diff options
Diffstat (limited to 'library/core/src/num/flt2dec/estimator.rs')
| -rw-r--r-- | library/core/src/num/flt2dec/estimator.rs | 14 |
1 files changed, 14 insertions, 0 deletions
diff --git a/library/core/src/num/flt2dec/estimator.rs b/library/core/src/num/flt2dec/estimator.rs new file mode 100644 index 000000000..50e2f7052 --- /dev/null +++ b/library/core/src/num/flt2dec/estimator.rs @@ -0,0 +1,14 @@ +//! 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 +} |