1//! The exponent estimator.
2
3/// Finds `k_0` such that `10^(k_0-1) < mant * 2^exp <= 10^(k_0+1)`.
4///
5/// This is used to approximate `k = ceil(log_10 (mant * 2^exp))`;
6/// the true `k` is either `k_0` or `k_0+1`.
7#[doc(hidden)]
8pub fn estimate_scaling_factor(mant: u64, exp: i16) -> i16 {
9 // 2^(nbits-1) < mant <= 2^nbits if mant > 0
10 let nbits: i64 = 64 - (mant - 1).leading_zeros() as i64;
11 // 1292913986 = floor(2^32 * log_10 2)
12 // therefore this always underestimates (or is exact), but not much.
13 (((nbits + exp as i64) * 1292913986) >> 32) as i16
14}
15