decimal.rs

//! Representation of a float as the significant digits and exponent.

use crate::num::dec2flt::float::RawFloat;
use crate::num::dec2flt::fpu::set_precision;

const INT_POW10: [u64; 16] = [
    1,
    10,
    100,
    1000,
    10000,
    100000,
    1000000,
    10000000,
    100000000,
    1000000000,
    10000000000,
    100000000000,
    1000000000000,
    10000000000000,
    100000000000000,
    1000000000000000,
];

/// A floating point number with up to 64 bits of mantissa and an `i64` exponent.
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
pub struct Decimal {
    pub exponent: i64,
    pub mantissa: u64,
    pub negative: bool,
    pub many_digits: bool,
}

impl Decimal {
    /// Detect if the float can be accurately reconstructed from native floats.
    #[inline]
    fn can_use_fast_path<F: RawFloat>(&self) -> bool {
        F::MIN_EXPONENT_FAST_PATH <= self.exponent
            && self.exponent <= F::MAX_EXPONENT_DISGUISED_FAST_PATH
            && self.mantissa <= F::MAX_MANTISSA_FAST_PATH
            && !self.many_digits
    }

    /// Try turning the decimal into an exact float representation, using machine-sized integers
    /// and floats.
    ///
    /// This is extracted into a separate function so that it can be attempted before constructing
    /// a Decimal. This only works if both the mantissa and the exponent
    /// can be exactly represented as a machine float, since IEE-754 guarantees
    /// no rounding will occur.
    ///
    /// There is an exception: disguised fast-path cases, where we can shift
    /// powers-of-10 from the exponent to the significant digits.
    pub fn try_fast_path<F: RawFloat>(&self) -> Option<F> {
        // Here we need to work around <https://github.com/rust-lang/rust/issues/114479>.
        // The fast path crucially depends on arithmetic being rounded to the correct number of bits
        // without any intermediate rounding. On x86 (without SSE or SSE2) this requires the precision
        // of the x87 FPU stack to be changed so that it directly rounds to 64/32 bit.
        // The `set_precision` function takes care of setting the precision on architectures which
        // require setting it by changing the global state (like the control word of the x87 FPU).
        let _cw = set_precision::<F>();

        if !self.can_use_fast_path::<F>() {
            return None;
        }

        let value = if self.exponent <= F::MAX_EXPONENT_FAST_PATH {
            // normal fast path
            let value = F::from_u64(self.mantissa);
            if self.exponent < 0 {
                value / F::pow10_fast_path((-self.exponent) as _)
            } else {
                value * F::pow10_fast_path(self.exponent as _)
            }
        } else {
            // disguised fast path
            let shift = self.exponent - F::MAX_EXPONENT_FAST_PATH;
            let mantissa = self.mantissa.checked_mul(INT_POW10[shift as usize])?;
            if mantissa > F::MAX_MANTISSA_FAST_PATH {
                return None;
            }
            F::from_u64(mantissa) * F::pow10_fast_path(F::MAX_EXPONENT_FAST_PATH as _)
        };

        if self.negative { Some(-value) } else { Some(value) }
    }
}