1 // Adapted from https://github.com/Alexhuszagh/rust-lexical.
2
3 //! Utilities to calculate exponents.
4
5 /// Convert usize into i32 without overflow.
6 ///
7 /// This is needed to ensure when adjusting the exponent relative to
8 /// the mantissa we do not overflow for comically-long exponents.
9 #[inline]
into_i32(value: usize) -> i3210 fn into_i32(value: usize) -> i32 {
11 if value > i32::max_value() as usize {
12 i32::max_value()
13 } else {
14 value as i32
15 }
16 }
17
18 // EXPONENT CALCULATION
19
20 // Calculate the scientific notation exponent without overflow.
21 //
22 // For example, 0.1 would be -1, and 10 would be 1 in base 10.
23 #[inline]
scientific_exponent( exponent: i32, integer_digits: usize, fraction_start: usize, ) -> i3224 pub(crate) fn scientific_exponent(
25 exponent: i32,
26 integer_digits: usize,
27 fraction_start: usize,
28 ) -> i32 {
29 if integer_digits == 0 {
30 let fraction_start = into_i32(fraction_start);
31 exponent.saturating_sub(fraction_start).saturating_sub(1)
32 } else {
33 let integer_shift = into_i32(integer_digits - 1);
34 exponent.saturating_add(integer_shift)
35 }
36 }
37
38 // Calculate the mantissa exponent without overflow.
39 //
40 // Remove the number of digits that contributed to the mantissa past
41 // the dot, and add the number of truncated digits from the mantissa,
42 // to calculate the scaling factor for the mantissa from a raw exponent.
43 #[inline]
mantissa_exponent(exponent: i32, fraction_digits: usize, truncated: usize) -> i3244 pub(crate) fn mantissa_exponent(exponent: i32, fraction_digits: usize, truncated: usize) -> i32 {
45 if fraction_digits > truncated {
46 exponent.saturating_sub(into_i32(fraction_digits - truncated))
47 } else {
48 exponent.saturating_add(into_i32(truncated - fraction_digits))
49 }
50 }
51