Day06.cs

using AdventOfCode.CSharp.Common;
using System;

namespace AdventOfCode.CSharp.Y2015.Solvers;

public class Day06 : ISolver
{
    public static void Solve(ReadOnlySpan<byte> input, Solution solution)
    {
        const int part1Mask = 1 << 16;
        int[,] grid = new int[1000, 1000];

        foreach (ReadOnlySpan<byte> instruction in input.SplitLines())
        {
            int x, y;
            int x1, y1, x2, y2;
            switch (instruction[6])
            {
                case (byte)' ':
                    // toggle
                    ParseArea(instruction[7..], out x1, out y1, out x2, out y2);

                    for (x = x1; x <= x2; x++)
                    {
                        for (y = y1; y <= y2; y++)
                        {
                            grid[x, y] = (grid[x, y] ^ part1Mask) + 2;
                        }
                    }

                    break;
                case (byte)'f':
                    // turn off
                    ParseArea(instruction[9..], out x1, out y1, out x2, out y2);

                    for (x = x1; x <= x2; x++)
                    {
                        for (y = y1; y <= y2; y++)
                        {
                            grid[x, y] = Math.Max((grid[x, y] & 0xFFFF) - 1, 0);
                        }
                    }

                    break;
                case (byte)'n':
                    // turn on
                    ParseArea(instruction[8..], out x1, out y1, out x2, out y2);

                    for (x = x1; x <= x2; x++)
                    {
                        for (y = y1; y <= y2; y++)
                        {
                            grid[x, y] = (grid[x, y] | part1Mask) + 1;
                        }
                    }

                    break;
            }
        }

        int part1 = 0;
        int part2 = 0;
        foreach (int gridValue in grid)
        {
            part1 += gridValue >> 16;
            part2 += gridValue & 0xFFFF;
        }

        solution.SubmitPart1(part1);
        solution.SubmitPart2(part2);
    }

    private static void ParseArea(ReadOnlySpan<byte> regionStr, out int x1, out int y1, out int x2, out int y2)
    {
        byte c;
        int i = 0;

        x1 = 0;
        while ((c = regionStr[i++]) != ',')
        {
            x1 = x1 * 10 + (c - '0');
        }

        y1 = 0;
        while ((c = regionStr[i++]) != ' ')
        {
            y1 = y1 * 10 + (c - '0');
        }

        i += 8; // length of "through "

        x2 = 0;
        while ((c = regionStr[i++]) != ',')
        {
            x2 = x2 * 10 + (c - '0');
        }

        y2 = 0;
        while (i < regionStr.Length)
        {
            y2 = y2 * 10 + (regionStr[i++] - '0');
        }
    }
}