rust/library/core/src/fmt/float.rs

use crate::fmt::{Debug, Display, Formatter, LowerExp, Result, UpperExp};
use crate::mem::MaybeUninit;
use crate::num::flt2dec;

// Don't inline this so callers don't use the stack space this function
// requires unless they have to.
#[inline(never)]
fn float_to_decimal_common_exact<T>(
    fmt: &mut Formatter<'_>,
    num: &T,
    sign: flt2dec::Sign,
    precision: usize,
) -> Result
where
    T: flt2dec::DecodableFloat,
{
    // SAFETY: Possible undefined behavior, see FIXME(#76092)
    unsafe {
        let mut buf = MaybeUninit::<[u8; 1024]>::uninit(); // enough for f32 and f64
        let mut parts = MaybeUninit::<[flt2dec::Part<'_>; 4]>::uninit();
        // FIXME(#76092): This is calling `assume_init_mut` on an uninitialized
        // `MaybeUninit` (here and elsewhere in this file). Revisit this once
        // we decided whether that is valid or not.
        // We can do this only because we are libstd and coupled to the compiler.
        // (FWIW, using `freeze` would not be enough; `flt2dec::Part` is an enum!)
        let formatted = flt2dec::to_exact_fixed_str(
            flt2dec::strategy::grisu::format_exact,
            *num,
            sign,
            precision,
            buf.assume_init_mut(),
            parts.assume_init_mut(),
        );
        fmt.pad_formatted_parts(&formatted)
    }
}

// Don't inline this so callers that call both this and the above won't wind
// up using the combined stack space of both functions in some cases.
#[inline(never)]
fn float_to_decimal_common_shortest<T>(
    fmt: &mut Formatter<'_>,
    num: &T,
    sign: flt2dec::Sign,
    precision: usize,
) -> Result
where
    T: flt2dec::DecodableFloat,
{
    // SAFETY: Possible undefined behavior, see FIXME(#76092)
    unsafe {
        // enough for f32 and f64
        let mut buf = MaybeUninit::<[u8; flt2dec::MAX_SIG_DIGITS]>::uninit();
        let mut parts = MaybeUninit::<[flt2dec::Part<'_>; 4]>::uninit();
        // FIXME(#76092)
        let formatted = flt2dec::to_shortest_str(
            flt2dec::strategy::grisu::format_shortest,
            *num,
            sign,
            precision,
            buf.assume_init_mut(),
            parts.assume_init_mut(),
        );
        fmt.pad_formatted_parts(&formatted)
    }
}

// Common code of floating point Debug and Display.
fn float_to_decimal_common<T>(
    fmt: &mut Formatter<'_>,
    num: &T,
    negative_zero: bool,
    min_precision: usize,
) -> Result
where
    T: flt2dec::DecodableFloat,
{
    let force_sign = fmt.sign_plus();
    let sign = match (force_sign, negative_zero) {
        (false, false) => flt2dec::Sign::Minus,
        (false, true) => flt2dec::Sign::MinusRaw,
        (true, false) => flt2dec::Sign::MinusPlus,
        (true, true) => flt2dec::Sign::MinusPlusRaw,
    };

    if let Some(precision) = fmt.precision {
        float_to_decimal_common_exact(fmt, num, sign, precision)
    } else {
        float_to_decimal_common_shortest(fmt, num, sign, min_precision)
    }
}

// Don't inline this so callers don't use the stack space this function
// requires unless they have to.
#[inline(never)]
fn float_to_exponential_common_exact<T>(
    fmt: &mut Formatter<'_>,
    num: &T,
    sign: flt2dec::Sign,
    precision: usize,
    upper: bool,
) -> Result
where
    T: flt2dec::DecodableFloat,
{
    // SAFETY: Possible undefined behavior, see FIXME(#76092)
    unsafe {
        let mut buf = MaybeUninit::<[u8; 1024]>::uninit(); // enough for f32 and f64
        let mut parts = MaybeUninit::<[flt2dec::Part<'_>; 6]>::uninit();
        // FIXME(#76092)
        let formatted = flt2dec::to_exact_exp_str(
            flt2dec::strategy::grisu::format_exact,
            *num,
            sign,
            precision,
            upper,
            buf.assume_init_mut(),
            parts.assume_init_mut(),
        );
        fmt.pad_formatted_parts(&formatted)
    }
}

// Don't inline this so callers that call both this and the above won't wind
// up using the combined stack space of both functions in some cases.
#[inline(never)]
fn float_to_exponential_common_shortest<T>(
    fmt: &mut Formatter<'_>,
    num: &T,
    sign: flt2dec::Sign,
    upper: bool,
) -> Result
where
    T: flt2dec::DecodableFloat,
{
    // SAFETY: Possible undefined behavior, see FIXME(#76092)
    unsafe {
        // enough for f32 and f64
        let mut buf = MaybeUninit::<[u8; flt2dec::MAX_SIG_DIGITS]>::uninit();
        let mut parts = MaybeUninit::<[flt2dec::Part<'_>; 6]>::uninit();
        // FIXME(#76092)
        let formatted = flt2dec::to_shortest_exp_str(
            flt2dec::strategy::grisu::format_shortest,
            *num,
            sign,
            (0, 0),
            upper,
            buf.assume_init_mut(),
            parts.assume_init_mut(),
        );
        fmt.pad_formatted_parts(&formatted)
    }
}

// Common code of floating point LowerExp and UpperExp.
fn float_to_exponential_common<T>(fmt: &mut Formatter<'_>, num: &T, upper: bool) -> Result
where
    T: flt2dec::DecodableFloat,
{
    let force_sign = fmt.sign_plus();
    let sign = match force_sign {
        false => flt2dec::Sign::Minus,
        true => flt2dec::Sign::MinusPlus,
    };

    if let Some(precision) = fmt.precision {
        // 1 integral digit + `precision` fractional digits = `precision + 1` total digits
        float_to_exponential_common_exact(fmt, num, sign, precision + 1, upper)
    } else {
        float_to_exponential_common_shortest(fmt, num, sign, upper)
    }
}

macro_rules! floating {
    ($ty:ident) => {
        #[stable(feature = "rust1", since = "1.0.0")]
        impl Debug for $ty {
            fn fmt(&self, fmt: &mut Formatter<'_>) -> Result {
                float_to_decimal_common(fmt, self, true, 1)
            }
        }

        #[stable(feature = "rust1", since = "1.0.0")]
        impl Display for $ty {
            fn fmt(&self, fmt: &mut Formatter<'_>) -> Result {
                float_to_decimal_common(fmt, self, false, 0)
            }
        }

        #[stable(feature = "rust1", since = "1.0.0")]
        impl LowerExp for $ty {
            fn fmt(&self, fmt: &mut Formatter<'_>) -> Result {
                float_to_exponential_common(fmt, self, false)
            }
        }

        #[stable(feature = "rust1", since = "1.0.0")]
        impl UpperExp for $ty {
            fn fmt(&self, fmt: &mut Formatter<'_>) -> Result {
                float_to_exponential_common(fmt, self, true)
            }
        }
    };
}

floating! { f32 }
floating! { f64 }