| //! The exponent estimator. |
| |
| /// Finds `k_0` such that `10^(k_0-1) < mant * 2^exp <= 10^(k_0+1)`. |
| /// |
| /// This is used to approximate `k = ceil(log_10 (mant * 2^exp))`; |
| /// the true `k` is either `k_0` or `k_0+1`. |
| #[doc(hidden)] |
| pub fn estimate_scaling_factor(mant: u64, exp: i16) -> i16 { |
| // 2^(nbits-1) < mant <= 2^nbits if mant > 0 |
| let nbits = 64 - (mant - 1).leading_zeros() as i64; |
| // 1292913986 = floor(2^32 * log_10 2) |
| // therefore this always underestimates (or is exact), but not much. |
| (((nbits + exp as i64) * 1292913986) >> 32) as i16 |
| } |
