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elliptic-curve: extract NonIdentity/NonZeroScalar casting methods #1903

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Merged
tarcieri merged 2 commits into from
Jun 14, 2025
Merged

elliptic-curve: extract NonIdentity/NonZeroScalar casting methods #1903

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01694ca
elliptic-curve: extract `NonIdentity` casting methods
tarcieri Jun 14, 2025
7126544
Add NonZeroScalar casting methods
tarcieri Jun 14, 2025
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73 changes: 35 additions & 38 deletions elliptic-curve/src/point/non_identity.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -51,6 +51,39 @@ where
}
}

impl<P> NonIdentity<P> {
/// Transform array reference containing [`NonIdentity`] points to an array reference to the
/// inner point type.
pub fn cast_array_as_inner<const N: usize>(points: &[Self; N]) -> &[P; N] {
// Ensure casting is safe.
// This always succeeds because `NonIdentity` is `repr(transparent)`.
debug_assert_eq!(size_of::<P>(), size_of::<NonIdentity<P>>());
debug_assert_eq!(align_of::<P>(), align_of::<NonIdentity<P>>());

// SAFETY: `NonIdentity` is a `repr(transparent)` newtype for `P` so it's safe to cast to
// the inner `P` type.
#[allow(unsafe_code)]
unsafe {
&*points.as_ptr().cast()
}
}

/// Transform slice containing [`NonIdentity`] points to a slice of the inner point type.
pub fn cast_slice_as_inner(points: &[Self]) -> &[P] {
// Ensure casting is safe.
// This always succeeds because `NonIdentity` is `repr(transparent)`.
debug_assert_eq!(size_of::<P>(), size_of::<NonIdentity<P>>());
debug_assert_eq!(align_of::<P>(), align_of::<NonIdentity<P>>());

// SAFETY: `NonIdentity` is a `repr(transparent)` newtype for `P` so it's safe to cast to
// the inner `P` type.
#[allow(unsafe_code)]
unsafe {
&*(points as *const [NonIdentity<P>] as *const [P])
}
}
}

impl<P: Copy> NonIdentity<P> {
/// Return wrapped point.
pub fn to_point(self) -> P {
Expand Down Expand Up @@ -114,26 +147,8 @@ where
type Output = [NonIdentity<P::AffineRepr>; N];

fn batch_normalize(points: &[Self; N]) -> [NonIdentity<P::AffineRepr>; N] {
// Ensure casting is safe.
// This always succeeds because `NonIdentity` is `repr(transparent)`.
debug_assert_eq!(size_of::<P>(), size_of::<NonIdentity<P>>());
debug_assert_eq!(align_of::<P>(), align_of::<NonIdentity<P>>());

#[allow(unsafe_code)]
// SAFETY: `NonIdentity` is `repr(transparent)`.
let points: &[P; N] = unsafe { &*points.as_ptr().cast() };
let points = Self::cast_array_as_inner::<N>(points);
let affine_points = <P as BatchNormalize<_>>::batch_normalize(points);

// Ensure `array::map()` can be optimized to a `memcpy`.
debug_assert_eq!(
size_of::<P::AffineRepr>(),
size_of::<NonIdentity<P::AffineRepr>>()
);
debug_assert_eq!(
align_of::<P::AffineRepr>(),
align_of::<NonIdentity<P::AffineRepr>>()
);
Comment on lines -127 to -135

@tarcieri tarcieri Jun 14, 2025

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I removed these because these properties should always hold true for a repr(transparent) newtype


affine_points.map(|point| NonIdentity { point })
}
}
Expand All @@ -146,26 +161,8 @@ where
type Output = Vec<NonIdentity<P::AffineRepr>>;

fn batch_normalize(points: &[Self]) -> Vec<NonIdentity<P::AffineRepr>> {
// Ensure casting is safe.
// This always succeeds because `NonIdentity` is `repr(transparent)`.
debug_assert_eq!(size_of::<P>(), size_of::<NonIdentity<P>>());
debug_assert_eq!(align_of::<P>(), align_of::<NonIdentity<P>>());

#[allow(unsafe_code)]
// SAFETY: `NonIdentity` is `repr(transparent)`.
let points: &[P] = unsafe { &*(points as *const [NonIdentity<P>] as *const [P]) };
let points = Self::cast_slice_as_inner(points);
let affine_points = <P as BatchNormalize<_>>::batch_normalize(points);

// Ensure `into_iter()` + `collect()` can be optimized away.
debug_assert_eq!(
size_of::<P::AffineRepr>(),
size_of::<NonIdentity<P::AffineRepr>>()
);
debug_assert_eq!(
align_of::<P::AffineRepr>(),
align_of::<NonIdentity<P::AffineRepr>>()
);

affine_points
.into_iter()
.map(|point| NonIdentity { point })
Expand Down