1 files changed, 62 insertions, 12 deletions

diff --git a/src/day21.cpp b/src/day21.cpp
index 91611c5..740c3ae 100644
--- a/src/day21.cpp
+++ b/src/day21.cpp
@@ -47,7 +47,7 @@ find_neighbours(position pos, const vector<string>& lines)
                 const auto nrow = row + drow;
                 const auto ncol = col + dcol;
 
-                if ( nrow < lines.size() && ncol < lines[0].size() && lines[nrow][ncol] == '.' ) {
+                if ( nrow < lines.size() && ncol < lines[0].size() && (lines[nrow][ncol] == '.' || lines[nrow][ncol] == 'S') ) {
                         neighbours.emplace(nrow, ncol);
                 }
         }
@@ -55,17 +55,14 @@ find_neighbours(position pos, const vector<string>& lines)
         return neighbours;
 }
 
-void
-part1()
+size_t
+count(vector<string> lines, position start, size_t rounds)
 {
-        auto lines          = read_file("data/day21.txt");
-        auto start_position = find_start_position(lines);
-
         queue<position> positions;
-        positions.emplace(start_position);
+        positions.emplace(start);
 
-        long sum = 0;
-        for ( int round = 0; round != 64; ++round ) {
+        size_t sum = 0;
+        for ( size_t round = 0; round != rounds; ++round ) {
                 set<position> next_positions;
 
                 sum = 0;
@@ -73,7 +70,7 @@ part1()
                         const auto [curr_row, curr_col] = positions.front();
                         lines[curr_row][curr_col]       = '.';
 
-                        auto neighbours = find_neighbours(positions.front(), lines);
+                        const auto neighbours = find_neighbours(positions.front(), lines);
                         positions.pop();
 
                         for ( const auto& [row, col]: neighbours ) {
@@ -87,11 +84,64 @@ part1()
                         positions.emplace(position);
                 }
         }
-        cout << sum << endl;
+        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()
 {
-        part1();
+        auto lines = read_file("data/day21.txt");
+        part1(lines);
+        part2(lines);
 }