AdventOfCode/Days/Day9.cs

using Spectre.Console;

namespace AdventOfCode.Days;

public class Day9 : Day
{
    public override int Number => 9;
    public override string Name => "Movie Theater";

    public override void RunPart1(bool display = true)
    {
        var tilePositions = ParseTilePositions(Input);

        var maxArea = 0L;

        for (var firstIndex = 0; firstIndex < tilePositions.Count - 1; firstIndex++)
        {
            for (var secondIndex = firstIndex + 1; secondIndex < tilePositions.Count; secondIndex++)
            {
                var firstTile = tilePositions[firstIndex];
                var secondTile = tilePositions[secondIndex];

                var area = (Math.Abs(secondTile.I - firstTile.I) + 1) *
                               (Math.Abs(secondTile.J - firstTile.J) + 1);

                if (area > maxArea)
                {
                    maxArea = area;
                }
            }
        }

        if (display)
        {
            AnsiConsole.MarkupLine($"[green]Max tile area is: [yellow]{maxArea}[/][/]");
        }
    }

    public override void RunPart2(bool display = true)
    {
        var tilePositions = ParseTilePositions(Input);

        // Make a list of possible areas in descending order
        var tileAreas = new List<TileArea>();

        for (var firstIndex = 0; firstIndex < tilePositions.Count - 1; firstIndex++)
        {
            for (var secondIndex = firstIndex + 1; secondIndex < tilePositions.Count; secondIndex++)
            {
                var firstTile = tilePositions[firstIndex];
                var secondTile = tilePositions[secondIndex];

                var area = (Math.Abs(secondTile.I - firstTile.I) + 1) *
                           (Math.Abs(secondTile.J - firstTile.J) + 1);

                tileAreas.Add(new TileArea(firstTile, secondTile, area));
            }
        }

        // Sort list in descending order (biggest area first)
        tileAreas.Sort((left, right) => right.Area.CompareTo(left.Area));

        // Compute polygon bounds with green tiles
        var jBoundsAtI = new Dictionary<long, PositionBounds>();
        var iBoundsAtJ = new Dictionary<long, PositionBounds>();
        foreach (var (firstTile, secondTile) in tilePositions.Zip(tilePositions.Skip(1).Append(tilePositions[0])))
        {
            // Tiles are on same line, set bounds for columns (j)
            if (firstTile.I == secondTile.I)
            {
                var start = Math.Min(firstTile.J, secondTile.J);
                var end = Math.Max(firstTile.J, secondTile.J);

                for (var j = start; j <= end; j++)
                {
                    // Update bounds
                    if (iBoundsAtJ.TryGetValue(j, out var bounds))
                    {
                        iBoundsAtJ[j] = new PositionBounds(
                            Math.Min(firstTile.I, bounds.Min),
                            Math.Max(firstTile.I, bounds.Max)
                        );
                    }
                    else
                    {
                        iBoundsAtJ[j] = new PositionBounds(
                            firstTile.I,
                            firstTile.I
                        );
                    }
                }
            }
            // Tiles are on same column, set bounds for rows (i)
            else if (firstTile.J == secondTile.J)
            {
                var start = Math.Min(firstTile.I, secondTile.I);
                var end = Math.Max(firstTile.I, secondTile.I);

                for (var i = start; i<= end; i++)
                {
                    // Update bounds
                    if (jBoundsAtI.TryGetValue(i, out var bounds))
                    {
                        jBoundsAtI[i] = new PositionBounds(
                            Math.Min(firstTile.J, bounds.Min),
                            Math.Max(firstTile.J, bounds.Max)
                        );
                    }
                    else
                    {
                        jBoundsAtI[i] = new PositionBounds(
                            firstTile.J,
                            firstTile.J
                        );
                    }
                }
            }
        }

        // Find the first area that is valid (composed of green or red tiles)
        var maxValidArea = -1L;
        foreach (var tileArea in tileAreas)
        {
            // For each point in this area, check if it is within bounds
            var iStart = Math.Min(tileArea.First.I, tileArea.Second.I);
            var iEnd = Math.Max(tileArea.First.I, tileArea.Second.I);

            var jStart = Math.Min(tileArea.First.J, tileArea.Second.J);
            var jEnd = Math.Max(tileArea.First.J, tileArea.Second.J);

            // Check if any point is out of bounds
            var outOfBounds = false;
            for (var i = iStart; i <= iEnd; i++)
            {
                for (var j = jStart; j <= jEnd; j++)
                {
                    var iBound = iBoundsAtJ[j];
                    var jBound = jBoundsAtI[i];

                    if (i < iBound.Min || i > iBound.Max ||
                        j < jBound.Min || j > jBound.Max)
                    {
                        outOfBounds = true;

                        break;
                    }
                }

                if (outOfBounds)
                {
                    break;
                }
            }

            // If no points are out of bounds, we have our biggest valid area
            if (!outOfBounds)
            {
                maxValidArea = tileArea.Area;

                break;
            }
        }

        if (display)
        {
            AnsiConsole.MarkupLine($"[green]Max valid tile area is: [yellow]{maxValidArea}[/][/]");
        }
    }

    private static List<TilePosition> ParseTilePositions(string input)
    {
        var tilePositions = new List<TilePosition>();

        foreach (var line in input.EnumerateLines())
        {
            var splitIndex = line.IndexOf(',');

            var j = long.Parse(line[..splitIndex]);
            var i = long.Parse(line[(splitIndex + 1)..]);

            tilePositions.Add(new TilePosition(i, j));
        }

        return tilePositions;
    }

    private record TilePosition(long I, long J);

    private record TileArea(TilePosition First, TilePosition Second, long Area);

    private record PositionBounds(long Min, long Max);
}