gvariant/

offset.rs

use crate::aligned_bytes::{Alignment, Misaligned, A1, A2, A4, A8};
use core::{borrow::Borrow, convert::TryFrom, fmt::Display, marker::PhantomData};

/// Represents a usize that is some multiple of [`Alignment::ALIGNMENT`].
///
/// `AlignedOffset<A2>` is essentially a usize that is multiple of 2.  This is
/// useful because you can slice a `AlignedSlice<A2>` and be statically
/// guaranteed that the slice will still be aligned.
///
/// Use `.try_new()`, `<usize>.try_into()` or `align_offset()` to construct
/// values of this type.
#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
pub struct AlignedOffset<A: Alignment>(usize, PhantomData<A>)
where
    Self: core::ops::BitOr,
    <Self as core::ops::BitOr>::Output: Into<Self>;
impl<A: Alignment> AlignedOffset<A> {
    /// Convert to usize
    pub fn to_usize(&self) -> usize {
        self.0
    }
    /// Construct an `AlignedOffset` from a usize.
    ///
    /// Result will be `Err(Misaligned)` if the passed `value` is not a multiple
    /// of the alignment of this type.  This is provided in addition to the
    /// `TryInto` impl because there is less scope for type ambiguity with this
    /// function.
    pub fn try_new(value: usize) -> Result<Self, Misaligned> {
        if value % A::ALIGNMENT == 0 {
            Ok(Self(value, PhantomData::<A> {}))
        } else {
            Err(Misaligned {})
        }
    }
}

/// Construct an [`AlignedOffset`] by rounding-up `idx` until it's a multiple of
/// [`A::ALIGNMENT`](Alignment).
///
/// This is useful for GVariant deserialisation because often we have an offset
/// representing the end of a value and we want to find the start of the next
/// one.  This involves padding according to the next value's alignment.
pub fn align_offset<A: Alignment>(idx: usize) -> AlignedOffset<A> {
    AlignedOffset::<A>(
        (idx + A::ALIGNMENT - 1) & !(A::ALIGNMENT - 1),
        PhantomData::<A> {},
    )
}

impl<A: Alignment> Display for AlignedOffset<A> {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        write!(f, "{}", self.0)
    }
}

impl<A: Alignment> PartialEq<usize> for AlignedOffset<A> {
    fn eq(&self, other: &usize) -> bool {
        *other == self.0
    }
}
impl<A: Alignment> PartialOrd<usize> for AlignedOffset<A> {
    fn partial_cmp(&self, other: &usize) -> Option<core::cmp::Ordering> {
        self.0.partial_cmp(other)
    }
}
impl<A: Alignment> PartialEq<AlignedOffset<A>> for usize {
    fn eq(&self, other: &AlignedOffset<A>) -> bool {
        *self == other.0
    }
}
impl<A: Alignment> PartialOrd<AlignedOffset<A>> for usize {
    fn partial_cmp(&self, other: &AlignedOffset<A>) -> Option<core::cmp::Ordering> {
        self.partial_cmp(&other.0)
    }
}
impl<A: Alignment> TryFrom<usize> for AlignedOffset<A> {
    type Error = Misaligned;
    fn try_from(value: usize) -> Result<Self, Self::Error> {
        Self::try_new(value)
    }
}
impl<A: Alignment> Borrow<usize> for AlignedOffset<A> {
    fn borrow(&self) -> &usize {
        &self.0
    }
}
impl<A: Alignment> From<AlignedOffset<A>> for usize {
    fn from(value: AlignedOffset<A>) -> Self {
        value.0
    }
}

// These are useful for implementing the field alignment algorithm described in
// the GVariant spec
impl<A: Alignment> core::ops::BitOr for AlignedOffset<A> {
    type Output = AlignedOffset<A>;
    fn bitor(self, rhs: Self) -> Self::Output {
        // This is safe because neither of A or B will have the bottom bits set,
        // so we'll end up with a multiple of neither:
        AlignedOffset::<A>(self.0 | rhs.0, PhantomData::<A> {})
    }
}

// These are useful for implementing the field alignment algorithm described in
// the GVariant spec
impl<A: Alignment> core::ops::Add for AlignedOffset<A> {
    type Output = AlignedOffset<A>;
    fn add(self, rhs: Self) -> Self::Output {
        // This is safe because neither of A or B will have the bottom bits set,
        // so we'll end up with a multiple of neither:
        AlignedOffset::<A>(self.0 + rhs.0, PhantomData::<A> {})
    }
}

macro_rules! impl_bitor {
    ($x:ty, $y:ty, $min:ty) => {
        impl core::ops::BitOr<AlignedOffset<$x>> for AlignedOffset<$y> {
            type Output = AlignedOffset<$min>;
            fn bitor(self, rhs: AlignedOffset<$x>) -> AlignedOffset<$min> {
                // This is safe because neither of A or B will have the bottom bits set,
                // so we'll end up with a multiple of neither:
                AlignedOffset::<$min>(self.0 | rhs.0, PhantomData::<$min> {})
            }
        }
    };
}
macro_rules! narrowing {
    ($big:ty, $small:ty) => {
        impl_bitor!($big, $small, $small);
        impl_bitor!($small, $big, $small);
        impl From<AlignedOffset<$big>> for AlignedOffset<$small> {
            fn from(v: AlignedOffset<$big>) -> Self {
                AlignedOffset::<$small>(v.0, PhantomData)
            }
        }
    };
}

narrowing!(A2, A1);
narrowing!(A4, A1);
narrowing!(A8, A1);
narrowing!(A4, A2);
narrowing!(A8, A2);
narrowing!(A8, A4);

#[cfg(test)]
#[cfg(feature = "alloc")]
mod tests {
    use super::*;
    use crate::aligned_bytes::{A2, A4};
    use core::convert::TryInto;

    #[test]
    fn test() {
        let x: Result<AlignedOffset<A2>, Misaligned> = 3usize.try_into();
        assert!(x.is_err());

        let x: AlignedOffset<A2> = 0usize.try_into().unwrap();
        assert_eq!(x.to_usize(), 0usize);

        let x: AlignedOffset<A4> = 8usize.try_into().unwrap();
        assert_eq!(x.to_usize(), 8usize);

        assert_eq!(align_offset::<A4>(0).to_usize(), 0);
        assert_eq!(align_offset::<A4>(1).to_usize(), 4);
        assert_eq!(align_offset::<A4>(4).to_usize(), 4);
        assert_eq!(align_offset::<A4>(6).to_usize(), 8);
        assert_eq!(align_offset::<A4>(7).to_usize(), 8);
        assert_eq!(align_offset::<A4>(8).to_usize(), 8);
        assert_eq!(align_offset::<A4>(9).to_usize(), 12);
    }
}