feat(*): Add 2021-10 part 2 files

2021/10/Program.cs

@@ -1,4 +1,7 @@
-namespace Day10
+using System.Numerics;
+using System.Text;
+
+namespace Day10
 {
     class Program
     {
@@ -10,8 +13,8 @@
             string[] lines = File.ReadAllLines("input.txt");
 
 
-            Part1(lines);
-            //Part2(lines);
+            //Part1(lines);
+            Part2(lines);
         }
 
         static void Part1(string[] lines)
@@ -30,6 +33,64 @@
             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>();
@@ -85,6 +146,43 @@
             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);