diff --git a/2017/18.cpp b/2017/18.cpp
index 552dafd..f1ef275 100644
--- a/2017/18.cpp
+++ b/2017/18.cpp
@@ -3,6 +3,7 @@
 #include <cstdint>
 #include <deque>
 #include <iostream>
+#include <sstream>
 #include <iterator>
 #include <map>
 #include <optional>
@@ -16,6 +17,7 @@
 
 namespace {
 
+
 using Name = std::string;
 using Value = std::int64_t;
 
@@ -23,6 +25,10 @@ struct Literal { Value value; };
 struct Variable { Name name; };
 using Expression = std::variant<Literal, Variable>;
 
+struct Send { Value sent; };
+struct Receive { Value & target; };
+using Effect = std::variant<Send, Receive>;
+
 struct Set { Name x; Expression y; };
 struct Add { Name x; Expression y; };
 struct Mul { Name x; Expression y; };
@@ -32,6 +38,11 @@ struct Rcv { Name x; };
 struct Snd { Expression x; };
 using Instruction = std::variant<Set, Add, Mul, Mod, Jgz, Rcv, Snd>;
 
+struct State {
+  std::map<Name, Value> registers;
+  std::size_t pc;
+};
+
 auto IsName(std::string const& word) {
   return std::all_of(word.begin(), word.end(), [](auto c) { return std::isalpha(c); });
 }
@@ -92,113 +103,140 @@ auto Parse(std::istream & in) -> std::vector<Instruction> {
   return result;
 }
 
-struct Send { Value sent; };
-struct Receive { Value & target; };
-struct Halt {};
-using Effect = std::variant<Send, Receive, Halt>;
+auto Eval(State const& state, Expression expression) -> Value {
+  return std::visit(overloaded{
+    [](Literal l) -> Value { return l.value; },
+    [&](Variable v) -> Value {
+      if (auto it = state.registers.find(v.name); it != state.registers.end()) {
+        return it->second;
+     } else {
+      return 0; }
+    },
+  }, expression);
+}
 
-struct Machine {
-  std::vector<Instruction> const& program_;
-  std::map<Name, Value> registers_;
-  std::size_t pc_;
+auto Step(State & state, Instruction const& instruction) -> std::optional<Effect> {
+  using R = std::optional<Effect>;
+  return std::visit(overloaded{
+    [&](Set instruction) -> R { state.registers[instruction.x]  = Eval(state, instruction.y); state.pc++; return {}; },
+    [&](Add instruction) -> R { state.registers[instruction.x] += Eval(state, instruction.y); state.pc++; return {}; },
+    [&](Mul instruction) -> R { state.registers[instruction.x] *= Eval(state, instruction.y); state.pc++; return {}; },
+    [&](Mod instruction) -> R { state.registers[instruction.x] %= Eval(state, instruction.y); state.pc++; return {}; },
+    [&](Jgz instruction) -> R { state.pc += Eval(state, instruction.x) > 0 ? Eval(state, instruction.y) : 1; return {}; },
+    [&](Snd instruction) -> R { state.pc++; return Send{Eval(state, instruction.x)}; },
+    [&](Rcv instruction) -> R { state.pc++; return Receive{state.registers[instruction.x]}; },
+    },
+    instruction);
+}
 
-public:
-  Machine(std::vector<Instruction> const& program)
-    : program_{program}, registers_{}, pc_{}
-  {}
-
-  auto Eval(Expression expression) -> Value {
-    return std::visit(overloaded{
-      [](Literal l) { return l.value; },
-      [&](Variable v) { return registers_[v.name]; },
-    }, expression);
-  }
-
-  auto Step() -> Effect {
-    while (pc_ < program_.size()) {
-      if (auto effect = std::visit(overloaded{
-        [&](Set instruction) -> std::optional<Effect> { registers_[instruction.x]  = Eval(instruction.y); pc_++; return {}; },
-        [&](Add instruction) -> std::optional<Effect> { registers_[instruction.x] += Eval(instruction.y); pc_++; return {}; },
-        [&](Mul instruction) -> std::optional<Effect> { registers_[instruction.x] *= Eval(instruction.y); pc_++; return {}; },
-        [&](Mod instruction) -> std::optional<Effect> { registers_[instruction.x] %= Eval(instruction.y); pc_++; return {}; },
-        [&](Jgz instruction) -> std::optional<Effect> { pc_ += Eval(instruction.x) > 0 ? Eval(instruction.y) : 1; return {}; },
-        [&](Snd instruction) -> std::optional<Effect> { pc_++; return Send{Eval(instruction.x)}; },
-        [&](Rcv instruction) -> std::optional<Effect> { pc_++; return Receive{registers_[instruction.x]}; },
-      },
-      program_[pc_]))
-      {
-        return *effect;
-      }
+auto BigStep(State & state, std::vector<Instruction> const& program) -> Effect {
+  while(state.pc < program.size()) {
+    if (auto effect = Step(state, program[state.pc])) {
+      return *effect;
     }
-    return Halt{};
   }
-};
+  throw std::runtime_error{"program crash"};
+}
 
