dashu_float/
root.rs

1use dashu_base::{Approximation, Sign, SquareRoot, SquareRootRem, UnsignedAbs};
2use dashu_int::IBig;
3
4use crate::{
5    error::{assert_finite, assert_limited_precision, panic_root_negative},
6    fbig::FBig,
7    repr::{Context, Repr, Word},
8    round::{Round, Rounded},
9    utils::{shl_digits, split_digits_ref},
10};
11
12impl<R: Round, const B: Word> SquareRoot for FBig<R, B> {
13    type Output = Self;
14    #[inline]
15    fn sqrt(&self) -> Self {
16        self.context.sqrt(self.repr()).value()
17    }
18}
19
20impl<R: Round> Context<R> {
21    /// Calculate the square root of the floating point number.
22    ///
23    /// # Examples
24    ///
25    /// ```
26    /// # use core::str::FromStr;
27    /// # use dashu_base::ParseError;
28    /// # use dashu_float::DBig;
29    /// use dashu_base::Approximation::*;
30    /// use dashu_float::{Context, round::{mode::HalfAway, Rounding::*}};
31    ///
32    /// let context = Context::<HalfAway>::new(2);
33    /// let a = DBig::from_str("1.23")?;
34    /// assert_eq!(context.sqrt(&a.repr()), Inexact(DBig::from_str("1.1")?, NoOp));
35    /// # Ok::<(), ParseError>(())
36    /// ```
37    ///
38    /// # Panics
39    ///
40    /// Panics if the precision is unlimited.
41    pub fn sqrt<const B: Word>(&self, x: &Repr<B>) -> Rounded<FBig<R, B>> {
42        assert_finite(x);
43        assert_limited_precision(self.precision);
44        if x.sign() == Sign::Negative {
45            panic_root_negative()
46        }
47
48        // adjust the signifcand so that the exponent is even
49        let digits = x.digits() as isize;
50        let shift = self.precision as isize * 2 - (digits & 1) + (x.exponent & 1) - digits;
51        let (signif, low, low_digits) = if shift > 0 {
52            (shl_digits::<B>(&x.significand, shift as usize), IBig::ZERO, 0)
53        } else {
54            let shift = (-shift) as usize;
55            let (hi, lo) = split_digits_ref::<B>(&x.significand, shift);
56            (hi, lo, shift)
57        };
58
59        let (root, rem) = signif.unsigned_abs().sqrt_rem();
60        let root = Sign::Positive * root;
61        let exp = (x.exponent - shift) / 2;
62
63        let res = if rem.is_zero() {
64            Approximation::Exact(root)
65        } else {
66            let adjust = R::round_low_part(&root, Sign::Positive, || {
67                (Sign::Positive * rem)
68                    .cmp(&root)
69                    .then_with(|| (low * 4u8).cmp(&Repr::<B>::BASE.pow(low_digits).into()))
70            });
71            Approximation::Inexact(root + adjust, adjust)
72        };
73        res.map(|signif| Repr::new(signif, exp))
74            .and_then(|v| self.repr_round(v))
75            .map(|v| FBig::new(v, *self))
76    }
77}
78
79// TODO(next): implement cbrt, nth_root
dashu_float/root.rs

dashu_float/
root.rs
