder charactersToCompleteLine = new StringBuilder();

            while (stackOfExpectedClosingCharacters.Count > 0)
            {
                char character = stackOfExpectedClosingCharacters.Pop();
                charactersToCompleteLine.Append(character);
            }

            Console.WriteLine($"Complete by adding {charactersToCompleteLine.ToString()}");

            return charactersToCompleteLine.ToString();
        }


        static char GetExpectedClosingCharacter(char openingCharacter)
        {
            int index = openingCharacters.FindIndex(x => x == openingCharacter);

            return closingCharacters[index];
        }
    }
}