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#include <fstream>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <tuple>
#include <vector>
using namespace std;
using pos_type = tuple<size_t, size_t>;
tuple<pos_type, pos_type, set<pos_type>>
read_file(string_view filename)
{
fstream input{ filename };
set<pos_type> walls;
pos_type start;
pos_type end;
size_t yPos = 0;
for ( string line; getline(input, line); ) {
for ( size_t xPos = 0; xPos != line.size(); ++xPos ) {
auto chr = line[xPos];
if ( chr == '#' ) {
walls.emplace(xPos, yPos);
}
else if ( chr == 'S' ) {
start = { xPos, yPos };
}
else if ( chr == 'E' ) {
end = { xPos, yPos };
}
}
++yPos;
}
return { start, end, walls };
}
map<pos_type, long>
bfs(const tuple<pos_type, pos_type, set<pos_type>>& data)
{
const auto& [start, end, walls] = data;
map<pos_type, long> path;
set<pos_type> seen;
queue<tuple<pos_type, long>> queue;
queue.push({ start, 0 });
while ( !queue.empty() ) {
auto [pos, dist] = queue.front();
queue.pop();
if ( seen.contains(pos) ) {
continue;
}
seen.insert(pos);
path[pos] = dist;
if ( pos == end ) {
break;
}
auto [x, y] = pos;
for ( const auto& new_pos: vector<pos_type>{ { x - 1, y }, { x, y - 1 }, { x + 1, y }, { x, y + 1 } } ) {
if ( walls.contains(new_pos) ) {
continue;
}
queue.push({ new_pos, dist + 1 });
}
}
return path;
}
void
part1(const tuple<pos_type, pos_type, set<pos_type>>& data)
{
auto path = bfs(data);
const auto& [start, end, walls] = data;
long count = 0;
for ( const auto& [pos, dist]: path ) {
auto [x, y] = pos;
for ( const auto& [dx, dy]: vector<pos_type>{ { -1, 0 }, { 0, -1 }, { 1, 0 }, { 0, 1 } } ) {
if ( !walls.contains({ x + dx, y + dy }) ) {
continue;
}
pos_type new_pos{ x + 2 * dx, y + 2 * dy };
if ( path.contains(new_pos) && dist < path.at(new_pos) && path.at(new_pos) - dist >= 100 + 2 ) {
++count;
}
}
}
cout << count << endl;
}
int
main()
{
auto data = read_file("data/day20.txt");
part1(data);
}
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