Adv2018/D11.2/Program.cs

using System;

namespace D11._2
{
    class Program
    {
        static void Main(string[] args)
        {
            if (args.Length == 0) return;
            int serial = int.Parse(args[0]);

            const int length = 300;
            var grid = new int[length, length];
            var summedGrid = new int[length, length];

            for (int x = 1; x <= length; ++x)
            {
                for (int y = 1; y <= length; ++y)
                {
                    int id = (x + 10);

                    int n = (id * y + serial) * id;
                    int c = (n / 100) % 10;


                    grid[x - 1, y - 1] = c - 5;
                }
            }

            int maxTotal = 0;
            (int x, int y, int g)? maxCoordinates = null;

            for (var size = 0; size < length; ++size)
            {
                for (int x = 0; x < length - size; ++x)
                    for (int y = 0; y < length - size; ++y)
                    {
                        int total = SumSquare(grid, summedGrid, size, x, y);

                        if (total > maxTotal)
                        {
                            maxTotal = total;
                            maxCoordinates = (x + 1, y + 1, size + 1);
                        }
                    }
            }

            Console.WriteLine($"The answer is : ({maxCoordinates.Value.x},{maxCoordinates.Value.y},{maxCoordinates.Value.g})");
        }

        private static int SumSquare(int[,] grid, int[,] summedGrid, int size, int x, int y)
        {
            // summedGrid contains the sum previously calculated for size - 1
            int total = summedGrid[x, y];

            // new sum is obtained by taking the previous sum and adding the edges plus the corner
            for (int i = 0; i < size; ++i)
                total += grid[x + i, y + size] + grid[x + size, y + i];
            total += grid[x + size, y + size];

            summedGrid[x, y] = total;
            return total;
        }
    }
}