add_int32_using_float32.cc

int32_t add_int32_using_float32(int32_t a, int32_t b) {
  const uint32_t SHIFT = 16;
  const uint32_t MASK = 0xFFFF;

  int32_t a_high = a >> SHIFT;  // Sign-extended
  uint32_t a_low = static_cast<uint32_t>(a) & MASK;
  int32_t b_high = b >> SHIFT;  // Sign-extended
  uint32_t b_low = static_cast<uint32_t>(b) & MASK;

  // Add low parts (this will fit in float32)
  float low_sum = static_cast<float>(a_low) + static_cast<float>(b_low);

  // Handle carry from low parts
  uint32_t carry = static_cast<uint32_t>(low_sum) >> SHIFT;
  uint32_t low_result = static_cast<uint32_t>(low_sum) & MASK;

  // Add high parts (this will fit in float32)
  float high_sum = static_cast<float>(a_high) + static_cast<float>(b_high) +
                   static_cast<float>(carry);
  uint32_t high_result = static_cast<uint32_t>(high_sum) & MASK;

  // Combine results
  uint32_t result = (high_result << SHIFT) | low_result;

  return static_cast<int32_t>(result);
}

int main() {
  std::vector<std::pair<int, int>> aa = {
      {-1000000000, -1000000001},
      {-1000000000, 1000000001},
      {1000000000, -1000000001},
      {1000000000, 1000000001},
      {1500000000, 500000000},
      {-500000000, 500000000},
      {-2147483648, 1},
      {-2147483648, -1},
      {2147483647, 1},
      {2147483647, -1},
      {2000000000, 147483647},
      {2000000000, 147483648},
      {-2000000000, -147483648},
      {-2000000000, -147483649},
      {2000000000, -147483648},
  };

  for (auto a : aa) {
    int32_t c = add_int32_using_float32(a.first, a.second);
    int32_t ec = a.first + a.second;
    std::cout << a.first << " + " << a.second << " = " << c << std::endl;
    if (c != ec) {
      std::cout << "Error: res / exp: " << c << " / " << ec << std::endl;
    }
  }

  return 0;
}