First Line. Second Line.

Line after a gap.

#include <iostream>
#include <vector>
#include <array>
#include <limits>


int third_digit(int n);
int power(int x, int y, int serial);
std::vector<int> summed_area_table(const int n, const std::vector<int>& array);
int get_sum(const std::vector<int>& i, int n, int x, int y, int size);

int main() {
  const int serial = 8772;
  const int n = 300;
  std::vector<int> powers(n  n);
  for(int x = 0; x < n; ++x) {
    for(int y = 0; y < n; ++y) {
      powers[x + n  y] = power(x + 1, y + 1, serial);
    }
  }

auto sat = summed_area_table(n, powers);
  int max_sum = std::numeric_limits<int>::min();
  int coords[2] = {-1, -1};
  int max_size = -1;
  for(int ksize = 1; ksize <= n; ++ksize) {
    for(int x = 1; x < n - ksize + 1 ; ++x) {
      for(int y = 1; y < n - ksize + 1; ++y) {
    //change from first exclusive last inclusive
    //to first invclusive last exclusive
    const int sum = get_sum(sat, n, x - 1, y - 1, ksize);
    if(sum > max_sum) {
      max_sum = sum;
      coords[0] = x;
      coords[1] = y;
      max_size = ksize;
    }
      }
    }
  }
  std::cout << "x " << coords[0] + 1 << " y " << coords[1] + 1 << " with sum " << max_sum << " and size " << max_size << std::endl;
}

int third_digit(int n) {
  n = n - (n % 100);
  n /= 100;
  return n % 10;
}

int power(int x, int y, int serial) {
  const int rack_id = x + 10;
  int power_level = y  rack_id;
  power_level += serial;
  power_level = rack_id;
  power_level = third_digit(power_level);
  return power_level - 5;
}

//https://en.wikipedia.org/wiki/Summed-area_table
std::vector<int> summed_area_table(const int n, const std::vector<int>& array) {
  std::vector<int> i(n  n);
  i[0 + n  0] = array[0 + n  0];
  if(n > 1) {
    i[1 + n  0] = array[0 + n  0] + array[1 + n  0];
    i[0 + n  1] = array[0 + n  0] + array[0 + n  1];
  }
  for(int y = 1; y < n; ++y) {
    for(int x = 1; x < n; ++x) {
      i[x + n  y] = array[x + n  y] + i[x + n  (y - 1)] + i[x - 1 + n  y] - i[x - 1 + n  (y - 1)];
    }
  }
  return i;
}

inline int get_sum(const std::vector<int>& i, int n, int x, int y, int size) {
  return i[x + size + n * (y + size)] + i[x + n  y] - i[x + size + n  y] - i[x + n * (y + size)];
}