CodeWars/5kyu/the_clockwise_spiral/solution.cpp

#include <vector>

static void next_diff(int &dy, int &dx) {
  int new_dx = -dy, new_dy = dx;
  dy = new_dy;
  dx = new_dx;
}

std::vector<std::vector<int>> create_spiral(int n) {
  if (n < 1) {
    return {};
  }

  auto result = std::vector<std::vector<int>>(
    static_cast<size_t>(n), std::vector<int>( static_cast<size_t>(n), 0 )
  );

  int y = 0, x = 0;
  int dy = 0, dx = 1;
  for (int i = 1, max = n * n; i <= max; i++) {
    result[y][x] = i;

    if (y + dy == n || y + dy < 0 || x + dx == n || x + dx < 0 || result[y + dy][x + dx] != 0) {
      next_diff(dy, dx);
    }

    y += dy;
    x += dx;
  }

  return result;
}