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ff: Add PrimeField::ReprEndianness associated type

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This enables generic code to reliably operate on the bits of an encoded
field element, by converting them to and from a known (little)
endianness.

The BitAnd and Shr bounds on PrimeField are now removed, as users can
perform these operations themselves as needed.
This commit is contained in:
Jack Grigg
2020-05-01 14:20:35 +12:00
parent 55568b4d6e
commit 38f87c2e73
12 changed files with 101 additions and 323 deletions
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59
ff/ff_derive/src/lib.rs
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@@ -31,6 +31,13 @@ impl FromStr for ReprEndianness {
}
impl ReprEndianness {
fn repr_endianness(&self) -> proc_macro2::TokenStream {
match self {
ReprEndianness::Big => quote! {::byteorder::BigEndian},
ReprEndianness::Little => quote! {::byteorder::LittleEndian},
}
}
fn from_repr(&self, name: &syn::Ident, limbs: usize) -> proc_macro2::TokenStream {
let read_repr = match self {
ReprEndianness::Big => quote! {
@@ -885,6 +892,7 @@ fn prime_field_impl(
let mont_reduce_self_params = mont_reduce_params(quote! {self}, limbs);
let mont_reduce_other_params = mont_reduce_params(quote! {other}, limbs);
let repr_endianness = endianness.repr_endianness();
let from_repr_impl = endianness.from_repr(name, limbs);
let into_repr_impl = endianness.into_repr(repr, &mont_reduce_self_params, limbs);
@@ -1117,58 +1125,9 @@ fn prime_field_impl(
}
}
impl ::core::ops::BitAnd<u64> for #name {
type Output = u64;
#[inline(always)]
fn bitand(mut self, rhs: u64) -> u64 {
self.mont_reduce(
#mont_reduce_self_params
);
self.0[0] & rhs
}
}
impl ::core::ops::Shr<u32> for #name {
type Output = #name;
#[inline(always)]
fn shr(mut self, mut n: u32) -> #name {
if n as usize >= 64 * #limbs {
return Self::from(0);
}
// Convert from Montgomery to native representation.
self.mont_reduce(
#mont_reduce_self_params
);
while n >= 64 {
let mut t = 0;
for i in self.0.iter_mut().rev() {
::core::mem::swap(&mut t, i);
}
n -= 64;
}
if n > 0 {
let mut t = 0;
for i in self.0.iter_mut().rev() {
let t2 = *i << (64 - n);
*i >>= n;
*i |= t;
t = t2;
}
}
// Convert back to Montgomery representation
self * R2
}
}
impl ::ff::PrimeField for #name {
type Repr = #repr;
type ReprEndianness = #repr_endianness;
fn from_repr(r: #repr) -> Option<#name> {
#from_repr_impl