Solver.cs

using AdventOfCode.PartSubmitter;
using AdventOfCode.Solver;
using AdventOfCode.Utils;

namespace AdventOfCode.Solutions.Y2023.D18;

public class Solver : ISolver<Line[], Line[]>
{
  public void Parse(string input, IPartSubmitter<Line[], Line[]> partSubmitter)
  {
    var lines = input
      .Split(Environment.NewLine, StringSplitOptions.RemoveEmptyEntries)
      .Select((line) => line.Split(' '));
    var instructions1 = lines
      .Select(
        (line) =>
        {
          var direction = line[0][0] switch
          {
            'U' => Direction.Up,
            'D' => Direction.Down,
            'L' => Direction.Left,
            'R' => Direction.Right,
            _ => throw new Exception("Invalid direction"),
          };
          var length = byte.Parse(line[1]);
          return new Instruction { Direction = direction, Length = length };
        }
      )
      .ToArray();

    partSubmitter.SubmitPart1(LinkInstructions(instructions1));

    var instructions2 = lines
      .Select(
        (line) =>
        {
          var hexString = line[2][2..^1];
          var length = int.Parse(hexString[0..5], System.Globalization.NumberStyles.HexNumber);
          var direction = hexString[5] switch
          {
            '0' => Direction.Right,
            '1' => Direction.Down,
            '2' => Direction.Left,
            '3' => Direction.Up,
            _ => throw new Exception("Invalid direction"),
          };

          return new Instruction { Direction = direction, Length = length };
        }
      )
      .ToArray();

    partSubmitter.SubmitPart2(LinkInstructions(instructions2));
  }

  private Line[] LinkInstructions(Instruction[] instructions)
  {
    Line[] lines = new Line[instructions.Length];
    Coordinate current = new(0, 0);
    for (int i = 0; i < instructions.Length; i++)
    {
      var instruction = instructions[i];
      Coordinate next = current + (instruction.Direction.ToCoordinate() * instruction.Length);
      lines[i] = new Line(current, next);
      current = next;
    }

    return lines;
  }

  public void Solve(Line[] input1, Line[] input2, IPartSubmitter partSubmitter)
  {
    partSubmitter.SubmitPart1(GetArea(input1));
    partSubmitter.SubmitPart2(GetArea(input2));
  }

  private long GetArea(Line[] lines)
  {
    // Implementation of the shoelace formula modified with picks theorem
    long interior = 0;
    ulong boundary = 0;

    foreach (var line in lines)
    {
      interior += checked(line.Start.X * line.End.Y) - checked(line.End.X * line.Start.Y);
      boundary += line.ManhattanLength;
    }

    interior /= 2;
    interior = Math.Abs(interior);

    return interior + ((long)boundary / 2) + 1;
  }
}