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Add ReprEndianness enum and PrimeField::REPR_ENDIANNESS #158

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Add ReprEndianness enum and PrimeField::REPR_ENDIANNESS #158

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19 changes: 17 additions & 2 deletions src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -37,6 +37,17 @@ use subtle::{Choice, ConditionallySelectable, ConstantTimeEq, CtOption};
#[cfg_attr(docsrs, doc(cfg(feature = "bits")))]
pub type FieldBits<V> = BitArray<V, Lsb0>;

/// Endianness of a field element's binary representation.
#[derive(Clone, Copy, Debug, Default, Eq, PartialEq)]
pub enum ReprEndianness {
/// Big endian: bytes are ordered most-to-least significant.
Big,

/// Little endian: bytes are ordered least-to-most significant.
#[default]
Little,
}

/// This trait represents an element of a field.
pub trait Field:
Sized
Expand Down Expand Up @@ -211,6 +222,9 @@ pub trait PrimeField: Field + From<u64> {
/// representation.
type Repr: Copy + Default + Send + Sync + 'static + AsRef<[u8]> + AsMut<[u8]>;

/// Endianness of the binary representation `Self::Repr`.
const REPR_ENDIANNESS: ReprEndianness = ReprEndianness::Little;

/// Interpret a string of numbers as a (congruent) prime field element.
/// Does not accept unnecessary leading zeroes or a blank string.
///
Expand Down Expand Up @@ -301,8 +315,9 @@ pub trait PrimeField: Field + From<u64> {
/// Converts an element of the prime field into the standard byte representation for
/// this field.
///
/// The endianness of the byte representation is implementation-specific. Generic
/// encodings of field elements should be treated as opaque.
/// The endianness of the byte representation is described by the associated constant
/// [`PrimeField::REPR_ENDIANNESS`]. Generic encodings of field elements should honor this
/// constant when using this representation in a non-opaque manner.
fn to_repr(&self) -> Self::Repr;

/// Returns true iff this element is odd.
Expand Down
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