 auto Part1(std::vector<Instruction> const& program) -> Value {
   Value last_sound = -1;
-  Machine m { program };
+  State state {};
 
-  for(;;) {
-    auto effect = m.Step();
+  for (;;) {
+    auto effect = BigStep(state, program);
     switch (effect.index()) {
       case 0: last_sound = std::get<0>(effect).sent; break;
       case 1: if (std::get<1>(effect).target > 0) { return last_sound; } break;
-      default: throw std::runtime_error{"program halted"};
     }
   }
+
+  throw std::runtime_error{"part 1 failed"};
 }
 
-auto Spin(Machine & m, std::deque<Value> & input, std::deque<Value> & output) -> Value* {
+auto Spin(
+  State & state,
+  std::vector<Instruction> const& program,
+  std::deque<Value> & input,
+  std::deque<Value> & output
+) -> std::pair<Value*, std::size_t>
+{
+  std::size_t n = 0;
   for(;;) {
-    auto effect = m.Step();
+    auto effect = BigStep(state, program);
     switch (effect.index()) {
-      case 0: output.push_back(std::get<0>(effect).sent); break;
+      case 0:
+        output.push_back(std::get<0>(effect).sent);
+        n++;
+        break;
       case 1: {
         auto & target = std::get<1>(effect).target;
         if (input.empty()) {
-          return &target;
+          return {&target, n};
         } else {
           target = input.front();
           input.pop_front();
         }
         break;
       }
-      default: throw std::runtime_error{"program halted"};
     }
   }  
 }
 
 auto Part2(std::vector<Instruction> const& program) -> std::size_t {
-  Machine m0 { program };
-  Machine m1 { program };
-  m1.registers_["p"] = 1;
+  State m0{};
+  State m1{{{"p",1}}};
 
   std::deque<Value> inputs0;
   std::deque<Value> inputs1;
 
-  auto stuck0 = Spin(m0, inputs0, inputs1);
-  auto stuck1 = Spin(m1, inputs1, inputs0);
-  
-  std::size_t result = inputs0.size();
+  auto [stuck0, _] = Spin(m0, program, inputs0, inputs1);
+  auto [stuck1, n] = Spin(m1, program, inputs1, inputs0);
 
   while(!(inputs0.empty() && inputs1.empty())) {
     if (!inputs0.empty()) {
       *stuck0 = inputs0.front(); inputs0.pop_front();
-      stuck0 = Spin(m0, inputs0, inputs1);
+      stuck0 = Spin(m0, program, inputs0, inputs1).first;
     }
     if (!inputs1.empty()) {
       *stuck1 = inputs1.front(); inputs1.pop_front();
-      result -= inputs0.size();
-      stuck1 = Spin(m1, inputs1, inputs0);
-      result += inputs0.size();
+      auto res = Spin(m1, program, inputs1, inputs0);
+      stuck1 = res.first;
+      n += res.second;
     }
   }
-
-  return result;
+  return n;
 }
 
 } // namespace
 
+TEST_SUITE("documented examples") {
+  TEST_CASE("part 1") {
+    std::istringstream in {
+      "set a 1\n"
+      "add a 2\n"
+      "mul a a\n"
+      "mod a 5\n"
+      "snd a\n"
+      "set a 0\n"
+      "rcv a\n"
+      "jgz a -1\n"
+      "set a 1\n"
+      "jgz a -2\n"};
+    REQUIRE(Part1(Parse(in)) == 4);
+  }
+  TEST_CASE("part 2") {
+    std::istringstream in {
+      "snd 1\n"
+      "snd 2\n"
+      "snd p\n"
+      "rcv a\n"
+      "rcv b\n"
+      "rcv c\n"
+      "rcv d\n"};
+    REQUIRE(Part2(Parse(in)) == 3);
+  }
+}
+
 auto main(int argc, char** argv) -> int {
   auto program = Parse(aocpp::Startup(argc, argv));
   std::cout << "Part 1: " << Part1(program) << std::endl;
diff --git a/2019/CMakeLists.txt b/2019/CMakeLists.txt
index 22f112d..37f9932 100644
--- a/2019/CMakeLists.txt
+++ b/2019/CMakeLists.txt
@@ -1,74 +1,74 @@
-add_executable(01 01.cpp)
-target_link_libraries(01 aocpp)
+add_executable(2019_01 01.cpp)
+target_link_libraries(2019_01 aocpp)
 
