aocpp/intcode/src/Interpreter.cpp

#include <intcode/Interpreter.hpp>

namespace intcode {

BadInstruction::BadInstruction(char const* const what)
    : std::runtime_error{what} {}

auto StepInput(Machine & m, ValueType const input) -> void {
  std::get<Input>(Step(m)).input = input;
}

auto StepOutput(Machine & m) -> ValueType {
  return std::get<Output>(Step(m)).output;
}

auto Step(Machine & m) -> std::variant<Input, Output, Halt> {
  for (;;) {
    auto next = m.Next();
    auto const instruction = next % 100; next /= 100;

    auto const arg = [&]() -> ValueType & {
      auto &v = m.Next();
      auto const mode = next % 10; next /= 10;
      switch (mode) {
      case 0: return m.At(v);
      case 1: return v;
      case 2: return m.Rel(v);
      default: throw BadInstruction{"invalid addressing mode"};
      }
    };

    switch (instruction) {
    case 1:
      arg() = arg() + arg();
      break;

    case 2:
      arg() = arg() * arg();
      break;

    case 3:
      return Input{arg()};

    case 4:
      return Output{arg()};

    case 5: {
      auto const a = arg();
      auto const b = arg();
      if (a) { m.Goto(b); }
      break;
    }

    case 6: {
      auto const a = arg();
      auto const b = arg();
      if (!a) { m.Goto(b); }
      break;
    }

    case 7: {
      auto const a = arg();
      arg() = a < arg();
      break;
    }

    case 8:
      arg() = arg() == arg();
      break;

    case 9:
      m.Rebase(arg());
      break;

    case 99:
      return Halt{};

    default:
    std::cout << instruction << std::endl;
      throw BadInstruction{"invalid opcode"};
    }
  }
}

} // namespace