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#include <iostream>
#include <map>
using namespace std;
namespace {
const int WIDTH = 300;
const int HEIGHT = 300;
using Grid = map<tuple<int, int>, int>;
int
power_level(int x, int y, int serial)
{
auto rack_id = x + 10;
auto level = rack_id * y;
level += serial;
level *= rack_id;
level /= 100;
level %= 10;
level -= 5;
return level;
}
Grid
build_grid(int serial)
{
Grid grid;
for ( int x = 1; x <= WIDTH; ++x ) {
for ( int y = 1; y <= HEIGHT; ++y ) {
grid[{ x, y }] = power_level(x, y, serial);
}
}
return grid;
}
int
sum3x3(const Grid& grid, int x, int y)
{
auto sum = 0;
for ( int dx = 0; dx != 3; ++dx ) {
for ( int dy = 0; dy != 3; ++dy ) {
sum += grid.at({ x + dx, y + dy });
}
}
return sum;
}
void
part1(const int serial)
{
auto grid = build_grid(serial);
auto best_sum = numeric_limits<int>::min();
auto best_x = 0;
auto best_y = 0;
for ( int x = 1; x <= WIDTH - 2; ++x ) {
for ( int y = 1; y <= HEIGHT - 2; ++y ) {
if ( auto sum = sum3x3(grid, x, y); sum > best_sum ) {
best_sum = sum;
best_x = x;
best_y = y;
}
}
}
cout << "Part 1: " << best_x << ',' << best_y << '\n';
}
Grid
pre_process(const Grid& grid)
{
Grid sat;
for ( int y = 1; y <= HEIGHT; ++y ) {
for ( int x = 1; x <= WIDTH; ++x ) {
const auto a = grid.at({ x, y });
const auto b = (x > 1) ? sat.at({ x - 1, y }) : 0;
const auto c = (y > 1) ? sat.at({ x, y - 1 }) : 0;
const auto d = (x > 1 && y > 1) ? sat.at({ x - 1, y - 1 }) : 0;
sat[{ x, y }] = a + b + c - d;
}
}
return sat;
}
int
get_safe(const Grid& grid, int x, int y)
{
return (x < 1 || y < 1) ? 0 : grid.at({ x, y });
}
int
square_sum(const Grid& grid, int x, int y, int s)
{
int x2 = x + s - 1;
int y2 = y + s - 1;
return get_safe(grid, x2, y2) - get_safe(grid, x - 1, y2) - get_safe(grid, x2, y - 1) + get_safe(grid, x - 1, y - 1);
}
void
part2(const int serial)
{
static const int MAX_SIZE = 100;
auto grid = pre_process(build_grid(serial));
auto best_sum = numeric_limits<int>::min();
auto best_x = 0;
auto best_y = 0;
auto best_s = 0;
for ( int s = 1; s <= MAX_SIZE; ++s ) {
for ( int x = 1; x <= WIDTH - s + 1; ++x ) {
for ( int y = 1; y <= HEIGHT - s + 1; ++y ) {
if ( auto sum = square_sum(grid, x, y, s); sum > best_sum ) {
best_sum = sum;
best_x = x;
best_y = y;
best_s = s;
}
}
}
}
cout << "Part 2: " << best_x << ',' << best_y << "," << best_s << '\n';
}
} // namespace
int
main()
{
static const int SERIAL = 7989;
part1(SERIAL);
part2(SERIAL);
}
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