-add_executable(02 02.cpp)
-target_link_libraries(02 aocpp intcode)
+add_executable(2019_02 02.cpp)
+target_link_libraries(2019_02 aocpp intcode)
 
-add_executable(03 03.cpp)
-target_link_libraries(03 aocpp)
+add_executable(2019_03 03.cpp)
+target_link_libraries(2019_03 aocpp)
 
-add_executable(04 04.cpp)
-target_link_libraries(04 aocpp)
+add_executable(2019_04 04.cpp)
+target_link_libraries(2019_04 aocpp)
 
-add_executable(05 05.cpp)
-target_link_libraries(05 aocpp intcode)
+add_executable(2019_05 05.cpp)
+target_link_libraries(2019_05 aocpp intcode)
 
-add_executable(06 06.cpp)
-target_link_libraries(06 aocpp)
+add_executable(2019_06 06.cpp)
+target_link_libraries(2019_06 aocpp)
 
-add_executable(07 07.cpp)
-target_link_libraries(07 aocpp intcode)
+add_executable(2019_07 07.cpp)
+target_link_libraries(2019_07 aocpp intcode)
 
-add_executable(08 08.cpp)
-target_link_libraries(08 aocpp)
+add_executable(2019_08 08.cpp)
+target_link_libraries(2019_08 aocpp)
 
-add_executable(09 09.cpp)
-target_link_libraries(09 aocpp intcode)
+add_executable(2019_09 09.cpp)
+target_link_libraries(2019_09 aocpp intcode)
 
-add_executable(10 10.cpp)
-target_link_libraries(10 aocpp)
+add_executable(2019_10 10.cpp)
+target_link_libraries(2019_10 aocpp)
 
-add_executable(11 11.cpp)
-target_link_libraries(11 aocpp intcode)
+add_executable(2019_11 11.cpp)
+target_link_libraries(2019_11 aocpp intcode)
 
-add_executable(12 12.cpp)
-target_link_libraries(12 aocpp)
+add_executable(2019_12 12.cpp)
+target_link_libraries(2019_12 aocpp)
 
-add_executable(13 13.cpp)
-target_link_libraries(13 aocpp intcode)
+add_executable(2019_13 13.cpp)
+target_link_libraries(2019_13 aocpp intcode)
 
-add_executable(14 14.cpp)
-target_link_libraries(14 aocpp)
+add_executable(2019_14 14.cpp)
+target_link_libraries(2019_14 aocpp)
 
-add_executable(15 15.cpp)
-target_link_libraries(15 aocpp intcode)
+add_executable(2019_15 15.cpp)
+target_link_libraries(2019_15 aocpp intcode)
 
-add_executable(16 16.cpp)
-target_link_libraries(16 aocpp)
+add_executable(2019_16 16.cpp)
+target_link_libraries(2019_16 aocpp)
 
-add_executable(17 17.cpp)
-target_link_libraries(17 aocpp intcode)
+add_executable(2019_17 17.cpp)
+target_link_libraries(2019_17 aocpp intcode)
 
-add_executable(18 18.cpp)
-target_link_libraries(18 aocpp)
+add_executable(2019_18 18.cpp)
+target_link_libraries(2019_18 aocpp)
 
-add_executable(19 19.cpp)
-target_link_libraries(19 aocpp intcode)
+add_executable(2019_19 19.cpp)
+target_link_libraries(2019_19 aocpp intcode)
 
-add_executable(20 20.cpp)
-target_link_libraries(20 aocpp)
+add_executable(2019_20 20.cpp)
+target_link_libraries(2019_20 aocpp)
 
-add_executable(21 21.cpp)
-target_link_libraries(21 aocpp intcode)
+add_executable(2019_21 21.cpp)
+target_link_libraries(2019_21 aocpp intcode)
 
-add_executable(22 22.cpp)
-target_link_libraries(22 aocpp zmod)
+add_executable(2019_22 22.cpp)
+target_link_libraries(2019_22 aocpp zmod)
 
