1 files changed, 97 insertions, 0 deletions

diff --git a/src/day21.cpp b/src/day21.cpp
new file mode 100644
index 0000000..91611c5
--- /dev/null
+++ b/src/day21.cpp
@@ -0,0 +1,97 @@
+#include <cstddef>
+#include <fstream>
+#include <iostream>
+#include <queue>
+#include <set>
+#include <string>
+#include <vector>
+using namespace std;
+
+using position = tuple<size_t, size_t>;
+
+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;
+}
+
+position
+find_start_position(const vector<string>& lines)
+{
+        for ( size_t row = 0; row != lines.size(); ++row ) {
+                for ( size_t col = 0; col != lines[row].size(); ++col ) {
+                        if ( lines[row][col] == 'S' ) {
+                                return { row, col };
+                        }
+                }
+        }
+        return {};
+}
+
+set<position>
+find_neighbours(position pos, const vector<string>& lines)
+{
+        static const vector<position> movements = { { -1, 0 }, { 0, 1 }, { 1, 0 }, { 0, -1 } };
+
+        set<position> neighbours;
+        const auto [row, col] = pos;
+
+        for ( const auto& [drow, dcol]: movements ) {
+                const auto nrow = row + drow;
+                const auto ncol = col + dcol;
+
+                if ( nrow < lines.size() && ncol < lines[0].size() && lines[nrow][ncol] == '.' ) {
+                        neighbours.emplace(nrow, ncol);
+                }
+        }
+
+        return neighbours;
+}
+
+void
+part1()
+{
+        auto lines          = read_file("data/day21.txt");
+        auto start_position = find_start_position(lines);
+
+        queue<position> positions;
+        positions.emplace(start_position);
+
+        long sum = 0;
+        for ( int round = 0; round != 64; ++round ) {
+                set<position> next_positions;
+
+                sum = 0;
+                while ( !positions.empty() ) {
+                        const auto [curr_row, curr_col] = positions.front();
+                        lines[curr_row][curr_col]       = '.';
+
+                        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);
+                }
+        }
+        cout << sum << endl;
+}
+
+int
+main()
+{
+        part1();
+}