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Implement sub_assign for radix_51

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Henry de Valence 2017-03-12 23:18:29 -07:00
parent 87001f9d6e
commit 4ef22b679c
1 changed files with 16 additions and 11 deletions
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27
src/field.rs
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@ -75,16 +75,6 @@ pub fn mul64(a: &[u64;5], b: &[u64;5]) -> [u64;5] {
[c0,c1,c2,c3,c4]
}
fn subtract(a: &[u64; 5], b: &[u64; 5]) -> [u64; 5] {
// add p to avoid underflow
[ (a[0] + 2251799813685229) - b[0]
, (a[1] + 2251799813685247) - b[1]
, (a[2] + 2251799813685247) - b[2]
, (a[3] + 2251799813685247) - b[3]
, (a[4] + 2251799813685247) - b[4]
]
}
fn from_bytes_64(bytes: &[u8;32]) -> [u64; 5] {
let low_51_bit_mask = (1u64 << 51) - 1;
// load bits [ 0, 64), no shift
@ -250,7 +240,22 @@ impl<'b> SubAssign<&'b FieldElement> for FieldElement {
self[i] -= _rhs[i];
}
}
// XXX_radix_51
#[cfg(feature="radix_51")]
fn sub_assign(&mut self, _rhs: &'b FieldElement) {
// To avoid underflow, first add p
// XXX how many copies should we add to preserve headroom?
self.0[0] += 2251799813685229;
self.0[1] += 2251799813685247;
self.0[2] += 2251799813685247;
self.0[3] += 2251799813685247;
self.0[4] += 2251799813685247;
// then subtract _rhs
self.0[0] -= _rhs.0[0];
self.0[1] -= _rhs.0[1];
self.0[2] -= _rhs.0[2];
self.0[3] -= _rhs.0[3];
self.0[4] -= _rhs.0[4];
}
}
impl<'a, 'b> Sub<&'b FieldElement> for &'a FieldElement {