core/num/dec2flt/
number.rs

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1//! Representation of a float as the significant digits and exponent.
2
3use crate::num::dec2flt::float::RawFloat;
4use crate::num::dec2flt::fpu::set_precision;
5
6#[rustfmt::skip]
7const INT_POW10: [u64; 16] = [
8    1,
9    10,
10    100,
11    1000,
12    10000,
13    100000,
14    1000000,
15    10000000,
16    100000000,
17    1000000000,
18    10000000000,
19    100000000000,
20    1000000000000,
21    10000000000000,
22    100000000000000,
23    1000000000000000,
24];
25
26#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
27pub struct Number {
28    pub exponent: i64,
29    pub mantissa: u64,
30    pub negative: bool,
31    pub many_digits: bool,
32}
33
34impl Number {
35    /// Detect if the float can be accurately reconstructed from native floats.
36    #[inline]
37    fn is_fast_path<F: RawFloat>(&self) -> bool {
38        F::MIN_EXPONENT_FAST_PATH <= self.exponent
39            && self.exponent <= F::MAX_EXPONENT_DISGUISED_FAST_PATH
40            && self.mantissa <= F::MAX_MANTISSA_FAST_PATH
41            && !self.many_digits
42    }
43
44    /// The fast path algorithm using machine-sized integers and floats.
45    ///
46    /// This is extracted into a separate function so that it can be attempted before constructing
47    /// a Decimal. This only works if both the mantissa and the exponent
48    /// can be exactly represented as a machine float, since IEE-754 guarantees
49    /// no rounding will occur.
50    ///
51    /// There is an exception: disguised fast-path cases, where we can shift
52    /// powers-of-10 from the exponent to the significant digits.
53    pub fn try_fast_path<F: RawFloat>(&self) -> Option<F> {
54        // Here we need to work around <https://github.com/rust-lang/rust/issues/114479>.
55        // The fast path crucially depends on arithmetic being rounded to the correct number of bits
56        // without any intermediate rounding. On x86 (without SSE or SSE2) this requires the precision
57        // of the x87 FPU stack to be changed so that it directly rounds to 64/32 bit.
58        // The `set_precision` function takes care of setting the precision on architectures which
59        // require setting it by changing the global state (like the control word of the x87 FPU).
60        let _cw = set_precision::<F>();
61
62        if self.is_fast_path::<F>() {
63            let mut value = if self.exponent <= F::MAX_EXPONENT_FAST_PATH {
64                // normal fast path
65                let value = F::from_u64(self.mantissa);
66                if self.exponent < 0 {
67                    value / F::pow10_fast_path((-self.exponent) as _)
68                } else {
69                    value * F::pow10_fast_path(self.exponent as _)
70                }
71            } else {
72                // disguised fast path
73                let shift = self.exponent - F::MAX_EXPONENT_FAST_PATH;
74                let mantissa = self.mantissa.checked_mul(INT_POW10[shift as usize])?;
75                if mantissa > F::MAX_MANTISSA_FAST_PATH {
76                    return None;
77                }
78                F::from_u64(mantissa) * F::pow10_fast_path(F::MAX_EXPONENT_FAST_PATH as _)
79            };
80            if self.negative {
81                value = -value;
82            }
83            Some(value)
84        } else {
85            None
86        }
87    }
88}
core/num/dec2flt/number.rs

core/num/dec2flt/
number.rs
