diff --git a/2017/18.cpp b/2017/18.cpp
new file mode 100644
index 0000000..552dafd
--- /dev/null
+++ b/2017/18.cpp
@@ -0,0 +1,206 @@
+#include <algorithm>
+#include <cctype>
+#include <cstdint>
+#include <deque>
+#include <iostream>
+#include <iterator>
+#include <map>
+#include <optional>
+#include <variant>
+#include <vector>
+
+#include <doctest.h>
+
+#include <aocpp/Startup.hpp>
+#include <aocpp/Overloaded.hpp>
+
+namespace {
+
+using Name = std::string;
+using Value = std::int64_t;
+
+struct Literal { Value value; };
+struct Variable { Name name; };
+using Expression = std::variant<Literal, Variable>;
+
+struct Set { Name x; Expression y; };
+struct Add { Name x; Expression y; };
+struct Mul { Name x; Expression y; };
+struct Mod { Name x; Expression y; };
+struct Jgz { Expression x; Expression y; };
+struct Rcv { Name x; };
+struct Snd { Expression x; };
+using Instruction = std::variant<Set, Add, Mul, Mod, Jgz, Rcv, Snd>;
+
+auto IsName(std::string const& word) {
+  return std::all_of(word.begin(), word.end(), [](auto c) { return std::isalpha(c); });
+}
+
+auto ParseName(std::istream & in) -> Name
+{
+  std::string word;
+  in >> word;
+  if (IsName(word)) { return word; }
+  throw std::runtime_error{"bad name"};
+}
+
+auto ParseExpression(std::istream & in) -> Expression
+{
+  std::string word;
+  in >> word;
+  if (IsName(word)) {
+    return Variable{word};
+  } else {
+    return Literal{std::stoll(word)};
+  }
+}
+
+auto Parse(std::istream & in) -> std::vector<Instruction> {
+  std::vector<Instruction> result;
+  std::string op;
+  while (in >> op) {
+    if ("set" == op) {
+      auto x = ParseName(in);
+      auto y = ParseExpression(in);
+      result.push_back(Set{x,y});
+    } else if ("add" == op) {
+      auto x = ParseName(in);
+      auto y = ParseExpression(in);
+      result.push_back(Add{x,y});
+    } else if ("mul" == op) {
+      auto x = ParseName(in);
+      auto y = ParseExpression(in);
+      result.push_back(Mul{x,y});
+    } else if ("mod" == op) {
+      auto x = ParseName(in);
+      auto y = ParseExpression(in);
+      result.push_back(Mod{x,y});
+    } else if ("jgz" == op) {
+      auto x = ParseExpression(in);
+      auto y = ParseExpression(in);
+      result.push_back(Jgz{x,y});
+    } else if ("rcv" == op) {
+      auto x = ParseName(in);
+      result.push_back(Rcv{x});
+    } else if ("snd" == op) {
+      auto x = ParseExpression(in);
+      result.push_back(Snd{x});
+    } else {
+      throw std::runtime_error{"unknown op"};
+    }
+  }
+  return result;
+}
+
+struct Send { Value sent; };
+struct Receive { Value & target; };
+struct Halt {};
+using Effect = std::variant<Send, Receive, Halt>;
+
+struct Machine {
+  std::vector<Instruction> const& program_;
+  std::map<Name, Value> registers_;
+  std::size_t pc_;
+
+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;
+      }
+    }
+    return Halt{};
+  }
+};
+
+auto Part1(std::vector<Instruction> const& program) -> Value {
+  Value last_sound = -1;
+  Machine m { program };
+
+  for(;;) {
+    auto effect = m.Step();
+    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"};
+    }
+  }
+}
+
+auto Spin(Machine & m, std::deque<Value> & input, std::deque<Value> & output) -> Value* {
+  for(;;) {
+    auto effect = m.Step();
+    switch (effect.index()) {
+      case 0: output.push_back(std::get<0>(effect).sent); break;
+      case 1: {
+        auto & target = std::get<1>(effect).target;
+        if (input.empty()) {
+          return &target;
+        } 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;
+
+  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();
+
+  while(!(inputs0.empty() && inputs1.empty())) {
+    if (!inputs0.empty()) {
+      *stuck0 = inputs0.front(); inputs0.pop_front();
+      stuck0 = Spin(m0, inputs0, inputs1);
+    }
+    if (!inputs1.empty()) {
+      *stuck1 = inputs1.front(); inputs1.pop_front();
+      result -= inputs0.size();
+      stuck1 = Spin(m1, inputs1, inputs0);
+      result += inputs0.size();
+    }
+  }
+
+  return result;
+}
+
+} // namespace
+
+auto main(int argc, char** argv) -> int {
+  auto program = Parse(aocpp::Startup(argc, argv));
+  std::cout << "Part 1: " << Part1(program) << std::endl;
+  std::cout << "Part 2: " << Part2(program) << std::endl;
+}
diff --git a/2017/CMakeLists.txt b/2017/CMakeLists.txt
new file mode 100644
index 0000000..0bac4c1
--- /dev/null
+++ b/2017/CMakeLists.txt
@@ -0,0 +1,2 @@
+add_executable(2017_18 18.cpp)
+target_link_libraries(2017_18 aocpp)
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5c33833..5a94da1 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,7 +1,7 @@
 cmake_minimum_required(VERSION 3.13)
 set(CMAKE_C_STANDARD 11)
 set(CMAKE_CXX_STANDARD 20)
-project(aocpp19
+project(aocpp
     VERSION 1
     LANGUAGES C CXX
 )
@@ -15,9 +15,10 @@ else()
 endif()
 
