diff --git a/2021/03.cpp b/2021/03.cpp
new file mode 100644
index 0000000..d8679d2
--- /dev/null
+++ b/2021/03.cpp
@@ -0,0 +1,134 @@
+#include <algorithm>
+#include <cstdint>
+#include <iostream>
+#include <iterator>
+#include <iterator>
+#include <sstream>
+#include <string>
+#include <vector>
+
+#include <boost/phoenix.hpp>
+#include <boost/range/algorithm.hpp>
+#include <boost/range/irange.hpp>
+#include <doctest.h>
+
+#include <aocpp/Startup.hpp>
+
+namespace
+{
+  namespace phx = boost::phoenix;
+  using phx::placeholders::arg1;
+
+  auto Parse(std::istream &in) -> std::vector<std::string>
+  {
+    using It = std::istream_iterator<std::string>;
+    std::vector<std::string> result;
+    std::move(It{in}, It{}, std::back_inserter(result));
+
+    if (result.empty())
+    {
+      throw std::runtime_error{"Empty input"};
+    }
+
+    if (std::any_of(std::next(result.begin()), result.end(),
+                    [W = result[0].size()](auto const &s)
+                    { return s.size() != W; }))
+    {
+      throw std::runtime_error{"input line length mismatch"};
+    }
+
+    return result;
+  }
+
+  auto Part1(std::vector<std::string> const &entries) -> std::uint64_t
+  {
+    auto const N{entries.size()};
+    auto const W{entries[0].size()};
+
+    std::uint64_t lo{};
+    std::uint64_t hi{};
+
+    for (auto const i : boost::irange(W))
+    {
+      auto const bit_test = '1' == arg1[i];
+
+      auto const n = boost::count_if(entries, bit_test);
+      lo <<= 1;
+      hi <<= 1;
+      (2 * n >= N ? hi : lo)++;
+    }
+
+    return lo * hi;
+  }
+
+  auto Part2_(std::vector<std::string> entries, bool const big) -> std::uint64_t
+  {
+    auto const N{entries.size()};
+    auto const W{entries[0].size()};
+
+    std::uint64_t result{};
+    auto begin{entries.begin()};
+    auto end{entries.end()};
+
+    for (auto const i : boost::irange(W))
+    {
+      auto const bit_test = '1' == arg1[i];
+
+      result <<= 1;
+
+      // If there's only one number left we should select it
+      if (std::distance(begin, end) == 1)
+      {
+        result |= bit_test(*begin);
+      }
+      else
+      {
+        auto const mid = std::partition(begin, end, bit_test);
+        auto const ones = std::distance(begin, mid);
+        auto const zeros = std::distance(mid, end);
+        auto const bit = big == (ones >= zeros);
+        result |= bit;
+        (bit ? end : begin) = mid;
+      }
+    }
+
+    return result;
+  }
+
+  auto Part2(std::vector<std::string> entries) -> std::uint64_t
+  {
+    return Part2_(entries, false) * Part2_(entries, true);
+  }
+}
+
+TEST_SUITE("2021-03")
+{
+  TEST_CASE("example")
+  {
+    std::istringstream in{
+        "00100\n"
+        "11110\n"
+        "10110\n"
+        "10111\n"
+        "10101\n"
+        "01111\n"
+        "00111\n"
+        "11100\n"
+        "10000\n"
+        "11001\n"
+        "00010\n"
+        "01010\n"};
+    auto entries = Parse(in);
+    CHECK(Part1(entries) == 198);
+    CHECK(Part2_(entries, false) == 10);
+    CHECK(Part2_(entries, true) == 23);
+    CHECK(Part2(std::move(entries)) == 230);
+  }
+}
+
+auto Main(std::istream &in, std::ostream &out) -> void
+{
+  auto entries{Parse(in)};
+  out << "Part 1: " << Part1(entries) << std::endl;
+  out << "Part 2: " << Part2(std::move(entries)) << std::endl;
+}
diff --git a/2021/CMakeLists.txt b/2021/CMakeLists.txt
new file mode 100644
index 0000000..fd0c11c
--- /dev/null
+++ b/2021/CMakeLists.txt
@@ -0,0 +1,2 @@
+add_executable(2021_03 03.cpp)
+target_link_libraries(2021_03 aocpp Boost::headers)
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 32a8bd0..8c276c5 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -29,4 +29,5 @@ add_subdirectory(2017)
 add_subdirectory(2018)
 add_subdirectory(2019)
 add_subdirectory(2020)
+add_subdirectory(2021)
 add_subdirectory(2022)