From acb278e7a61b13d14767e3a84c75d4f9f0ca6e7a Mon Sep 17 00:00:00 2001
From: Thomas Schmucker <ts@its1.de>
Date: Thu, 4 Dec 2025 12:56:24 +0100
Subject: aoc 2025, day 4

---
 2025/src/day04.cpp | 106 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 106 insertions(+)
 create mode 100644 2025/src/day04.cpp

(limited to '2025/src/day04.cpp')

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);
+}
-- 
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