 find_package(PkgConfig)
-pkg_check_modules(GMP REQUIRED IMPORTED_TARGET gmp)
+pkg_check_modules(GMP REQUIRED IMPORTED_TARGET gmpxx)
 
 add_subdirectory(lib)
 add_subdirectory(zmod)
 add_subdirectory(intcode)
+add_subdirectory(2017)
 add_subdirectory(2019)
diff --git a/lib/include/aocpp/Overloaded.hpp b/lib/include/aocpp/Overloaded.hpp
new file mode 100644
index 0000000..5bce333
--- /dev/null
+++ b/lib/include/aocpp/Overloaded.hpp
@@ -0,0 +1,9 @@
+#ifndef AOCPP_OVERLOADED_HPP_
+#define AOCPP_OVERLOADED_HPP_
+
+// helper type for the visitor #4
+template<class... Ts> struct overloaded : Ts... { using Ts::operator()...; };
+// explicit deduction guide (not needed as of C++20)
+template<class... Ts> overloaded(Ts...) -> overloaded<Ts...>;
+
+#endif // AOCPP_OVERLOADED_HPP_
diff --git a/zmod/include/zmod.hpp b/zmod/include/zmod.hpp
index b784e9d..e1cbcb4 100644
--- a/zmod/include/zmod.hpp
+++ b/zmod/include/zmod.hpp
@@ -21,6 +21,12 @@ public:
     return {value + rhs.value};
   }
 
+  auto operator+=(ZMod const& rhs) -> ZMod & {
+    value += rhs.value;
+    mpz_mod_ui(this->value.get_mpz_t(), value.get_mpz_t(), Mod);
+    return *this;
+  }
+
   auto operator-() const -> ZMod {
     return {-value};
   }
@@ -29,10 +35,22 @@ public:
     return {value - rhs.value};
   }
 
+  auto operator-=(ZMod const& rhs) -> ZMod & {
+    value -= rhs.value;
+    mpz_mod_ui(value.get_mpz_t(), value.get_mpz_t(), Mod);
+    return *this;
+  }
+
   auto operator*(ZMod const& rhs) const -> ZMod {
     return {value * rhs.value};
   }
 
+  auto operator*=(ZMod const& rhs) -> ZMod & {
+    value -= rhs.value;
+    mpz_mod_ui(value.get_mpz_t(), value.get_mpz_t(), Mod);
+    return *this;
+  }
+
   auto inverse() const -> ZMod {
     mpz_class m{Mod};
     ZMod result;
@@ -47,4 +65,4 @@ public:
 
 }
 
-#endif
\ No newline at end of file
+#endif