FMT_CONSTEXPR20 inline void format_dragon()

in thrift/compiler/detail/fmt/fmt/format-inl.h [2226:2338]

• 103 lines of code
• 28 McCabe index (conditional complexity)

FMT_CONSTEXPR20 inline void format_dragon(fp value, bool is_predecessor_closer,
                                          int num_digits, buffer<char>& buf,
                                          int& exp10) {
  bigint numerator;    // 2 * R in (FPP)^2.
  bigint denominator;  // 2 * S in (FPP)^2.
  // lower and upper are differences between value and corresponding boundaries.
  bigint lower;             // (M^- in (FPP)^2).
  bigint upper_store;       // upper's value if different from lower.
  bigint* upper = nullptr;  // (M^+ in (FPP)^2).
  // Shift numerator and denominator by an extra bit or two (if lower boundary
  // is closer) to make lower and upper integers. This eliminates multiplication
  // by 2 during later computations.
  int shift = is_predecessor_closer ? 2 : 1;
  uint64_t significand = value.f << shift;
  if (value.e >= 0) {
    numerator.assign(significand);
    numerator <<= value.e;
    lower.assign(1);
    lower <<= value.e;
    if (shift != 1) {
      upper_store.assign(1);
      upper_store <<= value.e + 1;
      upper = &upper_store;
    }
    denominator.assign_pow10(exp10);
    denominator <<= shift;
  } else if (exp10 < 0) {
    numerator.assign_pow10(-exp10);
    lower.assign(numerator);
    if (shift != 1) {
      upper_store.assign(numerator);
      upper_store <<= 1;
      upper = &upper_store;
    }
    numerator *= significand;
    denominator.assign(1);
    denominator <<= shift - value.e;
  } else {
    numerator.assign(significand);
    denominator.assign_pow10(exp10);
    denominator <<= shift - value.e;
    lower.assign(1);
    if (shift != 1) {
      upper_store.assign(1ULL << 1);
      upper = &upper_store;
    }
  }
  // Invariant: value == (numerator / denominator) * pow(10, exp10).
  if (num_digits < 0) {
    // Generate the shortest representation.
    if (!upper) upper = &lower;
    bool even = (value.f & 1) == 0;
    num_digits = 0;
    char* data = buf.data();
    for (;;) {
      int digit = numerator.divmod_assign(denominator);
      bool low = compare(numerator, lower) - even < 0;  // numerator <[=] lower.
      // numerator + upper >[=] pow10:
      bool high = add_compare(numerator, *upper, denominator) + even > 0;
      data[num_digits++] = static_cast<char>('0' + digit);
      if (low || high) {
        if (!low) {
          ++data[num_digits - 1];
        } else if (high) {
          int result = add_compare(numerator, numerator, denominator);
          // Round half to even.
          if (result > 0 || (result == 0 && (digit % 2) != 0))
            ++data[num_digits - 1];
        }
        buf.try_resize(to_unsigned(num_digits));
        exp10 -= num_digits - 1;
        return;
      }
      numerator *= 10;
      lower *= 10;
      if (upper != &lower) *upper *= 10;
    }
  }
  // Generate the given number of digits.
  exp10 -= num_digits - 1;
  if (num_digits == 0) {
    denominator *= 10;
    auto digit = add_compare(numerator, numerator, denominator) > 0 ? '1' : '0';
    buf.push_back(digit);
    return;
  }
  buf.try_resize(to_unsigned(num_digits));
  for (int i = 0; i < num_digits - 1; ++i) {
    int digit = numerator.divmod_assign(denominator);
    buf[i] = static_cast<char>('0' + digit);
    numerator *= 10;
  }
  int digit = numerator.divmod_assign(denominator);
  auto result = add_compare(numerator, numerator, denominator);
  if (result > 0 || (result == 0 && (digit % 2) != 0)) {
    if (digit == 9) {
      const auto overflow = '0' + 10;
      buf[num_digits - 1] = overflow;
      // Propagate the carry.
      for (int i = num_digits - 1; i > 0 && buf[i] == overflow; --i) {
        buf[i] = '0';
        ++buf[i - 1];
      }
      if (buf[0] == overflow) {
        buf[0] = '1';
        ++exp10;
      }
      return;
    }
    ++digit;
  }
  buf[num_digits - 1] = static_cast<char>('0' + digit);
}