#include <intcode/Interpreter.hpp>

namespace intcode {

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

auto Step(Machine & m) -> std::variant<Input, Output, Halt> {
  for (;;) {
    auto instruction = m.Next();
    auto modes = instruction / 10;

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

    switch (instruction % 100) {
    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 a = arg();
      auto b = arg();
      if (a) { m.Goto(b); }
      break;
    }

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

    case 7: {
      auto a = arg();
      auto b = arg();
      arg() = a < b; // order matters
      break;
    }

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

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

    case 99:
      return Halt{};

    default:
      throw BadInstruction{"invalid opcode"};
    }
  }
}

} // namespace