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#include <filesystem>
#include <fstream>
#include <iostream>
#include <string>
#include <vector>
using namespace std;
namespace {
enum class Direction : uint8_t {
Up,
Right,
Down,
Left
};
struct Cart {
size_t x;
size_t y;
Direction direction;
unsigned long intersection{};
};
using Grid = vector<string>;
using Carts = vector<Cart>;
tuple<Grid, Carts>
read_file(const filesystem::path& filename)
{
ifstream file{ filename };
Grid data;
Carts carts;
size_t y = 0;
for ( string line; getline(file, line); ) {
for ( size_t x = 0; x != line.size(); ++x ) {
using enum Direction;
const char chr = line[x];
if ( chr == '^' || chr == 'v' ) {
carts.emplace_back(x, y, chr == '^' ? Up : Down);
}
else if ( chr == '<' || chr == '>' ) {
carts.emplace_back(x, y, chr == '<' ? Left : Right);
}
}
data.emplace_back(line);
++y;
}
return { data, carts };
}
void
sort_carts(Carts& carts)
{
ranges::sort(carts,
[](auto& lhs, auto& rhs) {
return tie(lhs.y, lhs.x) < tie(rhs.y, rhs.x);
});
}
void
move_cart(Cart& cart)
{
using enum Direction;
switch ( cart.direction ) {
case Up:
--cart.y;
break;
case Right:
++cart.x;
break;
case Down:
++cart.y;
break;
case Left:
--cart.x;
break;
}
}
void
apply_direction_rules(const Grid& grid, Cart& cart)
{
using enum Direction;
const char chr = grid[cart.y][cart.x];
switch ( chr ) {
case '/':
if ( cart.direction == Up ) {
cart.direction = Right;
}
else if ( cart.direction == Right ) {
cart.direction = Up;
}
else if ( cart.direction == Down ) {
cart.direction = Left;
}
else if ( cart.direction == Left ) {
cart.direction = Down;
}
break;
case '\\':
if ( cart.direction == Up ) {
cart.direction = Left;
}
else if ( cart.direction == Right ) {
cart.direction = Down;
}
else if ( cart.direction == Down ) {
cart.direction = Right;
}
else if ( cart.direction == Left ) {
cart.direction = Up;
}
break;
case '+':
switch ( cart.intersection++ % 3 ) {
case 0:
cart.direction = static_cast<Direction>((static_cast<int>(cart.direction) + 3) % 4);
break;
case 2:
cart.direction = static_cast<Direction>((static_cast<int>(cart.direction) + 1) % 4);
break;
default:
break;
};
break;
default:
break;
}
}
void
part1(const Grid& grid, Carts carts)
{
while ( true ) {
sort_carts(carts);
for ( size_t i = 0; i != carts.size(); ++i ) {
auto& cart = carts[i];
move_cart(cart);
apply_direction_rules(grid, cart);
for ( size_t j = 0; j != carts.size(); ++j ) {
if ( i == j ) {
continue;
}
const auto& other = carts[j];
if ( cart.x == other.x && cart.y == other.y ) {
cout << "Part 1: " << cart.x << ',' << cart.y << '\n';
return;
}
}
}
}
}
void
part2(const Grid& grid, Carts carts)
{
while ( true ) {
sort_carts(carts);
vector<size_t> carts_to_be_removed;
for ( size_t i = 0; i != carts.size(); ++i ) {
auto& cart = carts[i];
move_cart(cart);
apply_direction_rules(grid, cart);
for ( size_t j = 0; j != carts.size(); ++j ) {
if ( i == j ) {
continue;
}
const auto& other = carts[j];
if ( cart.x == other.x && cart.y == other.y ) {
carts_to_be_removed.emplace_back(i);
carts_to_be_removed.emplace_back(j);
}
}
}
// remove carts
ranges::sort(carts_to_be_removed);
ranges::reverse(carts_to_be_removed);
for ( auto num: carts_to_be_removed ) {
carts.erase(carts.begin() + static_cast<long>(num));
}
if ( carts.size() == 1 ) {
cout << "Part 2: " << carts[0].x << ',' << carts[0].y << '\n';
return;
}
}
}
} // namespace
int
main()
{
auto [grid, carts] = read_file("data/day13.txt");
part1(grid, carts);
part2(grid, carts);
}
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