diff --git a/userspace/aoc2023/CMakeLists.txt b/userspace/aoc2023/CMakeLists.txt
index 8411a878..e76083f8 100644
--- a/userspace/aoc2023/CMakeLists.txt
+++ b/userspace/aoc2023/CMakeLists.txt
@@ -23,6 +23,7 @@ set(AOC2023_PROJECTS
 	day18
 	day19
 	day20
+	day21
 	full
 )
 
diff --git a/userspace/aoc2023/day21/CMakeLists.txt b/userspace/aoc2023/day21/CMakeLists.txt
new file mode 100644
index 00000000..aaa6ec5e
--- /dev/null
+++ b/userspace/aoc2023/day21/CMakeLists.txt
@@ -0,0 +1,22 @@
+cmake_minimum_required(VERSION 3.26)
+
+project(aoc2023_day21 CXX)
+
+set(SOURCES
+	main.cpp
+)
+
+add_executable(aoc2023_day21 ${SOURCES})
+target_compile_options(aoc2023_day21 PUBLIC -O2 -g)
+target_link_libraries(aoc2023_day21 PUBLIC libc ban)
+
+add_dependencies(aoc2023_day21 libc-install ban-install)
+
+add_custom_target(aoc2023_day21-install
+	COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_CURRENT_BINARY_DIR}/aoc2023_day21 ${BANAN_AOC2023_BIN}/day21
+	DEPENDS aoc2023_day21
+	DEPENDS aoc2023_always
+)
+
+add_dependencies(aoc2023 aoc2023_day21)
+add_dependencies(aoc2023-install aoc2023_day21-install)
diff --git a/userspace/aoc2023/day21/main.cpp b/userspace/aoc2023/day21/main.cpp
new file mode 100644
index 00000000..a037cf1a
--- /dev/null
+++ b/userspace/aoc2023/day21/main.cpp
@@ -0,0 +1,155 @@
+#include <BAN/Vector.h>
+#include <BAN/HashSet.h>
+
+#include <ctype.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;
+
+enum class Tile { Free, Rock };
+
+struct Position
+{
+	i64 x;
+	i64 y;
+
+	Position operator+(const Position& other) const
+	{
+		return {
+			.x = x + other.x,
+			.y = y + other.y
+		};
+	}
+
+	bool operator==(const Position& other) const
+	{
+		return x == other.x && y == other.y;
+	}
+};
+
+namespace BAN
+{
+	template<>
+	struct hash<Position>
+	{
+		hash_t operator()(const Position& position) const
+		{
+			return hash<u64>()(((u64)position.x << 32) | (u64)position.y);
+		}
+	};
+}
+
+struct Garden
+{
+	Position start;
+	BAN::Vector<BAN::Vector<Tile>> tiles;
+};
+
+Garden parse_garden(FILE* fp)
+{
+	Garden garden;
+
+	i64 row = 0;
+
+	char buffer[256];
+	while (fgets(buffer, sizeof(buffer), fp))
+	{
+		BAN::StringView line(buffer);
+		ASSERT(line.back() == '\n');
+		line = line.substring(0, line.size() - 1);
+		if (line.empty())
+			break;
+
+		MUST(garden.tiles.emplace_back(line.size(), Tile::Free));
+		for (size_t i = 0; i < line.size(); i++)
+		{
+			if (line[i] == '#')
+				garden.tiles.back()[i] = Tile::Rock;
+			if (line[i] == 'S')
+				garden.start = { .x = (i64)i, .y = row };
+		}
+
+		row++;
+	}
+
+	return garden;
+}
+
+i64 puzzle1(FILE* fp)
+{
+	auto garden = parse_garden(fp);
+
+	BAN::HashSetUnstable<Position> visited, reachable, pending;
+	MUST(pending.insert(garden.start));
+
+	for (i32 i = 0; i <= 64; i++)
+	{
+		auto temp = BAN::move(pending);
+		pending = BAN::HashSetUnstable<Position>();
+
+		while (!temp.empty())
+		{
+			auto position = *temp.begin();
+			temp.remove(position);
+
+			MUST(visited.insert(position));
+			if (i % 2 == 0)
+				MUST(reachable.insert(position));
+			
+			Position offsets[4] { { -1, 0 }, { 1, 0 }, { 0, -1 }, { 0, 1 } };
+			for (i32 j = 0; j < 4; j++)
+			{
+				auto next = position + offsets[j];
+				if (next.y < 0 || next.y >= (i64)garden.tiles.size())
+					continue;
+				if (next.x < 0 || next.x >= (i64)garden.tiles[0].size())
+					continue;
+				if (garden.tiles[next.y][next.x] == Tile::Rock)
+					continue;
+				if (visited.contains(next))
+					continue;
+				MUST(pending.insert(next));
+			}
+		}
+	}
+
+	return reachable.size();
+}
+
+i64 puzzle2(FILE* fp)
+{
+	(void)fp;
+	return -1;
+}
+
+int main(int argc, char** argv)
+{
+	const char* file_path = "/usr/share/aoc2023/day21_input.txt";
+
+	if (argc >= 2)
+		file_path = argv[1];
+
+	FILE* fp = fopen(file_path, "r");
+	if (fp == nullptr)
+	{
+		perror("fopen");
+		return 1;
+	}
+
+	printf("puzzle1: %" PRId64 "\n", puzzle1(fp));
+
+	fseek(fp, 0, SEEK_SET);
+
+	printf("puzzle2: %" PRId64 "\n", puzzle2(fp));
+
+	fclose(fp);
+}
diff --git a/userspace/aoc2023/full/main.cpp b/userspace/aoc2023/full/main.cpp
index c9371ece..a4a37288 100644
--- a/userspace/aoc2023/full/main.cpp
+++ b/userspace/aoc2023/full/main.cpp
@@ -4,7 +4,7 @@
 
 int main()
 {
-	for (int i = 1; i <= 20; i++)
+	for (int i = 1; i <= 21; i++)
 	{
 		printf("day %d:\n", i);