1 files changed, 8 insertions, 25 deletions

diff --git a/2023/src/day21.cpp b/2023/src/day21.cpp
index c633155..21a45f7 100644
--- a/2023/src/day21.cpp
+++ b/2023/src/day21.cpp
@@ -1,7 +1,5 @@
-#include <cstddef>
 #include <fstream>
 #include <iostream>
-#include <queue>
 #include <set>
 #include <string>
 #include <vector>
@@ -56,35 +54,20 @@ find_neighbours(position pos, const vector<string>& lines)
 }
 
 size_t
-count(vector<string> lines, position start, size_t rounds)
+count(const vector<string>& lines, position start, size_t rounds)
 {
-        queue<position> positions;
+        set<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);
+                set<position> new_positions;
+                for ( const auto& position: positions ) {
+                        const auto neighbours = find_neighbours(position, lines);
+                        new_positions.insert(neighbours.begin(), neighbours.end());
                 }
+                positions = std::move(new_positions);
         }
-        return sum;
+        return positions.size();
 }
 
 void