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#include <algorithm>
#include <array>
#include <fstream>
#include <iostream>
#include <iterator>
#include <queue>
#include <set>
#include <string>
#include <vector>
using namespace std;
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;
}
set<tuple<size_t, size_t>>
get_neighbours(const vector<string>& maze, tuple<size_t, size_t> possition)
{
static const tuple<size_t, size_t> deltas[] = { { -1, 0 }, { 0, 1 }, { 1, 0 }, { 0, -1 } }; // NOLINT
static const auto SYM_UP = '^';
static const auto SYM_RIGHT = '>';
static const auto SYM_DOWN = 'v';
static const auto SYM_LEFT = '<';
const auto [row, col] = possition;
switch ( maze[row][col] ) {
case SYM_UP:
return { make_tuple(row - 1, col) };
case SYM_RIGHT:
return { make_tuple(row, col + 1) };
case SYM_DOWN:
return { make_tuple(row + 1, col) };
case SYM_LEFT:
return { make_tuple(row, col - 1) };
}
set<tuple<size_t, size_t>> positions;
for ( const auto& delta: deltas ) {
const auto new_row = row + get<0>(delta);
const auto new_col = col + get<1>(delta);
if ( new_row >= maze.size() || new_col >= maze[0].size() ) {
continue;
}
if ( maze[new_row][new_col] == '#' ) {
continue;
}
if ( new_row < row && maze[new_row][new_col] == SYM_DOWN ) {
continue;
}
if ( new_row > row && maze[new_row][new_col] == SYM_UP ) {
continue;
}
if ( new_col < col && maze[new_row][new_col] == SYM_RIGHT ) {
continue;
}
if ( new_col > col && maze[new_row][new_col] == SYM_LEFT ) {
continue;
}
positions.emplace(new_row, new_col);
}
return positions;
}
void
part1(const vector<string>& maze)
{
const tuple<size_t, size_t> start = { 0, maze[0].find('.') };
const tuple<size_t, size_t> end = { maze.size() - 1, maze[maze.size() - 1].find('.') };
queue<tuple<tuple<size_t, size_t>, set<tuple<size_t, size_t>>>> queue;
queue.emplace(start, set<tuple<size_t, size_t>>{});
size_t longest = 0;
while ( !queue.empty() ) {
auto [position, seen] = queue.front();
queue.pop();
while ( position != end ) {
seen.emplace(position);
const auto neighbours = get_neighbours(maze, position);
vector<tuple<size_t, size_t>> next_positions;
// { neighbours \ seen }
set_difference(neighbours.begin(), neighbours.end(),
seen.begin(), seen.end(),
back_inserter(next_positions));
if ( next_positions.empty() ) {
break;
}
position = next_positions[0];
for ( size_t idx = 1; idx != next_positions.size(); ++idx ) {
queue.emplace(next_positions[idx], seen);
}
}
if ( position == end ) {
longest = max(longest, seen.size());
}
}
cout << longest << endl;
}
int
main()
{
const auto maze = read_file("data/day23.txt");
part1(maze);
}
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