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#include <cstddef>
#include <fstream>
#include <iostream>
#include <queue>
#include <set>
#include <string>
#include <vector>
using namespace std;
using position = tuple<size_t, size_t>;
vector<string>
read_file(string_view filename)
{
fstream input{ filename };
vector<string> data;
for ( string line; getline(input, line); ) {
data.emplace_back(line);
}
return data;
}
position
find_start_position(const vector<string>& lines)
{
for ( size_t row = 0; row != lines.size(); ++row ) {
for ( size_t col = 0; col != lines[row].size(); ++col ) {
if ( lines[row][col] == 'S' ) {
return { row, col };
}
}
}
return {};
}
set<position>
find_neighbours(position pos, const vector<string>& lines)
{
static const vector<position> movements = { { -1, 0 }, { 0, 1 }, { 1, 0 }, { 0, -1 } };
set<position> neighbours;
const auto [row, col] = pos;
for ( const auto& [drow, dcol]: movements ) {
const auto nrow = row + drow;
const auto ncol = col + dcol;
if ( nrow < lines.size() && ncol < lines[0].size() && (lines[nrow][ncol] == '.' || lines[nrow][ncol] == 'S') ) {
neighbours.emplace(nrow, ncol);
}
}
return neighbours;
}
size_t
count(vector<string> lines, position start, size_t rounds)
{
queue<position> positions;
positions.emplace(start);
size_t sum = 0;
for ( size_t round = 0; round != rounds; ++round ) {
set<position> next_positions;
sum = 0;
while ( !positions.empty() ) {
const auto [curr_row, curr_col] = positions.front();
lines[curr_row][curr_col] = '.';
const auto neighbours = find_neighbours(positions.front(), lines);
positions.pop();
for ( const auto& [row, col]: neighbours ) {
lines[row][col] = 'O';
next_positions.emplace(row, col);
++sum;
}
}
for ( const auto& position: next_positions ) {
positions.emplace(position);
}
}
return sum;
}
void
part1(const vector<string>& lines)
{
const auto start = find_start_position(lines);
cout << count(lines, start, 64) << endl;
}
void
part2(const vector<string>& lines)
{
const auto start = find_start_position(lines);
const auto [row, col] = start;
const auto pow2 = [](size_t val) -> size_t {
return val * val;
};
const auto size = lines.size();
const size_t steps = 26501365;
const auto grid_width = steps / size - 1;
const auto odd = pow2(grid_width / 2 * 2 + 1);
const auto even = pow2((grid_width + 1) / 2 * 2);
const auto odd_points = count(lines, start, size * 2 + 1);
const auto even_points = count(lines, start, size * 2);
const auto corner_t = count(lines, { size - 1, col }, size - 1);
const auto corner_r = count(lines, { row, 0 }, size - 1);
const auto corner_b = count(lines, { 0, col }, size - 1);
const auto corner_l = count(lines, { row, size - 1 }, size - 1);
const auto small_tr = count(lines, { size - 1, 0 }, size / 2 - 1);
const auto small_tl = count(lines, { size - 1, size - 1 }, size / 2 - 1);
const auto small_br = count(lines, { 0, 0 }, size / 2 - 1);
const auto small_bl = count(lines, { 0, size - 1 }, size / 2 - 1);
const auto large_tr = count(lines, { size - 1, 0 }, size * 3 / 2 - 1);
const auto large_tl = count(lines, { size - 1, size - 1 }, size * 3 / 2 - 1);
const auto large_br = count(lines, { 0, 0 }, size * 3 / 2 - 1);
const auto large_bl = count(lines, { 0, size - 1 }, size * 3 / 2 - 1);
auto total = odd * odd_points;
total += even * even_points;
total += corner_t + corner_r + corner_b + corner_l;
total += (grid_width + 1) * (small_tr + small_tl + small_br + small_bl);
total += grid_width * (large_tr + large_tl + large_br + large_bl);
cout << total << endl;
}
int
main()
{
auto lines = read_file("data/day21.txt");
part1(lines);
part2(lines);
}
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