AdventOfCode/Days/Day12.cs

using System.Collections.Frozen;
using System.Numerics;
using Spectre.Console;

namespace AdventOfCode.Days;

public class Day12 : Day
{
    public override int Number => 12;
    public override string Name => "Garden Groups";

    private const int GridSize = 140;

    public override void RunPart1(bool display = true)
    {
        var areas = ParseAreas();

        var visited = new HashSet<Point>();
        var totalPrice = 0;

        var currentArea = 0;
        var currentPerimeter = 0;

        for (var y = 0; y < GridSize; y++)
        {
            for (var x = 0; x < GridSize; x++)
            {
                // Skip already visited points
                if (visited.Contains(new Point(x, y)))
                {
                    continue;
                }

                var id = areas[x, y];

                ComputeAreaPrice(new Point(x, y), id, ref currentArea, ref currentPerimeter);

                // Add price of fencing this area
                totalPrice += currentArea * currentPerimeter;

                currentArea = 0;
                currentPerimeter = 0;
            }
        }

        if (display)
        {
            AnsiConsole.MarkupLine($"[green]Total pricing: [yellow]{totalPrice}[/][/]");
        }

        return;

        void ComputeAreaPrice(Point position, byte id, ref int area, ref int perimeter)
        {
            if (!visited.Add(position))
            {
                return;
            }

            // Update area and perimeter
            area++;
            perimeter += 4;

            var left = position with { X = position.X - 1 };
            var right = position with { X = position.X + 1 };
            var up = position with { Y = position.Y - 1 };
            var down = position with { Y = position.Y + 1 };

            if (left is { X: >= 0 and < GridSize } && areas[left.X, left.Y] == id)
            {
                perimeter--;

                ComputeAreaPrice(left, id, ref area, ref perimeter);
            }

            if (right is { X: >= 0 and < GridSize } && areas[right.X, right.Y] == id)
            {
                perimeter--;

                ComputeAreaPrice(right, id, ref area, ref perimeter);
            }

            if (up is { Y: >= 0 and < GridSize } && areas[up.X, up.Y] == id)
            {
                perimeter--;

                ComputeAreaPrice(up, id, ref area, ref perimeter);
            }

            if (down is { Y: >= 0 and < GridSize } && areas[down.X, down.Y] == id)
            {
                perimeter--;

                ComputeAreaPrice(down, id, ref area, ref perimeter);
            }
        }
    }

    public override void RunPart2(bool display = true)
    {

    }

    private byte[,] ParseAreas()
    {
        var areas = new byte[GridSize, GridSize];

        int y = 0;

        foreach (var line in Input.AsSpan().EnumerateLines())
        {
            int x = 0;

            foreach (var area in line)
            {
                areas[x, y] = (byte)(area - 'A');

                x++;
            }

            y++;
        }

        return areas;
    }

    private readonly record struct Point(int X, int Y)
        : IAdditionOperators<Point, Point, Point>,
            ISubtractionOperators<Point, Point, Point>,
            IMultiplyOperators<Point, int, Point>
    {
        public static Point operator +(Point left, Point right)
        {
            return new Point(left.X + right.X, left.Y + right.Y);
        }

        public static Point operator -(Point left, Point right)
        {
            return new Point(left.X - right.X, left.Y - right.Y);
        }

        public static Point operator *(Point left, int right)
        {
            return new Point(left.X * right, left.Y * right);
        }

        public static Point operator *(int left, Point right)
        {
            return new Point(right.X * left, right.Y * left);
        }

        public static implicit operator Point((int X, int Y) point)
            => new(point.X, point.Y);
    }
}