1 files changed, 106 insertions, 0 deletions

diff --git a/2025/src/day04.cpp b/2025/src/day04.cpp
new file mode 100644
index 0000000..295ed75
--- /dev/null
+++ b/2025/src/day04.cpp
@@ -0,0 +1,106 @@
+#include <filesystem>
+#include <fstream>
+#include <iostream>
+#include <set>
+#include <string>
+#include <tuple>
+
+using namespace std;
+
+namespace {
+
+using Pos = tuple<size_t, size_t>;
+
+set<Pos>
+read_file(const filesystem::path& filename)
+{
+        ifstream file{ filename };
+        set<Pos> grid;
+
+        size_t row = 0;
+        for ( string line; getline(file, line); ) {
+                for ( size_t col = 0; col != line.length(); ++col ) {
+                        if ( line[col] == '@' ) {
+                                grid.emplace(col, row);
+                        }
+                }
+                ++row;
+        }
+
+        return grid;
+}
+
+set<Pos>
+get_neighbours(const Pos& pos)
+{
+        auto [col, row] = pos;
+
+        return {
+                Pos{ col - 1, row },
+                Pos{ col - 1, row - 1 },
+                Pos{ col, row - 1 },
+                Pos{ col + 1, row - 1 },
+                Pos{ col + 1, row },
+                Pos{ col + 1, row + 1 },
+                Pos{ col, row + 1 },
+                Pos{ col - 1, row + 1 },
+        };
+}
+
+bool
+is_accessable(const set<Pos>& grid, const Pos& pos)
+{
+        long nneighbours = 0;
+        for ( const auto neighbour: get_neighbours(pos) ) {
+                if ( grid.contains(neighbour) ) {
+                        ++nneighbours;
+                }
+        }
+        return nneighbours < 4;
+}
+
+void
+part1(const set<Pos>& grid)
+{
+        long count = 0;
+        for ( const auto pos: grid ) {
+                if ( is_accessable(grid, pos) ) {
+                        ++count;
+                }
+        }
+        cout << "Part 1: " << count << '\n';
+}
+
+void
+part2(set<Pos> grid)
+{
+        auto old_size = grid.size();
+        while ( true ) {
+                set<Pos> remove;
+
+                for ( const auto& pos: grid ) {
+                        if ( is_accessable(grid, pos) ) {
+                                remove.insert(pos);
+                        }
+                }
+
+                if ( remove.empty() ) {
+                        break;
+                }
+
+                for ( const auto& pos: remove ) {
+                        grid.erase(pos);
+                }
+        }
+        cout << "Part 2: " << old_size - grid.size() << '\n';
+}
+
+} // namespace
+
+int
+main()
+{
+        auto grid = read_file("data/day04.txt");
+        part1(grid);
+        part2(grid);
+}