diff --git a/userspace/aoc2025/CMakeLists.txt b/userspace/aoc2025/CMakeLists.txt
index aa7962d3..68d2aa03 100644
--- a/userspace/aoc2025/CMakeLists.txt
+++ b/userspace/aoc2025/CMakeLists.txt
@@ -7,6 +7,7 @@ set(AOC2025_PROJECTS
 	day6
 	day7
 	day8
+	day9
 	full
 )
 
diff --git a/userspace/aoc2025/day9/CMakeLists.txt b/userspace/aoc2025/day9/CMakeLists.txt
new file mode 100644
index 00000000..baeb765e
--- /dev/null
+++ b/userspace/aoc2025/day9/CMakeLists.txt
@@ -0,0 +1,9 @@
+set(SOURCES
+	main.cpp
+)
+
+add_executable(aoc2025_day9 ${SOURCES})
+banan_include_headers(aoc2025_day9 ban)
+banan_link_library(aoc2025_day9 libc)
+
+install(TARGETS aoc2025_day9 OPTIONAL)
diff --git a/userspace/aoc2025/day9/main.cpp b/userspace/aoc2025/day9/main.cpp
new file mode 100644
index 00000000..fbe2b621
--- /dev/null
+++ b/userspace/aoc2025/day9/main.cpp
@@ -0,0 +1,188 @@
+#include <BAN/HashMap.h>
+#include <BAN/Sort.h>
+#include <BAN/Vector.h>
+
+#include <inttypes.h>
+#include <stdio.h>
+
+using i8  = int8_t;
+using i16 = int16_t;
+using i32 = int32_t;
+using i64 = int64_t;
+
+using u8  = uint8_t;
+using u16 = uint16_t;
+using u32 = uint32_t;
+using u64 = uint64_t;
+
+struct Point
+{
+	i64 x, y;
+};
+
+static BAN::Vector<Point> parse_points(FILE* fp)
+{
+	BAN::Vector<Point> points;
+
+	i64 x, y;
+	while (fscanf(fp, "%" SCNi64 ",%" SCNi64, &x, &y) == 2)
+		MUST(points.emplace_back(x, y));
+
+	return points;
+}
+
+i64 part1(FILE* fp)
+{
+	auto points = parse_points(fp);
+
+	i64 result = 0;
+
+	for (size_t i = 0; i < points.size(); i++)
+	{
+		for (size_t j = i + 1; j < points.size(); j++)
+		{
+			const i64 w = BAN::Math::abs(points[i].x - points[j].x) + 1;
+			const i64 h = BAN::Math::abs(points[i].y - points[j].y) + 1;
+			result = BAN::Math::max(result, w * h);
+		}
+	}
+
+	return result;
+}
+
+static BAN::Vector<i64> compress_coordinates(BAN::Vector<Point>& points)
+{
+	BAN::HashMap<i64, i64> coord_map;
+	MUST(coord_map.reserve(points.size() * 4));
+	for (size_t i = 0; i < points.size(); i++)
+	{
+		MUST(coord_map.emplace_or_assign(points[i].x));
+		MUST(coord_map.emplace_or_assign(points[i].y));
+	}
+
+	BAN::Vector<i64> coords;
+	MUST(coords.reserve(coord_map.size()));
+	for (auto [coord, _] : coord_map)
+		MUST(coords.push_back(coord));
+	BAN::sort::sort(coords.begin(), coords.end());
+
+	for (size_t i = 0; i < coords.size(); i++)
+		coord_map[coords[i]] = i;
+
+	for (auto& point : points)
+	{
+		point.x = coord_map[point.x];
+		point.y = coord_map[point.y];
+	}
+
+	return coords;
+}
+
+i64 part2(FILE* fp)
+{
+	auto points = parse_points(fp);
+	auto index2coord = compress_coordinates(points);
+
+	const size_t grid_size = index2coord.size();
+
+	BAN::Vector<bool> grid;
+	MUST(grid.resize(grid_size * grid_size, false));
+
+	for (size_t i = 0; i < points.size(); i++)
+	{
+		const auto curr = points[i];
+		const auto next = points[(i + 1) % points.size()];
+		ASSERT((curr.x == next.x) != (curr.y == next.y));
+
+		const i64 minx = BAN::Math::min(curr.x, next.x);
+		const i64 maxx = BAN::Math::max(curr.x, next.x);
+		const i64 miny = BAN::Math::min(curr.y, next.y);
+		const i64 maxy = BAN::Math::max(curr.y, next.y);
+
+		const i64 dx = (minx != maxx);
+		const i64 dy = (miny != maxy);
+
+		for (i64 x = minx, y = miny; x <= maxx && y <= maxy; x += dx, y += dy)
+			grid[y * grid_size + x] = true;
+	}
+
+	for (size_t y = 0; y < grid_size; y++)
+	{
+		bool is_inside = false;
+		for (size_t x = 0; x < grid_size; x++)
+		{
+			if (!grid[y * grid_size + x])
+				grid[y * grid_size + x] = is_inside;
+			else if (x + 1 >= grid_size || !grid[y * grid_size + x + 1])
+				is_inside = !is_inside;
+			else
+			{
+				const bool from_below1 = (y + 1 < grid_size) && grid[(y + 1) * grid_size + x];
+				while (x + 1 < grid_size && grid[y * grid_size + x + 1])
+					x++;
+				const bool from_below2 = (y + 1 < grid_size) && grid[(y + 1) * grid_size + x];
+
+				if (from_below1 != from_below2)
+					is_inside = !is_inside;
+			}
+		}
+	}
+
+	const auto is_usable_area =
+		[&grid, grid_size](const Point& a, const Point& b) -> bool
+		{
+			const i64 minx = BAN::Math::min(a.x, b.x);
+			const i64 maxx = BAN::Math::max(a.x, b.x);
+			const i64 miny = BAN::Math::min(a.y, b.y);
+			const i64 maxy = BAN::Math::max(a.y, b.y);
+			for (i64 x = minx; x <= maxx; x++)
+				if (!grid[miny * grid_size + x] || !grid[maxy * grid_size + x])
+					return false;
+			for (i64 y = miny; y <= maxy; y++)
+				if (!grid[y * grid_size + minx] || !grid[y * grid_size + maxy])
+					return false;
+			return true;
+		};
+
+	i64 result = 0;
+
+	for (size_t i = 0; i < points.size(); i++)
+	{
+		for (size_t j = i + 1; j < points.size(); j++)
+		{
+			const auto& a = points[i];
+			const auto& b = points[j];
+			if (!is_usable_area(a, b))
+				continue;
+
+			const i64 w = BAN::Math::abs(index2coord[a.x] - index2coord[b.x]) + 1;
+			const i64 h = BAN::Math::abs(index2coord[a.y] - index2coord[b.y]) + 1;
+			result = BAN::Math::max(result, w * h);
+		}
+	}
+
+	return result;
+}
+
+int main(int argc, char** argv)
+{
+	const char* file_path = "/usr/share/aoc2025/day9_input.txt";
+
+	if (argc >= 2)
+		file_path = argv[1];
+
+	FILE* fp = fopen(file_path, "r");
+	if (fp == nullptr)
+	{
+		perror("fopen");
+		return 1;
+	}
+
+	printf("part1: %" PRId64 "\n", part1(fp));
+
+	fseek(fp, 0, SEEK_SET);
+
+	printf("part2: %" PRId64 "\n", part2(fp));
+
+	fclose(fp);
+}