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

core/num/dec2flt/
decimal.rs
