This year, I’m posting my solutions to the puzzles in the Advent of Code.
Click the link to view the puzzle and code, along with a link to the solution on Github.
And now, on with the solution:
Day 21, part 1
Simplified Problem:
Modify a string based on the instructions in your puzzle input. Valid instructions are:
swap position X with position Ymeans that the letters at indexesXandY(counting from0) should be swapped.swap letter X with letter Ymeans that the lettersXandYshould be swapped (regardless of where they appear in the string).rotate left/right X stepsmeans that the whole string should be rotated; for example, one right rotation would turnabcdintodabc.rotate based on position of letter Xmeans that the whole string should be rotated to the right based on the index of letterX(counting from0) as determined before this instruction does any rotations. Once the index is determined, rotate the string to the right one time, plus a number of times equal to that index, plus one additional time if the index was at least4.reverse positions X through Ymeans that the span of letters at indexesXthroughY(including the letters atXandY) should be reversed in order.move position X to position Ymeans that the letter which is at indexXshould be removed from the string, then inserted such that it ends up at indexY.
Following the instructions in the puzzle input, what is the output of scrambling abcdefgh?
Day 21, Part 2
Using the same instructions, what was the input that produced fbgdceah?
Solution
For part 2, the inverse rotation code was taken from Reddit:
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class Program { #region Fields private static List<char> Input = "abcdefgh".ToList(); private static readonly string[] Instructions = File.ReadAllLines("input.txt"); private static readonly Regex NumberRegex = new Regex(@"\d+"); private static bool _isPartTwo; #endregion static void Main(string[] args) { // Part 1 foreach (string instruction in Instructions) { ProcessInstruction(instruction); } string message = $"Scrambled password: {new string(Input.ToArray())}"; Console.WriteLine(message); Debug.WriteLine(message); // Part 2 Input = "fbgdceah".ToList(); _isPartTwo = true; foreach (string instruction in Instructions.Reverse()) { ProcessInstruction(instruction); } message = $"Decrypted password: {new string(Input.ToArray())}"; Console.WriteLine(message); Debug.WriteLine(message); Console.ReadKey(); } static void ProcessInstruction(string instruction) { MatchCollection matches = NumberRegex.Matches(instruction); // Can't use a switch statement on substring. if (instruction.StartsWith("swap position")) { SwapPosXPosY(Convert.ToInt32(matches[0].Value), Convert.ToInt32(matches[1].Value)); return; } if (instruction.StartsWith("swap letter")) { string[] words = instruction.Split(new[] {' '}, StringSplitOptions.RemoveEmptyEntries); SwapLetterWithLetter(words[2][0], words[5][0]); return; } if (instruction.StartsWith("rotate left")) { int steps = Convert.ToInt32(matches[0].Value); if (_isPartTwo) { RotateRight(steps); } else { RotateLeft(steps); } return; } if (instruction.StartsWith("rotate right")) { int steps = Convert.ToInt32(matches[0].Value); if (_isPartTwo) { RotateLeft(steps); } else { RotateRight(steps); } return; } if (instruction.StartsWith("rotate based")) { RotateOnPosition(instruction.Last()); return; } if (instruction.StartsWith("reverse")) { ReversePositionsXThroughY(Convert.ToInt32(matches[0].Value), Convert.ToInt32(matches[1].Value)); return; } // Only one instruction left - move position x to position y MovePosXToPosY(Convert.ToInt32(matches[_isPartTwo ? 1 : 0].Value), Convert.ToInt32(matches[_isPartTwo ? 0 : 1].Value)); } #region Scramblers static void SwapPosXPosY(int x, int y) { char temp = Input[x]; Input[x] = Input[y]; Input[y] = temp; } static void SwapLetterWithLetter(char a, char b) { int indexA = Input.IndexOf(a); int indexB = Input.IndexOf(b); char temp = Input[indexA]; Input[indexA] = Input[indexB]; Input[indexB] = temp; } static void RotateLeft(int steps) { for (var i = 0; i < steps; i++) { char temp = Input[0]; Input.RemoveAt(0); Input.Add(temp); } } static void RotateRight(int steps) { int index = Input.Count - 1; for (var i = 0; i < steps; i++) { char temp = Input[index]; Input.RemoveAt(index); Input.Insert(0, temp); } } static void RotateOnPosition(char c) { int index = Input.IndexOf(c); int steps = index + (index >= 4 ? 2 : 1); // Rotate index+1, +1 again if index>= 4 if (_isPartTwo) { // Inverse rotation - https://www.reddit.com/r/adventofcode/comments/5ji29h/2016_day_21_solutions/dbgkbpv/ switch (index) { case 0: case 1: RotateLeft(1); break; case 2: RotateRight(2); break; case 3: RotateLeft(2); break; case 4: RotateRight(1); break; case 5: RotateLeft(3); break; case 6: RotateRight(steps); break; case 7: RotateRight(4); break; } } else { RotateRight(steps); } } static void ReversePositionsXThroughY(int x, int y) { // Reverse parameters are index,count. // Without the +1, Reverse() misses a character. Input.Reverse(x, (y - x) + 1); } static void MovePosXToPosY(int x, int y) { char temp = Input[x]; Input.RemoveAt(x); Input.Insert(y, temp); } #endregion } |
You can find the full solution on Github