Function get_cached_power

extlibs/fmt/include/fmt/format-inl.h:463–477

Returns a cached power of 10 `c_k = c_k.f * pow(2, c_k.e)` such that its (binary) exponent satisfies `min_exponent <= c_k.e <= min_exponent + 28`.

// Returns a cached power of 10 `c_k = c_k.f * pow(2, c_k.e)` such that its
// (binary) exponent satisfies `min_exponent <= c_k.e <= min_exponent + 28`.
inline fp get_cached_power(int min_exponent, int& pow10_exponent) {
  const int64_t one_over_log2_10 = 0x4d104d42;  // round(pow(2, 32) / log2(10))
  int index = static_cast<int>(
      ((min_exponent + fp::significand_size - 1) * one_over_log2_10 +
       ((int64_t(1) << 32) - 1))  // ceil
      >> 32                       // arithmetic shift
  );
  // Decimal exponent of the first (smallest) cached power of 10.
  const int first_dec_exp = -348;
  // Difference between 2 consecutive decimal exponents in cached powers of 10.
  const int dec_exp_step = 8;
  index = (index - first_dec_exp - 1) / dec_exp_step + 1;
  pow10_exponent = first_dec_exp + index * dec_exp_step;
  return {data::pow10_significands[index], data::pow10_exponents[index]};
}

// A simple accumulator to hold the sums of terms in bigint::square if uint128_t
// is not available.