-add_executable(23 23.cpp)
-target_link_libraries(23 aocpp intcode)
+add_executable(2019_23 23.cpp)
+target_link_libraries(2019_23 aocpp intcode)
 
-add_executable(24 24.cpp)
-target_link_libraries(24 aocpp)
+add_executable(2019_24 24.cpp)
+target_link_libraries(2019_24 aocpp)
 
-add_executable(25 25.cpp)
-target_link_libraries(25 aocpp intcode)
+add_executable(2019_25 25.cpp)
+target_link_libraries(2019_25 aocpp intcode)
diff --git a/2020/01.cpp b/2020/01.cpp
new file mode 100644
index 0000000..dfc9dac
--- /dev/null
+++ b/2020/01.cpp
@@ -0,0 +1,86 @@
+#include <algorithm>
+#include <bitset>
+#include <cstdint>
+#include <iostream>
+#include <sstream>
+#include <iterator>
+#include <vector>
+
+#include <doctest.h>
+
+#include <aocpp/Startup.hpp>
+
+namespace {
+
+auto Parse(std::istream & in) -> std::vector<std::int64_t>
+{
+  std::vector<int64_t> result;
+  std::int64_t x;
+  while (in >> x) {
+    result.push_back(x);
+  }
+  return result;
+}
+
+auto Part1(std::vector<std::int64_t> entries) {
+  std::bitset<2020> numbers;
+  std::sort(entries.begin(), entries.end());
+  for (auto const i : entries) {
+    if (i >= 2020) {
+      break;
+    }
+    numbers.set(i);
+  }
+  for (auto const i : entries) {
+    if (numbers.test(2020 - i)) {
+      return (2020 - i) * i;
+    }
+  }
+  throw std::runtime_error{"no part 1 solution"};
+}
+
+auto Part2(std::vector<std::int64_t> entries) {
+  std::bitset<2020> numbers;
+  std::sort(entries.begin(), entries.end());
+  for (auto const i : entries) {
+    if (i >= 2020) { break; }
+    if (i < 2020) { numbers.set(i); }
+  }
+
+  for (auto it1 = entries.begin(); it1 != entries.end(); it1++) {
+    for (auto it2 = std::next(it1); it2 != entries.end(); it2++) {
+      auto target = 2020 - (*it1) - (*it2);
+      if (target < 0) break;
+      if (target < 2020 && numbers.test(target)) { return target * *it1 * *it2;}
+    }
+  }
+  throw std::runtime_error{"no part 2 solution"};
+}
+
+}
+
+TEST_SUITE("documented examples") {
+  std::istringstream in {
+    "1721\n"
+    "979\n"
+    "366\n"
+    "299\n"
+    "675\n"
+    "1456\n"};
+  auto entries = Parse(in);
+
+TEST_CASE("part 1") {
+  REQUIRE(Part1(entries) == 514579);
+}
+
+TEST_CASE("part 2") {
+  REQUIRE(Part2(entries) == 241861950);
+}
+
+}
+
+auto main(int argc, char** argv) -> int {
+  auto entries = Parse(aocpp::Startup(argc, argv));
+  std::cout << "Part 1: " << Part1(entries) << std::endl;
+  std::cout << "Part 2: " << Part2(entries) << std::endl;
+}
diff --git a/2020/CMakeLists.txt b/2020/CMakeLists.txt
new file mode 100644
index 0000000..93f0586
--- /dev/null
+++ b/2020/CMakeLists.txt
@@ -0,0 +1,2 @@
+add_executable(2020_01 01.cpp)
+target_link_libraries(2020_01 aocpp)
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5a94da1..16016d0 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -22,3 +22,4 @@ add_subdirectory(zmod)
 add_subdirectory(intcode)
 add_subdirectory(2017)
 add_subdirectory(2019)
+add_subdirectory(2020)
diff --git a/CMakePresets.json b/CMakePresets.json
index 155daee..3571f09 100644
--- a/CMakePresets.json
+++ b/CMakePresets.json
@@ -8,7 +8,7 @@
             "generator": "Ninja",
             "binaryDir": "${sourceDir}/out/build/${presetName}",
             "cacheVariables": {
-                "CMAKE_BUILD_TYPE": "RelWithDebInfo",
+                "CMAKE_BUILD_TYPE": "Debug",
                 "CMAKE_TOOLCHAIN_FILE": "",
                 "CMAKE_INSTALL_PREFIX": "${sourceDir}/out/install/${presetName}"
             }