From e43fca9377964876bd40f954307ca85bad58044c Mon Sep 17 00:00:00 2001
From: Matthew Gretton-Dann <matt@gretton-dann.org.uk>
Date: Mon, 13 Dec 2021 11:36:48 +0000
Subject: [PATCH] Add 2016 day 25 puzzle

---
 2016/puzzle-25-01.cc | 227 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 227 insertions(+)
 create mode 100644 2016/puzzle-25-01.cc

diff --git a/2016/puzzle-25-01.cc b/2016/puzzle-25-01.cc
new file mode 100644
index 0000000..57fc5d9
--- /dev/null
+++ b/2016/puzzle-25-01.cc
@@ -0,0 +1,227 @@
+//
+// Created by Matthew Gretton-Dann on 12/12/2021.
+//
+
+#include <array>
+#include <cassert>
+#include <iostream>
+#include <string>
+#include <utility>
+#include <variant>
+#include <vector>
+
+enum class Register { a, b, c, d };
+auto to_register(char c) -> Register
+{
+  switch (c) {
+  case 'a':
+    return Register::a;
+  case 'b':
+    return Register::b;
+  case 'c':
+    return Register::c;
+  case 'd':
+    return Register::d;
+  default:
+    abort();
+  }
+}
+
+enum class Opcode { cpy, inc, dec, jnz, tgl, out };
+
+auto to_opcode(std::string const& s) -> Opcode
+{
+  if (s == "cpy") {
+    return Opcode::cpy;
+  }
+  if (s == "inc") {
+    return Opcode::inc;
+  }
+  if (s == "dec") {
+    return Opcode::dec;
+  }
+  if (s == "jnz") {
+    return Opcode::jnz;
+  }
+  if (s == "tgl") {
+    return Opcode::tgl;
+  }
+  if (s == "out") {
+    return Opcode::out;
+  }
+  abort();
+}
+
+using Int = long;  // NOLINT(google-runtime-int)
+using Operand = std::variant<Register, Int, std::monostate>;
+
+struct Instruction
+{
+  Instruction(Opcode opcode, Operand const& op1, Operand const& op2)
+      : opcode_(opcode), op1_(op1), op2_(op2)
+  {
+  }
+
+  [[nodiscard]] auto opcode() const noexcept -> Opcode { return opcode_; }
+  [[nodiscard]] auto op1() const noexcept -> Operand const& { return op1_; }
+  [[nodiscard]] auto op2() const noexcept -> Operand const& { return op2_; }
+
+  void toggle()
+  {
+    switch (opcode_) {
+    case Opcode::cpy:
+      opcode_ = Opcode::jnz;
+      break;
+    case Opcode::jnz:
+      opcode_ = Opcode::cpy;
+      break;
+    case Opcode::inc:
+      opcode_ = Opcode::dec;
+      break;
+    case Opcode::dec:
+    case Opcode::tgl:
+    case Opcode::out:
+      opcode_ = Opcode::inc;
+      break;
+    }
+  }
+
+  static auto instruction(std::string const& s) -> Instruction
+  {
+    Opcode opcode = to_opcode(s.substr(0, 3));
+    Operand op1{std::monostate()};
+    Operand op2{std::monostate()};
+    std::size_t idx{4};
+
+    {
+      char* end{nullptr};
+      auto val{
+        std::strtol(s.data() + idx,  // NOLINT(cppcoreguidelines-pro-bounds-constant-array-index)
+                    &end, 10)};
+      if (end != s.data() + idx) {  // NOLINT(cppcoreguidelines-pro-bounds-pointer-arithmetic)
+        op1 = val;
+        idx = end - s.data();
+      }
+      else {
+        op1 = to_register(s[idx]);
+        idx += 1;
+      }
+    }
+
+    if (idx < s.size()) {
+      assert(s[idx] == ' ');
+      ++idx;
+      char* end{nullptr};
+      auto val{
+        std::strtol(s.data() + idx,  // NOLINT(cppcoreguidelines-pro-bounds-pointer-arithmetic)
+                    &end, 10)};
+      if (end != s.data() + idx) {  // NOLINT(cppcoreguidelines-pro-bounds-pointer-arithmetic)
+        op2 = val;
+      }
+      else {
+        op2 = to_register(s[idx]);
+      }
+    }
+
+    return {opcode, op1, op2};
+  }
+
+private:
+  Opcode opcode_;
+  Operand op1_;
+  Operand op2_;
+};
+
+struct State
+{
+  explicit State(std::vector<Instruction> instructions) : instructions_(std::move(instructions)) {}
+
+  [[nodiscard]] auto reg(Register r) const noexcept -> Int
+  {
+    return registers_
+      [static_cast<unsigned>(  // NOLINT(cppcoreguidelines-pro-bounds-constant-array-index)
+        r)];
+  }
+  void reg(Register r, Int v) noexcept
+  {
+    registers_[static_cast<unsigned>(  // NOLINT(cppcoreguidelines-pro-bounds-constant-array-index)
+      r)] = v;
+  }
+
+  void execute()
+  {
+    while (pc_ < instructions_.size()) {
+      auto& i{instructions_[pc_]};
+      switch (i.opcode()) {
+      case Opcode::cpy:
+        set(i.op2(), value(i.op1()));
+        break;
+      case Opcode::inc:
+        set(i.op1(), value(i.op1()) + 1);
+        break;
+      case Opcode::dec:
+        set(i.op1(), value(i.op1()) - 1);
+        break;
+      case Opcode::jnz:
+        if (value(i.op1()) != 0) {
+          pc_ += value(i.op2()) - 1;
+        }
+        break;
+      case Opcode::tgl: {
+        auto dest{pc_ + value(i.op1())};
+        if (dest >= 0 && dest < instructions_.size()) {
+          instructions_[dest].toggle();
+        }
+      } break;
+      case Opcode::out:
+        std::cout << value(i.op1()) << '\n';
+        break;
+      }
+
+      ++pc_;
+    }
+  }
+
+private:
+  auto value(Operand const& o) -> Int
+  {
+    if (std::holds_alternative<Register>(o)) {
+      return registers_
+        [static_cast<unsigned>(  // NOLINT(cppcoreguidelines-pro-bounds-constant-array-index)
+          std::get<Register>(o))];
+    }
+    if (std::holds_alternative<Int>(o)) {
+      return std::get<Int>(o);
+    }
+    abort();
+  }
+
+  void set(Operand const& dest, Int value)
+  {
+    if (std::holds_alternative<Register>(dest)) {
+      registers_[  // NOLINT(cppcoreguidelines-pro-bounds-constant-array-index)
+        static_cast<unsigned>(std::get<Register>(dest))] = value;
+    }
+  }
+
+  std::vector<Instruction> instructions_;
+  std::array<Int, 4> registers_{0, 0, 0, 0};
+  Int pc_{0};
+};
+
+auto main() -> int
+{
+  std::vector<Instruction> instructions;
+  std::string line;
+  while (std::getline(std::cin, line) && !line.empty()) {
+    instructions.push_back(Instruction::instruction(line));
+  }
+
+  State state(instructions);
+  // So it looks as if the code we're given takes the value in register A adds 2548 to it and then
+  // outputs the binary representation starting at the least significant bit.  So we want
+  // (a + 2548) to have the binary representation 1010101010 - which turns out to be 182.
+  Int ra = 2730 - 2548;
+  state.reg(Register::a, ra);
+  state.execute();
+}